NAVSTAR GLOBAL POSITIONING SYSTEM INTERFACE SPECIFICATION IS-GPS-200 Revision D IRN-200D-001 7 March 2006 Navstar GPS Space Segment/Navigation User Interfaces // SIGNED // Deputy System Program Director GPS JOINT PROGRAM OFFICE Headquarters Space and Missile Systems Center (SMC) Navstar GPS Joint Program Office (SMC/GP) 2420 Vela Way, Suite 1866 El Segundo, CA 90245-4659 U.S.A. By ARINC Engineering Services, LLC 2250 E. Imperial Highway, Suite 450 El Segundo, CA 90245 U.S.A. Cage Code: 0VYX1 DISTRIBUTION STATEMENT A. APPROVED FOR PUBLIC RELEASE; DISTRIBUTION IS UNLIMITED (This page intentionally left blank.) IS-GPS-200D 7 Dec 2004 ii REVISION RECORD REV DESCRIPTION APPROVED DATE DOCUMENT NC Initial Release 25 Jan 1983 A Incorporates IRN-200NC-001, IRN-200NC-002, and IRN 25 Sep 1984 200NC-003 B 30 Nov 1987 Incorporates IRN-200A-001A C Incorporates IRN-200B-001 thru IRN-200B-007 10 Oct 1993 C Re-formatted in Microsoft Word 6.0 in GEMS compatible format 10 Oct 1993 12 Jan 1996 C Changed distribution status to Public Release 25 Sep 1997 20 Oct 1997 D 7 Dec 2004 23 Nov 2004 GPS-200 to IS-GPS-200, introduce and specify the requirements of Improved Clock and Ephemeris (ICE) message for L2 C signal, and other additional updates Incorporates IRN-200C-001 thru IRN-200C-005R1, change ICD IRN-200D-001 Adds additional PRN sequences to Section 6 7 Mar 2006 9 Mar 2006 IRN-200D-001 IS-GPS-200D 7 Mar 2006 iii (This page intentionally left blank.) IS-GPS-200D 7 Dec 2004 iv Page Revision Record Pages Revision Pages Revision i IRN-200D-001 ii D iii IRN-200D-001 iv D v IRN-200D-001 vi D vii -xi IRN-200D-001 xii - xiii D xiv - xvi IRN-200D-001 1 – 5 D 6 IRN-200D-001 7 – 17 D 18 IRN-200D-001 19 – 56 D 56a – 56l IRN-200D-001 57 – 193 D IRN-200D-001 IS-GPS-200D 7 Mar 2006 v (This page intentionally left blank.) IS-GPS-200D 7 Dec 2004 vi TABLE OF CONTENTS 1. SCOPE....................................................................................................................................................................1 1.1 Scope ..............................................................................................................................................................1 1.2 IS Approval and Changes...............................................................................................................................1 2. APPLICABLE DOCUMENTS.............................................................................................................................3 2.1 Government Documents.................................................................................................................................3 2.2 Non-Government Documents.........................................................................................................................3 3. REQUIREMENTS ................................................................................................................................................5 3.1 Interface Definition ........................................................................................................................................5 3.2 Interface Identification ...................................................................................................................................5 3.2.1 Ranging Codes .....................................................................................................................................5 3.2.1.1 P-Code........................................................................................................................................6 3.2.1.2 Y-Code.......................................................................................................................................6 3.2.1.3 C/A-Code ...................................................................................................................................6 3.2.1.4 L2 CM-Code (IIR-M, IIF, and subsequent blocks).....................................................................6 3.2.1.5 L2 CL-Code (IIR-M, IIF, and subsequent blocks)......................................................................6 3.2.1.6 Non-Standard Codes .................................................................................................................11 3.2.2 NAV Data...........................................................................................................................................11 3.2.3 L1/L2 Signal Structure.......................................................................................................................12 3.3 Interface Criteria...........................................................................................................................................14 3.3.1 Composite Signal ...............................................................................................................................14 3.3.1.1 Frequency Plan..........................................................................................................................14 3.3.1.2 Correlation Loss........................................................................................................................14 3.3.1.3 Carrier Phase Noise...................................................................................................................14 3.3.1.4 Spurious Transmissions.............................................................................................................14 3.3.1.5 Phase Quadrature......................................................................................................................15 3.3.1.6 User-Received Signal Levels ....................................................................................................15 3.3.1.7 Equipment Group Delay............................................................................................................17 3.3.1.7.1 Group Delay Uncertainty ................................................................................................17 3.3.1.7.2 Group Delay Differential ................................................................................................17 3.3.1.8 Signal Coherence......................................................................................................................17 3.3.1.9 Signal Polarization ....................................................................................................................17 3.3.2 PRN Code Characteristics..................................................................................................................18 3.3.2.1 Code Structure..........................................................................................................................18 IRN-200D-001 IS-GPS-200D 7 Mar 2006 vii 3.3.2.2 P-Code Generation ....................................................................................................................20 3.3.2.3 C/A-Code Generation................................................................................................................30 3.3.2.4 L2 CM-/L2 CL-Code Generation..............................................................................................35 3.3.3 Navigation Data..................................................................................................................................38 3.3.3.1 Navigation Data Modulation (L2 CM)......................................................................................38 3.3.3.1.1 Forward Error Correction................................................................................................38 3.3.4 GPS Time and SV Z-Count.................................................................................................................40 4. NOT APPLICABLE............................................................................................................................................43 5. NOT APPLICABLE............................................................................................................................................45 6. NOTES..................................................................................................................................................................47 6.1 Acronyms .....................................................................................................................................................47 6.2 Definitions....................................................................................................................................................51 6.2.1 User Range Accuracy.........................................................................................................................51 6.2.2 SV Block Definitions ..........................................................................................................................51 6.2.2.1 Developmental SVs...................................................................................................................51 6.2.2.2 Operational SVs ........................................................................................................................51 6.2.2.2.1 Block II SVs....................................................................................................................51 6.2.2.2.2 Block IIA SVs.................................................................................................................51 6.2.2.2.3 Block IIR SVs .................................................................................................................52 6.2.2.2.4 Block IIR-M SVs ............................................................................................................52 6.2.2.2.5 Block IIF SVs..................................................................................................................52 6.2.3 Operational Interval Definitions..........................................................................................................52 6.2.3.1 Normal Operations ....................................................................................................................52 6.2.3.2 Short-term Extended Operations ...............................................................................................52 6.2.3.3 Long-term Extended Operations ...............................................................................................52 6.2.4 GPS Week Number............................................................................................................................53 6.2.5 L5 Civil Signal...................................................................................................................................53 6.3 Supporting Material......................................................................................................................................53 6.3.1 Received Signals ................................................................................................................................53 6.3.2 Extended Navigation Mode (Block II/IIA)..........................................................................................55 6.3.3 Block IIA Mode (Block IIR/IIR-M)....................................................................................................56 6.3.4 Autonomous Navigation Mode ...........................................................................................................56 6.3.5 PRN Code sequences expansion ....................................................................................................... 56a 6.3.5.1 Additional C/A-code PRN sequences ..................................................................................... 56a 6.3.5.2 Additional P-Code PRN sequences.........................................................................................56b IRN-200D-001 IS-GPS-200D 7 Mar 2006 viii 6.3.5.2.1 Additional P-code Generation.......................................................................................56b 6.3.5.3 Additional L2 CM-/L2 CL-Code PRN sequences....................................................................56i 10. APPENDIX I. LETTERS OF EXCEPTION...................................................................................................57 10.1 Scope ..........................................................................................................................................................57 10.2 Applicable Documents ...............................................................................................................................57 10.3 Letters of Exception ...................................................................................................................................57 20. APPENDIX II. GPS NAVIGATION DATA STRUCTURE FOR DATA, D(t) ...........................................65 20.1 Scope ..........................................................................................................................................................65 20.2 Applicable Documents. ...............................................................................................................................65 20.2.1 Government Documents....................................................................................................................65 20.2.2 Non-Government Documents............................................................................................................65 20.3 Requirements..............................................................................................................................................67 20.3.1 Data Characteristics..........................................................................................................................67 20.3.2 Message Structure .............................................................................................................................67 20.3.3 Message Content ...............................................................................................................................81 20.3.3.1 Telemetry Word ......................................................................................................................81 20.3.3.2 Handover Word (HOW)..........................................................................................................81 20.3.3.3 Subframe 1 ..............................................................................................................................83 20.3.3.3.1 Subframe 1 Content.......................................................................................................83 20.3.3.3.2 Subframe 1 Parameter Characteristics ..........................................................................86 20.3.3.3.3 User Algorithms for Subframe 1 Data ..........................................................................86 20.3.3.4 Subframes 2 and 3...................................................................................................................93 20.3.3.4.1 Content of Subframes 2 and 3.......................................................................................93 20.3.3.4.2 Subframe 2 and 3 Parameter Characteristics.................................................................95 20.3.3.4.3 User Algorithm for Ephemeris Determination..............................................................95 20.3.3.4.4 NMCT Validity Time..................................................................................................101 20.3.3.5 Subframes 4 and 5.................................................................................................................102 20.3.3.5.1 Content of Subframes 4 and 5.....................................................................................102 20.3.3.5.2 Algorithms Related to Subframe 4 and 5 Data............................................................117 20.3.4 Timing Relationships ......................................................................................................................126 20.3.4.1 Paging and Cutovers..............................................................................................................126 20.3.4.2 SVTime vs. GPS Time.........................................................................................................126 20.3.4.3 Speed of Light.......................................................................................................................126 20.3.4.4 Data Sets...............................................................................................................................127 20.3.4.5 Reference Times....................................................................................................................130 IRN-200D-001 IS-GPS-200D 7 Mar 2006 ix 20.3.5 Data Frame Parity...........................................................................................................................133 20.3.5.1 SV/CS Parity Algorithm........................................................................................................133 20.3.5.2 User Parity Algorithm...........................................................................................................133 30. APPENDIX III. GPS NAVIGATION DATA STRUCTURE FOR CNAV DATA, DC(t)..........................137 30.1 Scope ........................................................................................................................................................137 30.2 Applicable Documents. .............................................................................................................................137 30.2.1 Government Documents..................................................................................................................137 30.2.2 Non-Government Documents..........................................................................................................137 30.3 Requirements............................................................................................................................................139 30.3.1 Data Characteristics........................................................................................................................139 30.3.2 Message Structure ...........................................................................................................................139 30.3.3 Message Content .............................................................................................................................139 30.3.3.1 Message Type 10 and 11 Ephemeris and Health Parameters...............................................155 30.3.3.1.1 Message Type 10 and 11 Ephemeris and Health Parameter Content.........................155 30.3.3.1.2 Message Type 10 and 11 Ephemeris Parameter Characteristics.................................158 30.3.3.1.3 User Algorithm for Determination of SV Position......................................................158 30.3.3.2 Message Types 30 Through 37 SV Clock Correction Parameters. .......................................163 30.3.3.2.1 Message Type 30 Through 37 SV Clock Correction Parameter Content....................163 30.3.3.2.2 Clock Parameter Characteristics .................................................................................163 30.3.3.2.3 User Algorithms for SV Clock Correction Data .........................................................163 30.3.3.2.4 SV Clock Accuracy Estimates ....................................................................................165 30.3.3.3 Message Type 30 Ionospheric and Group Delay Correction Parameters..............................168 30.3.3.3.1 Message Type 30 Ionospheric and Group Delay Correction Parameter Content........168 30.3.3.4 Message Types 31, 12, and 37 Almanac Parameters.............................................................171 30.3.3.4.1 Almanac Reference Week...........................................................................................171 30.3.3.4.2 Almanac Reference Time............................................................................................171 30.3.3.4.3 SV PRN Number.........................................................................................................171 30.3.3.4.4 Signal Health (L1/L2/L5)............................................................................................171 30.3.3.4.5 Midi Almanac Parameter Content...............................................................................172 30.3.3.4.6 Reduced Almanac Parameter Content.........................................................................172 30.3.3.5 Message Type 32 Earth Orientation Parameters (EOP)........................................................175 30.3.3.5.1 EOP Content ...............................................................................................................175 30.3.3.6 Message Type 33 Coordinated Universal Time (UTC) Parameters......................................179 30.3.3.6.1 UTC Parameter Content..............................................................................................179 30.3.3.6.2 UTC and GPS Time ....................................................................................................179 IRN-200D-001 IS-GPS-200D 7 Mar 2006 x 30.3.3.7 Message Types 34, 13, and 14 Differential Correction Parameters ......................................181 30.3.3.7.1 Differential Correction Parameters Content................................................................181 30.3.3.7.2 DC Data Packet...........................................................................................................181 30.3.3.7.3 Application of Clock-Related DC Data.......................................................................184 30.3.3.7.4 Application of Orbit-Related DC Data........................................................................184 30.3.3.7.5 SV Differential Range Accuracy Estimates................................................................186 30.3.3.8 Message Type 35 GPS/GNSS Time Offset...........................................................................187 30.3.3.8.1 GPS/GNSS Time Offset Parameter Content...............................................................187 30.3.3.8.2 GPS and GNSS Time..................................................................................................187 30.3.3.9 Message Types 36 and 15 Text Messages.............................................................................188 30.3.4 Timing Relationships ......................................................................................................................189 30.3.4.1 Paging and Cutovers..............................................................................................................189 30.3.4.2 SVTime vs. GPS Time.........................................................................................................190 30.3.4.3 Speed of Light.......................................................................................................................190 30.3.5 Data Frame Parity...........................................................................................................................191 30.3.5.1 Parity Algorithm....................................................................................................................191 IRN-200D-001 IS-GPS-200D 7 Mar 2006 xi LIST OF FIGURES Figure 3-1. Generation of P-, C/A-Codes and Modulating Signals...................................................................19 Figure 3-2. X1A Shift Register Generator Configuration..................................................................................21 Figure 3-3. X1B Shift Register Generator Configuration..................................................................................22 Figure 3-4. X2A Shift Register Generator Configuration..................................................................................23 Figure 3-5. X2B Shift Register Generator Configuration..................................................................................24 Figure 3-6. P-Code Generation.........................................................................................................................26 Figure 3-7. P-Code Signal Component Timing .................................................................................................27 Figure 3-8. G1 Shift Register Generator Configuration ....................................................................................31 Figure 3-9. G2 Shift Register Generator Configuration ....................................................................................32 Figure 3-10. Example C/A-Code Generation ......................................................................................................33 Figure 3-11. C/A-Code Timing Relationships.....................................................................................................34 Figure 3-12. L2 CM-/L2 CL-Code Timing Relationships...................................................................................36 Figure 3-13. L2 CM/L2 CL Shift Register Generator Configuration..................................................................37 Figure 3-14. Convolutional Encoder ...................................................................................................................39 Figure 3-15. Convolutional Transmit/Decoding Timing Relationships...............................................................39 Figure 3-16. Time Line Relationship of HOW Message.....................................................................................42 Figure 6-1. User Received Minimum Signal Level Variations (Example, Block II/IIA/IIR)............................54 Figure 10-1. Letters of Exception.......................................................................................................................59 Figure 20-1. Data Format ...................................................................................................................................69 Figure 20-2. TLM and HOW Formats.................................................................................................................82 Figure 20-3. Sample Application of Correction Parameters................................................................................92 Figure 20-4. Ionospheric Model........................................................................................................................123 Figure 20-5. Example Flow Chart for User Implementation of Parity Algorithm.............................................135 Figure 30-1. Message Type 10 - Ephemeris 1...................................................................................................141 Figure 30-2. Message Type 11 - Ephemeris 2...................................................................................................142 Figure 30-3. Message Type 30 - Clock, IONO & Group Delay........................................................................143 Figure 30-4. Message Type 31 - Clock & Reduced Almanac ...........................................................................144 Figure 30-5. Message Type 32 - Clock & EOP.................................................................................................145 Figure 30-6. Message Type 33 - Clock & UTC.................................................................................................146 Figure 30-7. Message Type 34 - Clock & Differential Correction....................................................................147 Figure 30-8. Message Type 35 -Clock & GGTO .............................................................................................148 Figure 30-9. Message Type 36 - Clock & Text.................................................................................................149 Figure 30-10. Message Type 37 - Clock & Midi Almanac .................................................................................150 IS-GPS-200D 7 Dec 2004 xii Figure 30-11. Message Type 12 - Reduced Almanac..........................................................................................151 Figure 30-12. Message Type 13 – Clock Differential Correction .......................................................................152 Figure 30-13. Message Type 14 – Ephemeris Differential Correction................................................................153 Figure 30-14. Message Type 15 - Text................................................................................................................154 Figure 30-15. ReducedAlmanac Packet Content................................................................................................174 Figure 30-16. Differential Correction Data Packet..............................................................................................182 IS-GPS-200D 7 Dec 2004 xiii LIST OF TABLES Table 3-I. Code Phase Assignments..................................................................................................................7 Table 3-II. Code Phase Assignments (IIR-M, IIF, and subsequent blocks only)................................................9 Table 3-III. Signal Configuration......................................................................................................................13 Table 3-IV. Composite L1 Transmitted Signal Phase........................................................................................16 Table 3-V. Received Minimum RF Signal Strength .........................................................................................16 Table 3-VI. P-Code Reset Timing......................................................................................................................28 Table 3-VII. Final Code Vector States.................................................................................................................29 Table 6-I Additional C/A-/P-Code Phase Assignments................................................................................ 56c Table 6-II. Additional L2 CM-/L2 CL-Code Phase Assignments ...................................................................56i Table 20-I. Subframe 1 Parameters ...................................................................................................................87 Table 20-II. Ephemeris Data Definitions............................................................................................................94 Table 20-III. Ephemeris Parameters.....................................................................................................................96 Table 20-IV. Elements of Coordinate Systems ....................................................................................................97 Table 20-V. Data IDs and SV IDs in Subframes 4 and 5..................................................................................105 Table 20-VI. Almanac Parameters .....................................................................................................................107 Table 20-VII. NAV Data Health Indications .......................................................................................................109 Table 20-VIII. Codes for Health of SV Signal Components.................................................................................110 Table 20-IX. UTC Parameters...........................................................................................................................113 Table 20-X. Ionospheric Parameters.................................................................................................................114 Table 20-XI. IODC Values and Data Set Lengths (Block II/IIA).....................................................................128 Table 20-XII. IODC Values and Data Set Lengths (Block IIR/IIR-M/IIF)........................................................129 Table 20-XIII. Reference Times ...........................................................................................................................132 Table 20-XIV. Parity Encoding Equations............................................................................................................134 Table 30-I. Message Types 10 and 11 Parameters...........................................................................................159 Table 30-II. Elements of Coordinate System....................................................................................................161 Table 30-III. Clock Correction and Accuracy Parameters .................................................................................164 Table 30-IV. Group Delay Differential Parameters............................................................................................168 Table 30-V. Midi Almanac Parameters.............................................................................................................173 Table 30-VI. Reduced Almanac Parameters.......................................................................................................174 Table 30-VII. Earth Orientation Parameters.........................................................................................................176 Table 30-VIII. Application of EOP Parameters .....................................................................................................177 Table 30-IX. UTC Parameters...........................................................................................................................180 Table 30-X. Differential Correction Parameters.................................................................................................183 IRN-200D-001 IS-GPS-200D 7 Mar 2006 xiv Table 30-XI. GPS/GNSS Time Offset Parameters.............................................................................................188 Table 30-XII. Message Broadcast Intervals.........................................................................................................189 IRN-200D-001 IS-GPS-200D 7 Mar 2006 xv (This page intentionally left blank.) IRN-200D-001 IS-GPS-200D 7 Mar 2006 xvi 1. SCOPE 1.1 Scope. This Interface Specification (IS) defines the requirements related to the interface between the Space Segment (SS) of the Global Positioning System (GPS) and the navigation User Segment (US) of the GPS for radio frequency (RF) link 1 (L1) and link 2 (L2). 1.2 IS Approval and Changes. ARINC Engineering Services, LLC has been designated the Interface Control Contractor (ICC), and is responsible for the basic preparation, approval, distribution, retention, and Interface Control Working Group (ICWG) coordination of the IS in accordance with GP-03-001. The Navstar GPS Joint Program Office is the necessary authority to make this IS effective. The Joint Program Office (JPO) administers approvals under the auspices of the Configuration Control Board (CCB), which is governed by the appropriate JPO Operating Instruction (OI). Military organizations and contractors are represented at the CCB by their respective segment member. All civil organizations and public interest are represented by the Department of Transportation representative of the GPS JPO. A proposal to change the approved version of this IS can be submitted by any ICWG participating organization to the GPS JPO and/or the ICC. The ICC is responsible for the preparation of the change paper and change coordination, in accordance with GP-03-001. The ICC prepares the change paper as a Proposed Interface Revision Notice (PIRN) and is responsible for coordination of PIRNs with the ICWG. The ICWG coordinated PIRN must be submitted to the GPS JPO CCB for review and approval. The ICWG review period for all Proposed Interface Revisions Notices (PIRNs) is 45 days after receipt by individual addressees. A written request to extend the review period may be submitted to the ICC for consideration. IS-GPS-200D 7 Dec 2004 1 (This page intentionally left blank.) IS-GPS-200D 7 Dec 2004 2 2. APPLICABLE DOCUMENTS 2.1 Government Documents. The following documents of the issue specified contribute to the definition of the interfaces between the GPS Space Segment and the GPS navigation User Segment, and form a part of this IS to the extent specified herein. Specifications Federal None Military None Other Government Activity None Standards Federal None Military None Other Publications GP-03-001 GPS Interface Control Working Group Charter 14 Nov 2003 2.2 Non-Government Documents. The following documents of the issue specified contribute to the definition of the interfaces between the GPS Space Segment and the GPS Navigation User Segment and form a part of this IS to the extent specified herein. Specifications None Other Publications None IS-GPS-200D 7 Dec 2004 3 (This page intentionally left blank.) IS-GPS-200D 7 Dec 2004 4 3. REQUIREMENTS 3.1 Interface Definition. The interface between the GPS Space Segment (SS) and the GPS navigation User Segment (US) includes two RF links, L1 and L2. Utilizing these links, the space vehicles (SVs) of the SS shall provide continuous earth coverage signals that provide to the US the ranging codes and the system data needed to accomplish the GPS navigation (NAV) mission. These signals shall be available to a suitably equipped user with RF visibility to an SV. 3.2 Interface Identification. The carriers of L1 and L2 are typically modulated by one or more bit trains, each of which normally is a composite generated by the modulo-2 addition of a pseudo-random noise (PRN) ranging code and the downlink system data (referred to as NAV data). 3.2.1 Ranging Codes. Three PRN ranging codes are transmitted: the precision (P) code which is the principal NAV ranging code; the Y-code, used in place of the P-code whenever the anti-spoofing (A-S) mode of operation is activated; and the coarse/acquisition (C/A) code which is used for acquisition of the P (or Y) code (denoted as P(Y)) and as a civil ranging signal. Code-division-multiple-access techniques allow differentiating between the SVs even though they may transmit at the same frequencies. The SVs will transmit intentionally "incorrect" versions of the C/A and the P(Y) codes where needed to protect the users from receiving and utilizing anomalous NAV signals as a result of a malfunction in the SV's reference frequency generation system. These two "incorrect" codes are termed non-standard C/A (NSC) and non-standard Y (NSY) codes. For Block IIR-M, IIF, and subsequent blocks of SVs, two additional PRN ranging codes are transmitted. They are the L2 civil-moderate (L2 CM) code and the L2 civil-long (L2 CL) code. The SVs will transmit intentionally "incorrect" versions of the L2 CM and L2 CL codes where needed to protect the users from receiving and utilizing anomalous NAV signals as a result of a malfunction in the SV's reference frequency generation system. These "incorrect" codes are termed non-standard L2 CM (NSCM) and non-standard L2 CL (NSCL) codes. The SVs shall also be capable of initiating and terminating the broadcast of NSCM and/or NSCL code(s) independently of each other, in response to CS command. IS-GPS-200D 7 Dec 2004 5 3.2.1.1 P-Code. The PRN P-code for SV ID number i is a ranging code, Pi(t), of 7 days in length at a chipping rate of 10.23 Mbps. The 7 day sequence is the modulo-2 sum of two sub-sequences referred to as X1 and X2i; their lengths are 15,345,000 chips and 15,345,037 chips, respectively. The X2i sequence is an X2 sequence selectively delayed by 1 to 37 chips thereby allowing the basic code generation technique to produce a set of 37 mutually exclusive P-code sequences of 7 days in length. Of these, 32 are designated for use by SVs and 5 are reserved for other purposes (e.g. ground transmitters, etc.). Assignment of these code phase segments by SV-ID number (or other use) is given in Table 3-I. Additional PRN P-code sequences with assigned PRN numbers are provided in Section 6.3.5.2, Table 6-I 3.2.1.2 Y-Code. The PRN Y-code is used in place of the P-code when the A-S mode of operation is activated. 3.2.1.3 C/A-Code. The PRN C/A-Code for SV ID number i is a Gold code, Gi(t), of 1 millisecond in length at a chipping rate of 1023 Kbps. The Gi(t) sequence is a linear pattern generated by the modulo-2 addition of two subsequences, G1 and G2i, each of which is a 1023 chip long linear pattern. The epochs of the Gold code are synchronized with the X1 epochs of the P-code. As shown in Table 3-I, the G2i sequence is a G2 sequence selectively delayed by pre-assigned number of chips, thereby generating a set of different C/A-codes. Assignment of these by GPS PRN signal number is given in Table 3-I. Additional PRN C/A-code sequences with assigned PRN numbers are provided in Section 6.3.5.1, Table 6-I 3.2.1.4 L2 CM-Code (IIR-M, IIF, and subsequent blocks). The PRN L2 CM-code for SV ID number i is a ranging code, CM,i(t), which is 20 milliseconds in length at a chipping rate of 511.5 Kbps. The epochs of the L2 CM-code are synchronized with the X1 epochs of the P-code. The CM,i(t) sequence is a linear pattern which is short cycled every count of 10230 chips by resetting with a specified initial state. Assignment of initial states by GPS PRN signal number is given in Table 3-II. Additional PRN L2 CM-code sequence pairs are provided in Section 6.3.5.3, Table 6-II 3.2.1.5 L2 CL-Code (IIR-M, IIF, and subsequent blocks). The PRN L2 CL-code for SV ID number i is a ranging code, CL,i(t), which is 1.5 seconds in length at a chipping rate of 511.5 Kbps. The epochs of the L2 CL-code are synchronized with the X1 epochs of the P-code. The CL,i(t) sequence is a linear pattern which is generated using the same code generator polynomial as the one used for CM,i(t). However, the CL,i(t) sequence is short cycled by resetting with a specified initial state every code count of 767250 chips. Assignment of initial states by GPS PRN signal number is given in Table 3-II. Additional PRN L2 CL-code sequence pairs are provided in Section 6.3.5.3, Table 6-II IRN-200D-001 IS-GPS-200D 7 Mar 2006 6 Table 3-I. Code Phase Assignments (sheet 1 of 2) SV ID No. GPS PRN Signal No. Code Phase Selection Code Delay Chips First 10 Chips Octal* C/A First 12 Chips Octal PC/A(G2i)**** (X2i) C/A P 1 1 2⊕6 1 5 1 1440 4444 2 2 3⊕7 2 6 2 1620 4000 3 3 4⊕8 3 7 3 1710 4222 4 4 5⊕9 4 8 4 1744 4333 5 5 1⊕9 5 17 5 1133 4377 6 6 2 ⊕ 10 6 18 6 1455 4355 7 7 1⊕8 7 139 7 1131 4344 8 8 2⊕9 8 140 8 1454 4340 9 9 3 ⊕ 10 9 141 9 1626 4342 10 10 2⊕3 10 251 10 1504 4343 11 12 13 14 15 16 17 18 11 12 13 14 15 16 17 18 3⊕4 5⊕6 6⊕7 7⊕8 8⊕9 9 ⊕ 10 1⊕4 2⊕5 11 12 13 14 15 16 17 18 252 254 255 256 257 258 469 470 11 12 13 14 15 16 17 18 1642 1750 1764 1772 1775 1776 1156 1467 ⏐ ⏐ ⏐ ⏐ ⏐ ⏐ ⏐ ⏐ 19 19 3⊕6 19 471 19 1633 4343 * In the octal notation for the first 10 chips of the C/A code as shown in this column, the first digit (1) represents a "1" for the first chip and the last three digits are the conventional octal representation of the remaining 9 chips. (For example, the first 10 chips of the C/A code for PRN Signal Assembly No. 1 are: 1100100000). ** C/A codes 34 and 37 are common. *** PRN sequences 33 through 37 are reserved for other uses (e.g. ground transmitters). **** The two-tap coder utilized here is only an example implementation that generates a limited set of valid C/A codes. ⊕ = "exclusive or" NOTE: The code phase assignments constitute inseparable pairs, each consisting of a specific C/A and a specific P code phase, as shown above. IS-GPS-200D 7 Dec 2004 7 Table 3-I. Code Phase Assignments (sheet 2 of 2) SV ID No. GPS PRN Signal No. Code Phase Selection Code Delay Chips First 10 Chips Octal* C/A First 12 Chips Octal PC/A(G2i)**** (X2i) C/A P 20 21 22 23 24 25 26 27 28 29 30 31 32 *** *** *** *** *** 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34** 35 36 37** 4⊕7 5⊕8 6⊕9 1⊕3 4⊕6 5⊕7 6⊕8 7⊕9 8 ⊕ 10 1⊕6 2⊕7 3⊕8 4⊕9 5 ⊕ 10 4 ⊕ 10 1⊕7 2⊕8 4 ⊕ 10 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 472 473 474 509 512 513 514 515 516 859 860 861 862 863 950 947 948 950 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 1715 1746 1763 1063 1706 1743 1761 1770 1774 1127 1453 1625 1712 1745 1713 1134 1456 1713 4343 ⏐ ⏐ ⏐ ⏐ ⏐ ⏐ ⏐ ⏐ ⏐ ⏐ ⏐ ⏐ ⏐ ⏐ ⏐ ⏐ 4343 * In the octal notation for the first 10 chips of the C/A code as shown in this column, the first digit (1) represents a "1" for the first chip and the last three digits are the conventional octal representation of the remaining 9 chips. (For example, the first 10 chips of the C/A code for PRN Signal Assembly No. 1 are: 1100100000). ** C/A codes 34 and 37 are common. *** PRN sequences 33 through 37 are reserved for other uses (e.g. ground transmitters). **** The two-tap coder utilized here is only an example implementation that generates a limited set of valid C/A codes. ⊕ = "exclusive or" NOTE: The code phase assignments constitute inseparable pairs, each consisting of a specific C/A and a specific P code phase, as shown above. IS-GPS-200D 7 Dec 2004 8 Table 3-II. Code Phase Assignments (IIR-M, IIF, and subsequent blocks only) (sheet 1 of 2) SV ID No. GPS PRN Signal No. Initial Shift Register State (Octal) End Shift Register State (Octal) L2 CM L2 CL L2 CM * L2 CL ** 1 1 742417664 624145772 552566002 267724236 2 2 756014035 506610362 034445034 167516066 3 3 002747144 220360016 723443711 771756405 4 4 066265724 710406104 511222013 047202624 5 5 601403471 001143345 463055213 052770433 6 6 703232733 053023326 667044524 761743665 7 7 124510070 652521276 652322653 133015726 8 8 617316361 206124777 505703344 610611511 9 9 047541621 015563374 520302775 352150323 10 10 733031046 561522076 244205506 051266046 11 11 713512145 023163525 236174002 305611373 12 12 024437606 117776450 654305531 504676773 13 13 021264003 606516355 435070571 272572634 14 14 230655351 003037343 630431251 731320771 15 15 001314400 046515565 234043417 631326563 16 16 222021506 671511621 535540745 231516360 17 17 540264026 605402220 043056734 030367366 18 18 205521705 002576207 731304103 713543613 19 19 064022144 525163451 412120105 232674654 * Short cycled period = 10230 ** Short cycled period = 767250 *** PRN sequences 33 through 37 are reserved for other uses (e.g. ground transmitters). NOTE: There are many other available initial register states which can be used for other signal transmitters including any additional SVs in future. IS-GPS-200D 7 Dec 2004 9 Table 3-II. Code Phase Assignments (IIR-M, IIF, and subsequent blocks only) (sheet 2 of 2) SV ID No. GPS PRN Signal No. Initial Shift Register State (Octal) End Shift Register State (Octal) L2 CM L2 CL L2 CM * L2 CL ** 20 20 120161274 266527765 365636111 641733155 21 21 044023533 006760703 143324657 730125345 22 22 724744327 501474556 110766462 000316074 23 23 045743577 743747443 602405203 171313614 24 24 741201660 615534726 177735650 001523662 25 25 700274134 763621420 630177560 023457250 26 26 010247261 720727474 653467107 330733254 27 27 713433445 700521043 406576630 625055726 28 28 737324162 222567263 221777100 476524061 29 29 311627434 132765304 773266673 602066031 30 30 710452007 746332245 100010710 012412526 31 31 722462133 102300466 431037132 705144501 32 32 050172213 255231716 624127475 615373171 *** 33 500653703 437661701 154624012 041637664 *** 34 755077436 717047302 275636742 100107264 *** 35 136717361 222614207 644341556 634251723 *** 36 756675453 561123307 514260662 257012032 *** 37 435506112 240713073 133501670 703702423 * Short cycled period = 10230 ** Short cycled period = 767250 *** PRN sequences 33 through 37 are reserved for other uses (e.g. ground transmitters). NOTE: There are many other available initial register states which can be used for other signal transmitters including any additional SVs in future. IS-GPS-200D 7 Dec 2004 10 3.2.1.6 Non-Standard Codes. The NSC, NSCM, NSCL, and NSY codes, used to protect the user from a malfunction in the SV's reference frequency system (reference paragraph 3.2.1), are not for utilization by the user and, therefore, are not defined in this document. 3.2.2 NAV Data. The NAV data, D(t), includes SV ephemerides, system time, SV clock behavior data, status messages and C/A to P (or Y) code handover information, etc. The 50 bps data is modulo-2 added to the P(Y)and C/A- codes; the resultant bit-trains are used to modulate the L1 and L2 carriers. For a given SV, the data train D(t), if present, is common to the P(Y)- and C/A- codes on both the L1 and L2 channels. The content and characteristics of the NAV data, D(t), are given in Appendix II of this document. For Block IIR-M, Block IIF, and subsequent blocks of SVs, civil navigation (CNAV) data, DC(t), also includes SV ephemerides, system time, SV clock behavior, status messages, etc. The DC(t) is a 25 bps data stream which is coded by a rate ½ convolutional coder. When selected by ground command, the resulting 50 sps symbol stream is modulo-2 added to the L2 CM-code; the resultant bit-train is combined with L2 CL-code using chip by chip time- division multiplexing method (i.e. alternating between L2 CM ⊕ data and L2 CL chips); the multiplexed bit-train is used to modulate the L2 carrier. The content and characteristics of the CNAV data, DC(t), are given in Appendix III of this document. During the initial period of Block IIR-M SVs operation, prior to Initial Operational Capability of L2 C signal, Block IIR-M may modulo-2 add the NAV data, D(t), to the L2 CM-code instead of CNAV data, DC(t). Moreover, the NAV data, D(t), can be used in one of two different data rates which are selectable by ground command. D(t) with a data rate of 50 bps can be commanded to be modulo-2 added to the L2 CM-code, or D(t) with a symbol rate of 50 symbols per second (sps) (rate ½ convolutional encode of a 25 bps NAV data) can be commanded to be modulo-2 added to the L2 CM-code. The resultant bit-train is combined with L2 CL-code using chip by chip time-division multiplexing method (i.e. alternating between L2 CM ⊕ data and L2 CL chips). This multiplexed bit-train is used to modulate the L2 carrier. IS-GPS-200D 7 Dec 2004 11 3.2.3 L1/L2 Signal Structure. The L1 consists of two carrier components which are in phase quadrature with each other. Each carrier component is bi-phase shift key (BPSK) modulated by a separate bit train. One bit train is the modulo-2 sum of the P(Y)-code and NAV data, D(t), while the other is the modulo-2 sum of the C/A-code and the NAV data, D(t). For Block II/IIA and IIR, the L2 is BPSK modulated by only one of those two bit trains; the bit train to be used for L2 modulation is selected by ground command. A third modulation mode is also selectable on the L2 channel by ground command: it utilizes the P(Y)-code without the NAV data as the modulating signal. For a particular SV, all transmitted signal elements (carriers, codes and data) are coherently derived from the same onboard frequency source. For Block IIR-M, Block IIF, and subsequent blocks of SVs, the L2 consists of two carrier components. One carrier component is BPSK modulated by the bit train which is the modulo-2 sum of the P(Y)-code with or without NAV data D(t), while the other is BPSK modulated by any one of three other bit trains which are selectable by ground command. The three possible bit trains are: (1) the modulo-2 sum of the C/A-code and D(t); (2) the C/A-code with no data and; (3) a chip-by-chip time multiplex combination of bit trains consisting of the L2 CM-code with DC(t) and the L2 CL-code with no data. The L2 CM-code with the 50 sps symbol stream of DC(t) is time-multiplexed with L2 CL-code at a 1023 kHz rate as described in paragraph 3.2.2. The first L2 CM-code chip starts synchronously with the end/start of week epoch. During the initial period of Block IIR-M SVs operation, prior to Initial Operational Capability of L2 C signal, Block IIR-M may modulo-2 add the NAV data, D(t), to the L2 CM-code instead of CNAV data, DC(t). In such configuration, the data rate of D(t) may be 50 bps (i.e. without convolution encoding) or it may be 25 bps. The D(t) of 25 bps shall be convolutionally encoded resulting in 50 sps. The different configuration and combination of codes/signals specified in this section are shown in Table 3-III. IS-GPS-200D 7 Dec 2004 12 Table 3-III. Signal Configuration SV Blocks L1 L2** In-Phase* Quadrature-Phase* In-Phase* Quadrature-Phase* Block II/IIA/IIR P(Y) ⊕ D(t) C/A ⊕ D(t) P(Y) ⊕ D(t) or P(Y) or C/A ⊕ D(t) Not Applicable Block IIR-M*** P(Y) ⊕ D(t) C/A ⊕ D(t) P(Y) ⊕ D(t) or P(Y) L2 CM ⊕ D(t) with L2 CL or L2 CM ⊕ D′(t) with L2 CL or C/A ⊕ D(t) or C/A Block IIR-M/IIF P(Y) ⊕ D(t) C/A ⊕ D(t) P(Y) ⊕ D(t) or P(Y) L2 CM ⊕ DC(t) with L2 CL or C/A ⊕ D(t) or C/A Notes: 1) The configuration identified in this table reflects only the content of Section 3.2.3 and does not show all available codes/signals on L1/L2. 2) It should be noted that there are no flags or bits in the navigation message to directly indicate which signal option is broadcast for L2 Civil (L2 C) signal. ⊕ = “exclusive-or” (modulo-2 addition) D(t) = NAV data at 50 bps D′(t) = NAV data at 25 bps with FEC encoding resulting in 50 sps DC(t) = CNAV data at 25 bps with FEC encoding resulting in 50 sps * Terminology of “in-phase” and “quadrature-phase” is used only to identify the relative phase quadrature relationship of the carrier components (i.e. 90 degrees offset of each other). ** The two carrier components on L2 may not have the phase quadrature relationship. They may be broadcast on same phase (ref. Section 3.3.1.5). *** Possible signal configuration for Block IIR-M only during the initial period of Block IIR-M SVs operation, prior to Initial Operational Capability of L2 C signal. See paragraph 3.2.2. IS-GPS-200D 7 Dec 2004 13 3.3 Interface Criteria. The criteria specified in the following define the requisite characteristics of the SS/US interface for the L1 and L2. 3.3.1 Composite Signal. The following criteria define the characteristics of the composite signals. 3.3.1.1 Frequency Plan. The signals shall be contained within two 20.46-MHz bands centered about L1 and L2. The carrier frequencies for the L1 and L2 signals shall be coherently derived from a common frequency source within the SV. The nominal frequency of this source --as it appears to an observer on the ground --is 10.23 MHz. The SV carrier frequency and clock rates -- as they would appear to an observer located in the SV --are offset to compensate for relativistic effects. The clock rates are offset by ∆ f/f = -4.4647E-10, equivalent to a change in the P-code chipping rate of 10.23 MHz offset by a ∆ f = -4.5674E-3 Hz. This is equal to 10.22999999543 MHz. The nominal carrier frequencies (f0) shall be 1575.42 MHz, and 1227.6 MHz for L1 and L2, respectively. 3.3.1.2 Correlation Loss. Correlation loss is defined as the difference between the SV power received in a 20.46 MHz bandwidth and the signal power recovered in an ideal correlation receiver of the same bandwidth. On the L1 and L2 channels, the worst case correlation loss occurs when the carrier is modulated by the sum of the P(Y) code and the NAV data stream. For this case, the correlation loss apportionment shall be as follows: 1. SV modulation imperfections 0.6 dB 2. Ideal UE receiver waveform distortion 0.4 dB (due to 20.46 MHz filter) 3.3.1.3 Carrier Phase Noise. The phase noise spectral density of the unmodulated carrier shall be such that a phase locked loop of 10 Hz one-sided noise bandwidth shall be able to track the carrier to an accuracy of 0.1 radians rms. 3.3.1.4 Spurious Transmissions. In-band spurious transmissions shall be at least 40 dB below the unmodulated L1 and L2 carriers over the allocated 20.46 MHz channel bandwidth. IS-GPS-200D 7 Dec 2004 14 3.3.1.5 Phase Quadrature. The two L1 carrier components modulated by the two separate bit trains (C/A-code plus data and P(Y)-code plus data) shall be in phase quadrature (within ±100 milliradians) with the C/A signal carrier lagging the P signal by 90 degrees. Referring to the phase of the P carrier when Pi(t) equals zero as the "zero phase angle", the P(Y)- and C/A-code generator output shall control the respective signal phases in the following manner: when Pi(t) equals one, a 180-degree phase reversal of the P-carrier occurs; when Gi(t) equals one, the C/A carrier advances 90 degrees; when the Gi(t) equals zero, the C/A carrier shall be retarded 90 degrees (such that when Gi(t) changes state, a 180-degree phase reversal of the C/A carrier occurs). The resultant nominal composite transmitted signal phases as a function of the binary state of only the two modulating signals are as shown in Table 3-IV. For Block IIR-M, IIF, and subsequent blocks of SVs, phase quadrature relationship between the two L2 carrier components can be the same as for the two L1 carrier components as described above. However, for the L2 case, the civil signal carrier component is modulated by any one of three (IIF) or four (IIR-M) different bit trains as described in paragraph 3.2.3. Moreover, the two L2 carrier components can be in same phase. The resultant composite transmitted signal phases will vary as a function of the binary state of the modulating signals as well as the signal power ratio and phase quadrature relationship. Beyond these considerations, additional carrier components in Block IIR-M, IIF, and subsequent blocks of SVs will result in composite transmitted signal phase relationships other than the nominal special case of Table 3-IV. For Block IIF, the crosstalk between the C/A, when selected, and P(Y) signals shall not exceed –20 dB in the L1 and L2. The crosstalk is the relative power level of the undesired signal to the desired reference signal. 3.3.1.6 User-Received Signal Levels. The SV shall provide L1 and L2 navigation signal strength at end-of-life (EOL), worst-case, in order to meet the minimum levels specified in Table 3-V. The minimum received power is measured at the output of a 3 dBi linearly polarized user receiving antenna (located near ground) at worst normal orientation, when the SV is above a 5-degree elevation angle. The received signal levels are observed within the in- band allocation defined in para. 3.3.1.1. The Block IIF SV shall provide L1 and L2 signals with the following characteristic: the L1 off-axis power gain shall not decrease by more than 2 dB from the Edge-of-Earth (EOE) to nadir, nor more than 10 dB from EOE to 20 degrees off nadir, and no more than 18 dB from EOE to 23 degrees off nadir; the L2 off-axis power gain shall not decrease by more than 2 dB from EOE to nadir, and no more than 10 dB from EOE to 23 degrees off nadir; the power drop off between EOE and ±23 degrees shall be in a monotonically decreasing fashion. Additional related data is provided as supporting material in paragraph 6.3.1. IS-GPS-200D 7 Dec 2004 15 Table 3-IV. Composite L1 Transmitted Signal Phase ** (Block II/IIA and IIR SVs Only) Nominal Composite L1 Signal Phase* Code State P C/A 0° -70.5° +109.5° 180° 0 1 0 1 0 0 1 1 * ** Relative to 0, 0 code state with positive angles leading and negative angles lagging. Based on the composite of two L1 carrier components with 3 dB difference in the power levels of the two. Table 3-V. Received Minimum RF Signal Strength SV Blocks Channel Signal P(Y) C/A or L2 C II/IIA/IIR L1 -161.5 dBW -158.5 dBW L2 -164.5 dBW or -164.5 dBW IIR-M/IIF L1 -161.5 dBW -158.5 dBW L2 -161.5 dBW -160.0 dBW IS-GPS-200D 7 Dec 2004 16 3.3.1.7 Equipment Group Delay. Equipment group delay is defined as the delay between the signal radiated output of a specific SV (measured at the antenna phase center) and the output of that SV's on-board frequency source; the delay consists of a bias term and an uncertainty. The bias term is of no concern to the US since it is included in the clock correction parameters relayed in the NAV data, and is therefore accounted for by the user computations of system time (reference paragraphs 20.3.3.3.3.1, 30.3.3.2.3). The uncertainty (variation) of this delay as well as the group delay differential between the signals of L1 and L2 are defined in the following. 3.3.1.7.1 Group Delay Uncertainty. The effective uncertainty of the group delay shall not exceed 3.0 nanoseconds (two sigma). 3.3.1.7.2 Group Delay Differential. The group delay differential between the radiated L1 and L2 signals (i.e. L1 P(Y) and L2 P(Y), L1 P(Y) and L2 C) is specified as consisting of random plus bias components. The mean differential is defined as the bias component and will be either positive or negative. For a given navigation payload redundancy configuration, the absolute value of the mean differential delay shall not exceed 15.0 nanoseconds. The random variations about the mean shall not exceed 3.0 nanoseconds (two sigma). Corrections for the bias components of the group delay differential are provided to the US in the Nav message using parameters designated as TGD (reference paragraph 20.3.3.3.3.2) and Inter-Signal Correction (ISC) (reference paragraph 30.3.3.3.1.1). 3.3.1.8 Signal Coherence. All transmitted signals for a particular SV shall be coherently derived from the same on-board frequency standard; all digital signals shall be clocked in coincidence with the PRN transitions for the P- signal and occur at the P-signal transition speed. On the L1 channel the data transitions of the two modulating signals (i.e., that containing the P(Y)-code and that containing the C/A-code), L1 P(Y) and L1 C/A, shall be such that the average time difference between the transitions does not exceed 10 nanoseconds (two sigma). 3.3.1.9 Signal Polarization. The transmitted signal shall be right-hand circularly polarized (RHCP). For the angular range of ±14.3 degrees from boresight, L1 ellipticity shall be no worse than 1.2 dB for Block II/IIA and shall be no worse than 1.8 dB for Block IIR/IIR-M/IIF SVs. L2 ellipticity shall be no worse than 3.2 dB for Block II/IIA SVs and shall be no worse than 2.2 dB for Block IIR/IIR-M/IIF over the angular range of ±14.3 degrees from boresight. IS-GPS-200D 7 Dec 2004 17 3.3.2 PRN Code Characteristics. The characteristics of the P-, L2 CM-, L2 CL-, and the C/A-codes are defined below in terms of their structure and the basic method used for generating them. Figure 3-1 depicts a simplified block diagram of the scheme for generating the 10.23 Mbps Pi(t) and the 1.023 Mbps Gi(t) patterns (referred to as P- and C/A-codes respectively), and for modulo-2 summing these patterns with the NAV bit train, D(t), which is clocked at 50 bps. The resultant composite bit trains are then used to modulate the signal carriers. 3.3.2.1 Code Structure. The Pi(t) pattern (P-code) is generated by the modulo-2 summation of two PRN codes, X1(t) and X2(t - iT), where T is the period of one P-code chip and equals (1.023 x 107)-1 seconds, while i is an integer from 1 through 37. This allows the generation of 37 unique P(t) code phases (identified in Table 3-I) using the same basic code generator. The linear Gi(t) pattern (C/A-code) is the modulo-2 sum of two 1023-bit linear patterns, G1 and G2i. The latter sequence is selectively delayed by an integer number of chips to produce many different G(t) patterns (defined in Table 3-I). The CM,i(t) pattern (L2 CM-code) is a linear pattern which is reset with a specified initial state every code count of 10230 chips. Different initial states are used to generate different CM,i(t) patterns (defined in Table 3-II). The CL,i(t) pattern (L2 CL-code) is also a linear pattern but with a longer reset period of 767250 chips. Different initial states are used to generate different CL,i(t) patterns (defined in Table 3-II). For a given SV-ID, two different initial states are used to generate different CL,i(t) and CM,i(t) patterns. Section 6.3.5 provides a selected subset of additional P-, L2 CM-, L2 CL-, and the C/A-code sequences with assigned PRN numbers. IRN-200D-001 IS-GPS-200D 7 Mar 2006 18 Z- COUNTER RESET COMMAND GENERATOR X1 CODE GENERATOR CODE SELECT DEVICE X2 CODE GENERATOR RECLOCKING DEVICE 10.23 MHz FREQUENCY SOURCE GOLD CODE GENERATOR EPOCH RESET EPOCH DETECT EPOCH RESET EPOCH DETECT 10 20X1 EPOCH DATA ENCODER D(t) Pi(t) D(t) Pi(t) FORMATTED DATA Pi(t) X2i(t) X1(t) Gi(t) REMOTE COMMAND Z-COUNT 1.023 MHz 1 KHz 50 Hz Gi(t) D(t) Z- COUNTER RESET COMMAND GENERATOR X1 CODE GENERATOR CODE SELECT DEVICE X2 CODE GENERATOR RECLOCKING DEVICE 10.23 MHz FREQUENCY SOURCE GOLD CODE GENERATOR EPOCH RESET EPOCH DETECT EPOCH RESET EPOCH DETECT 10 20X1 EPOCH DATA ENCODER D(t) Pi(t) D(t) Pi(t) FORMATTED DATA Pi(t) X2i(t) X1(t) Gi(t) REMOTE COMMAND Z-COUNT 1.023 MHz 1 KHz 50 Hz Gi(t) D(t) Figure 3-1. Generation of P-, C/A-Codes and Modulating Signals IS-GPS-200D 7 Dec 2004 19 3.3.2.2 P-Code Generation. Each Pi(t) pattern is the modulo-2 sum of two extended patterns clocked at 10.23 Mbps (X1 and X2i). X1 itself is generated by the modulo-2 sum of the output of two 12-stage registers (X1A and X1B) short cycled to 4092 and 4093 chips respectively. When the X1A short cycles are counted to 3750, the X1 epoch is generated. The X1 epoch occurs every 1.5 seconds after 15,345,000 chips of the X1 pattern have been generated. The polynomials for X1A and X1B, as referenced to the shift register input, are: X1A: 1 + X6 + X8 + X11 + X12, and X1B: 1 + X1 + X2 + X5 + X8 + X9 + X10 + X11 + X12. Samples of the relationship between shift register taps and the exponents of the corresponding polynomial, referenced to the shift register input, are as shown in Figures 3-2, 3-3, 3-4 and 3-5. The state of each generator can be expressed as a code vector word which specifies the binary sequence constant of each register as follows: (a) the vector consists of the binary state of each stage of the register, (b) the stage 12 value appears at the left followed by the values of the remaining states in order of descending stage numbers, and (c) the shift direction is from lower to higher stage number with stage 12 providing the current output. This code vector convention represents the present output and 11 future outputs in sequence. Using this convention, at each X1 epoch, the X1A shift register is initialized to code vector 001001001000 and the X1B shift register is initialized to code vector 010101010100. The first chip of the X1A sequence and the first chip of the X1B sequence occur simultaneously in the first chip interval of any X1 period. The natural 4095 chip cycles of these generating sequences are shortened to cause precession of the X1B sequence with respect to the X1A sequence during subsequent cycles of the X1A sequence in the X1 period. Re- initialization of the X1A shift register produces a 4092 chip sequence by omitting the last 3 chips (001) of the natural 4095 chip X1A sequence. Re-initialization of the X1B shift register produces a 4093 chip sequence by omitting the last 2 chips (01) of the natural 4095 chip X1B sequence. This results in the phase of the X1B sequence lagging by one chip for each X1A cycle in the X1 period. The X1 period is defined as the 3750 X1A cycles (15,345,000 chips) which is not an integer number of X1B cycles. To accommodate this situation, the X1B shift register is held in the final state (chip 4093) of its 3749th cycle. It remains in this state until the X1A shift register completes its 3750th cycle (343 additional chips). The completion of the 3750th X1A cycle establishes the next X1 epoch which re-initializes both the X1A and X1B shift registers starting a new X1 cycle. IS-GPS-200D 7 Dec 2004 20 POLYNOMIAL X1A: 1 + X 6 + X8 + X11 + X12 STAGE NUMBERS OUTPUT 0 123456789101112 INITIAL TAP CONDITIONS NUMBERS SHIFT DIRECTION Figure 3-2. X1A Shift Register Generator Configuration IS-GPS-200D 7 Dec 2004 21 1 0 2 0 3 1 4 0 5 1 6 0 7 1 8 0 9 1 10 0 11 1 12 0 STAGE NUMBERS INITIAL 0 1 2 3 4 5 6 7 8 9 10 11 12 OUTPUT TAP POLYNOMIAL X1B: 1 + X1 + X2 + X5 + X8 + X9 + X10 + X11 + X12 1 0 2 0 3 1 4 0 5 1 6 0 7 1 8 0 9 1 10 0 11 1 12 0 STAGE NUMBERS INITIAL 0 1 2 3 4 5 6 7 8 9 10 11 12 OUTPUT TAP POLYNOMIAL X1B: 1 + X1 + X2 + X5 + X8 + X9 + X10 + X11 + X12 CONDITIONS NUMBERS SHIFT DIRECTION Figure 3-3. X1B Shift Register Generator Configuration IS-GPS-200D 7 Dec 2004 22 1 1 2 0 3 1 4 0 5 0 6 1 7 0 8 0 9 1 10 0 11 0 12 1 STAGE NUMBERS INITIAL 0 1 2 3 4 5 6 7 8 9 10 11 12 OUTPUT TAP POLYNOMIAL X2A: 1 + X1 + X3 + X4 + X5 + X7 + X8 + X9 + X10 + X11 + X12 1 1 2 0 3 1 4 0 5 0 6 1 7 0 8 0 9 1 10 0 11 0 12 1 STAGE NUMBERS INITIAL 0 1 2 3 4 5 6 7 8 9 10 11 12 OUTPUT TAP POLYNOMIAL X2A: 1 + X1 + X3 + X4 + X5 + X7 + X8 + X9 + X10 + X11 + X12 CONDITIONS NUMBERS SHIFT DIRECTION Figure 3-4. X2A Shift Register Generator Configuration IS-GPS-200D 7 Dec 2004 23 1 0 2 0 3 1 4 0 5 1 6 0 7 1 8 0 9 1 10 0 11 1 12 0 STAGE NUMBERS INITIAL 0 1 2 3 4 5 6 7 8 9 10 11 12 OUTPUT TAP POLYNOMIAL X2B: 1 + X2 + X3 + X4 + X8 + X9 + X12 1 0 2 0 3 1 4 0 5 1 6 0 7 1 8 0 9 1 10 0 11 1 12 0 STAGE NUMBERS INITIAL 0 1 2 3 4 5 6 7 8 9 10 11 12 OUTPUT TAP POLYNOMIAL X2B: 1 + X2 + X3 + X4 + X8 + X9 + X12 CONDITIONS NUMBERS SHIFT DIRECTION Figure 3-5. X2B Shift Register Generator Configuration IS-GPS-200D 7 Dec 2004 24 The X2i sequences are generated by first producing an X2 sequence and then delaying it by a selected integer number of chips, i, ranging from 1 to 37. Each of the X2i sequences is then modulo-2 added to the X1 sequence thereby producing up to 37 unique P(t) sequences. The X2A and X2B shift registers, used to generate X2, operate in a similar manner to the X1A and X1B shift registers. They are short-cycled, X2A to 4092 and X2B to 4093, so that they have the same relative precession rate as the X1 shift registers. X2A epochs are counted to include 3750 cycles and X2B is held in the last state at 3749 cycle until X2A completes its 3750th cycle. The polynomials for X2A and X2B, as referenced to the shift register input, are: X2A: 1 + X1 + X3 + X4 + X5 + X7 + X8 + X9 + X10 + X11 + X12, and X2B: 1 + X2 + X3 + X4 + X8 + X9 + X12. (The initialization vector for X2A is 100100100101 and for X2B is 010101010100). The X2A and X2B epochs are made to precess with respect to the X1A and X1B epochs by causing the X2 period to be 37 chips longer than the X1 period. When the X2A is in the last state of its 3750th cycle and X2B is in the last state of its 3749th cycle, their transitions to their respective initial states are delayed by 37 chip time durations. At the beginning of the GPS week, X1A, X1B, X2A and X2B shift registers are initialized to produce the first chip of the week. The precession of the shift registers with respect to X1A continues until the last X1A period of the GPS week interval. During this particular X1A period, X1B, X2A and X2B are held when reaching the last state of their respective cycles until that X1A cycle is completed (see Table 3-VI). At this point, all four shift registers are initialized and provide the first chip of the new week. Figure 3-6 shows a functional P-code mechanization. Signal component timing is shown in Figure 3-7, while the end-of-week reset timing and the final code vector states are given in Tables 3-VI and 3-VII, respectively. IS-GPS-200D 7 Dec 2004 25 10.23 MHz X1 EPOCH X1A REGISTER C I 1 6 12 R 4093 DECODE 4092 DECODE 4092 DECODE 4093 DECODE C CLOCK CONTROL 3750 Z-COUNTER 403,200 X1B REGISTER C I 1 12 R X2A REGISTER C I 1 12 R X2B REGISTER C I 1 2 12 R 7 DAY RESET SHIFT REGISTER A 1, 2, 5, 8, 9, 10, 11, 12 1, 3, 4, 5, 7, 8, 9, 10, 11, 12 2, 3, 4, 8, 9, 12 6, 8, 11, 12 A CLOCK CONTROL B 3749 3750 37 C 3749 B CLOCK CONTROL 1 i C -CLOCK I -INPUT R -RESET TO INITIAL CONDITIONS ON NEXT CLOCK REGISTER INPUTS SET X1A EPOCH RESUME HALT SET X1B EPOCH END/WEEK HALT START/WEEK ENABLE X2 EPOCH RESUME HALT END/WEEK SET X2B EPOCH X2 SET X2A EPOCH X1 X2i Pi Figure 3-6. P-Code Generation IS-GPS-200D 7 Dec 2004 26 012301230 X1 EPOCHS X2 EPOCHS * 37 Chips 74 Chips P Epoch TIME 0 1.5 sec 3.0 sec 4.5 sec 7 days 14 days * Does not include any offset due to PRN delay. Figure 3-7. P-Code Signal Component Timing IS-GPS-200D 7 Dec 2004 27 Table 3-VI. P-Code Reset Timing (Last 400 μsec of 7-day period) ** Code Chip X1A-Code X1B-Code X2A-Code X2B-Code 1 345 1070 967 • • • • • • • • • • • • 3023 3367 3989 4092 • • • • • • • • • • • • 3127 3471 4092 4093 • • • • • • • • • • • • 3749 4092 4093 4093 • • • • • • • • • • • • 4093 4092 4093 4092* * Last Chip of Week. ** Does not include any X2 offset due to PRN delay. IS-GPS-200D 7 Dec 2004 28 Table 3-VII. Final Code Vector States Code Chip Number Vector State Vector State for 1st Chip following Epoch X1A 4091 100010010010 001001001000 4092 000100100100 X1B 4092 100101010101 010101010100 4093 001010101010 X2A 4091 111001001001 100100100101 4092 110010010010 X2B 4092 000101010101 010101010100 4093 001010101010 NOTE: First Chip in each sequence is output bit whose leading edge occurs simultaneously with the epoch. IS-GPS-200D 7 Dec 2004 29 3.3.2.3 C/A-Code Generation. Each Gi(t) sequence is a 1023-bit Gold-code which is itself the modulo-2 sum of two 1023-bit linear patterns, G1 and G2i. The G2i sequence is formed by effectively delaying the G2 sequence by an integer number of chips. The G1 and G2 sequences are generated by 10-stage shift registers having the following polynomials as referred to in the shift register input (see Figures 3-8 and 3-9). G1 = X10 + X3 + 1, and G2 = X10 + X9 + X8 + X6 + X3 +X2+ 1. The initialization vector for the G1 and G2 sequences is 1111111111. The G1 and G2 shift registers are initialized at the P-coder X1 epoch. The G1 and G2 registers are clocked at 1.023 MHz derived from the 10.23 MHz P-coder clock. The initialization by the X1 epoch phases the 1.023 MHz clock to insure that the first chip of the C/A code begins at the same time as the first chip of the P-code. The effective delay of the G2 sequence to form the G2i sequence may be accomplished by combining the output of two stages of the G2 shift register by modulo-2 addition (see Figure 3-10). However, this two-tap coder implementation generates only a limited set of valid C/A codes. Table 3-I contains a tabulation of the G2 shift register taps selected and their corresponding P-code X2i and PRN signal numbers together with the first several chips of each resultant PRN code. Timing relationships related to the C/A code are shown in Figure 3-11. IS-GPS-200D 7 Dec 2004 30 STAGE NUMBERS INPUT 0 123 INITIAL CONDITIONS Figure 3-8. POLYNOMIAL G1: 1 + X 3 + X10 OUTPUT 4 5 6 7 8 910 TAP NUMBERS SHIFT DIRECTION G1 Shift Register Generator Configuration IS-GPS-200D 7 Dec 2004 31 1 1 2 1 3 1 4 1 5 1 6 1 7 1 8 1 9 1 10 1 STAGE NUMBERS INITIAL 0 1 2 3 4 5 6 7 8 9 10 OUTPUT TAP POLYNOMIAL G2: 1 + X 2 + X 3 +X6 + X 8 + X 9 + X 10 INPUT 1 1 2 1 3 1 4 1 5 1 6 1 7 1 8 1 9 1 10 1 STAGE NUMBERS INITIAL 0 1 2 3 4 5 6 7 8 9 10 OUTPUT TAP POLYNOMIAL G2: 1 + X 2 + X 3 +X6 + X 8 + X 9 + X 10 INPUT CONDITIONS NUMBERS SHIFT DIRECTION Figure 3-9. G2 Shift Register Generator Configuration IS-GPS-200D 7 Dec 2004 32 10 S C I C S I G1 REGISTER 2 3 6 8 9 10 G2 REGISTER 3 X1 EPOCH 20 G EPOCH 1 Kbps 1023 DECODE PHASE SELECT LOGIC G2i G1 10 S C I C S I G1 REGISTER 2 3 6 8 9 10 G2 REGISTER 3 X1 EPOCH 20 G EPOCH 1 Kbps 1023 DECODE PHASE SELECT LOGIC G2i G1 SYNCH 10 10.23 MHz SYNCH Gi 50 bps TO DATA ENCODER REGISTER INPUTS C -CLOCK I -INPUT S -SET ALL ONES Figure 3-10. Example C/A-Code Generation IS-GPS-200D 7 Dec 2004 33 X1 Epoch @ 2/3 bps 0 1 2 18 19 0 X1 Epoch @ 2/3 bps 0 1 2 18 19 0 1023 1023 1023 1023 1023 etc. 1023 BIT Gold Code @ 1023 Kbps 1 msec Gold Code Epochs @ 1000/sec Data @ 50 cps 20 msec Figure 3-11. C/A-Code Timing Relationships IS-GPS-200D 7 Dec 2004 34 3.3.2.4 L2 CM-/L2 CL-Code Generation. Each CM,i(t) pattern (L2 CM-code) and CL,i(t) pattern (L2 CL-code) are generated using the same code generator polynomial each clocked at 511.5 Kbps. Each pattern is initiated and reset with a specified initial state (defined in Table 3-II). CM,i(t) pattern is reset after 10230 chips resulting in a code period of 20 milliseconds, and CL,i(t) pattern is reset after 767250 chips resulting in a code period of 1.5 seconds. The L2 CM and L2 CL shift registers are initialized at the P-coder X1 epoch. The first L2 CM-code chip starts synchronously with the end/start of week epoch. Timing relationships related to the L2 CM-/L2 CL- codes are shown in Figure 3-12. The maximal polynomial used for L2 CM- and L2 CL-codes is 1112225171 (octal) of degree 27. The L2 CM and L2 CL code generator is conceptually described in Figure 3-13 using modular-type shift register generator. IS-GPS-200D 7 Dec 2004 35 End/start of week X1 Epoch @ 2/3 bps 1.5 second 767250 Chips 767250 BIT L2 CL-Code @ 511.5 Kbps 10230 10230 BIT L2 CM-Code @ 511.5 Kbps 10230 10230 10230 10230 10230 10230 1 2 43 73 74 75 etc. etc. 20 msec Data @ 50 cps L2 CM @ 511.5 Kbps L2 C @ 1023 Kbps L2 CL @ 511.5 Kbps End/start of week X1 Epoch @ 2/3 bps 1.5 second 767250 Chips 767250 BIT L2 CL-Code @ 511.5 Kbps 10230 10230 BIT L2 CM-Code @ 511.5 Kbps 10230 10230 10230 10230 10230 10230 1 2 43 73 74 75 etc. etc. 20 msec Data @ 50 cps L2 CM @ 511.5 Kbps L2 C @ 1023 Kbps L2 CL @ 511.5 Kbps Figure 3-12. L2 CM-/L2 CL-Code Timing Relationships IS-GPS-200D 7 Dec 2004 36 Figure 3-13. L2 CM/L2 CL Shift Register Generator Configuration IS-GPS-200D 7 Dec 2004 37 3 3 2 POLYNOMIAL: DELAY1 + X3 + X 4 +X5 + X 6 + X 9 + X 11 + X13 + X 16 + X19 + X 21 + X 24 + X27 NUMBERS 3 3 2 12 3 1 1 OUTPUT INITIAL CONDITIONS ARE A FUNCTION OF PRN AND CODE PERIOD (MODERATE/LONG) SHIFT DIRECTION 3.3.3 Navigation Data. The content and format of the NAV data, D(t), and the CNAV data, DC(t), are given in Appendices II and III, respectively, of this document. 3.3.3.1 Navigation Data Modulation (L2 CM). For Block IIR-M, Block IIF, and subsequent blocks of SVs, the CNAV bit train, DC(t), is rate ½ encoded and, thus, clocked at 50 sps. The resultant symbol sequence is then modulo-2 added to the L2 CM-code. During the initial period of Block IIR-M SVs operation, prior to Initial Operational Capability of L2 C signal, and upon ground command, the NAV bit train, D(t), at one of two data rates, may be modulo-2 added to the L2 CM-code instead of CNAV data, DC(t), as further described in Section 3.2.2. 3.3.3.1.1 Forward Error Correction. The CNAV bit train, DC(t), will always be Forward Error Correction (FEC) encoded by a rate 1/2 convolutional code. For Block IIR-M, the NAV bit train, D(t), can be selected to be convolutionally encoded. The resulting symbol rate is 50 sps. The convolutional coding will be constraint length 7, with a convolutional encoder logic arrangement as illustrated in Figure 3-14. The G1 symbol is selected on the output as the first half of a 40-millisecond data bit period. Twelve-second navigation messages broadcast by the SV are synchronized with every eighth of the SV's P(Y)-code X1 epochs. However, the navigation message is FEC encoded in a continuous process independent of message boundaries (i.e. at the beginning of each new message, the encoder registers illustrated in Figure 3-14 contains the last six bits of the previous message). Because the FEC encoding convolves successive messages, it is necessary to define which transmitted symbol is synchronized to SV time, as follows. The beginning of the first symbol that contains any information about the first bit of a message will be synchronized to every eighth X1 epoch (referenced to end/start of week). The users’ convolutional decoders will introduce a fixed delay that depends on their respective algorithms (usually 5 constraint lengths, or 35 bits), for which they must compensate to determine system time from the received signal. This convolutional decoding delay and the various relationships with the start of the data block transmission and SV time are illustrated in Figure 3-15. IS-GPS-200D 7 Dec 2004 38 G2 (133 OCTAL) DATA INPUT (25 BPS) OUTPUT SYMBOLS (50 SPS) (ALTERNATING G1/G2) G1 (171 OCTAL) SYMBOL CLOCK Figure 3-14. Convolutional Encoder USER’S DECODING DELAY DOWNLINK DELAY LATER ENCODED DATA BLOCK TRANSMITTED ON L2 EARLY SV 12 SECOND EPOCHS ENCODED DATA BLOCK RECEIVED BY USER DATA BLOCK DECODED BY USER Figure 3-15. Convolutional Transmit/Decoding Timing Relationships IS-GPS-200D 7 Dec 2004 39 3.3.4 GPS Time and SV Z-Count. GPS time is established by the Control Segment and is referenced to Coordinated Universal Time (UTC) as maintained by the U.S. Naval Observatory (UTC(USNO)) zero time-point defined as midnight on the night of January 5, 1980/morning of January 6, 1980. The largest unit used in stating GPS time is one week defined as 604,800 seconds. GPS time may differ from UTC because GPS time shall be a continuous time scale, while UTC is corrected periodically with an integer number of leap seconds. There also is an inherent but bounded drift rate between the UTC and GPS time scales. The OCS shall control the GPS time scale to be within one microsecond of UTC (modulo one second). The NAV data contains the requisite data for relating GPS time to UTC. The accuracy of this data during the transmission interval shall be such that it shall relate GPS time (maintained by the MCS of the CS) to UTC (USNO) within 90 nanoseconds (one sigma). This data is generated by the CS; therefore, the accuracy of this relationship may degrade if for some reason the CS is unable to upload data to a SV. At this point, it is assumed that alternate sources of UTC are no longer available, and the relative accuracy of the GPS/UTC relationship will be sufficient for users. Range error components (e.g. SV clock and position) contribute to the GPS time transfer error, and under normal operating circumstances (two frequency time transfers from SV(s) whose navigation message indicates a URA of eight meters or less), this corresponds to a 97 nanosecond (one sigma) apparent uncertainty at the SV. Propagation delay errors and receiver equipment biases unique to the user add to this time transfer uncertainty. IS-GPS-200D 7 Dec 2004 40 In each SV the X1 epochs of the P-code offer a convenient unit for precisely counting and communicating time. Time stated in this manner is referred to as Z-count, which is given as a 29-bit binary number consisting of two parts as follows: a. The binary number represented by the 19 least significant bits of the Z-count is referred to as the time of week (TOW) count and is defined as being equal to the number of X1 epochs that have occurred since the transition from the previous week. The count is short-cycled such that the range of the TOW-count is from 0 to 403,199 X1 epochs (equaling one week) and is reset to zero at the end of each week. The TOWcount's zero state is defined as that X1 epoch which is coincident with the start of the present week. This epoch occurs at (approximately) midnight Saturday night-Sunday morning, where midnight is defined as 0000 hours on the UTC scale which is nominally referenced to the Greenwich Meridian. Over the years the occurrence of the "zero state epoch" may differ by a few seconds from 0000 hours on the UTC scale since UTC is periodically corrected with leap seconds while the TOW-count is continuous without such correction. To aid rapid ground lock-on to the P-code signal, a truncated version of the TOW-count, consisting of its 17 most significant bits, is contained in the hand-over word (HOW) of the L1 and L2 NAV data (D(t)) stream; the relationship between the actual TOW-count and its truncated HOW version is illustrated by Figure 3-16. b. The ten most significant bits of the Z-count are a modulo 1024 binary representation of the sequential number assigned to the current GPS week (see paragraph 6.2.4). The range of this count is from 0 to 1023 with its zero state being defined as the GPS week number zero and every integer multiple of 1024 weeks, thereafter (i.e. 0, 1024, 2048, etc.). IS-GPS-200D 7 Dec 2004 41 P(Y)-CODE EPOCH (END/START OF WEEK) X1 EPOCHS 1.5 sec 01234567 8 403,192 403,196 403,199 DECIMAL EQUIVALENTS OF ACTUAL TOW COUNTS SUBFRAME EPOCHS 6 sec 100,7990 1 2 3 DECIMAL EQUIVALENTS OF HOW-MESSAGE TOW COUNTS NOTES: 1. TO AID IN RAPID GROUND LOCK-ON THE HAND-OVER WORD (HOW ) OF EACH SUBFRAME CONTAINS A TRUNCATED TIME-OF-WEEK (TOW) COUNT 2. THE HOW IS THE SECOND WORD IN EACH SUBFRAME (REFERENCE PARAGRAPH 20.3.3.2). 3. THE HOW-MESSAGE TOW COUNT CONSISTS OF THE 17 MSBs OF THE ACTUAL TOW COUNT AT THE START OF THE NEXT SUBFRAME. 4. TO CONVERT FROM THE HOW-MESSAGE TOW COUNT TO THE ACTUAL TOW COUNT AT THE START OF THE NEXT SUBFRAME, MULTIPLY BY FOUR. 5. THE FIRST SUBFRAME STARTS SYNCHRONOUSLY WITH THE END/START OF WEEK EPOCH. Figure 3-16. Time Line Relationship of HOW Message IS-GPS-200D 7 Dec 2004 42 4. NOT APPLICABLE IS-GPS-200D 7 Dec 2004 43 (This page intentionally left blank.) IS-GPS-200D 7 Dec 2004 44 5. NOT APPLICABLE IS-GPS-200D 7 Dec 2004 45 (This page intentionally left blank.) IS-GPS-200D 7 Dec 2004 46 6. NOTES 6.1 Acronyms AI -Availability Indicator AODO -Age of Data Offset A-S -Anti-Spoofing Autonav -Autonomous Navigation BPSK -Bi-Phase Shift Key CDC -Clock Differential Correction CNAV -Civil Navigation cps -cycles per second CRC -Cyclic Redundancy Check CS -Control Segment DC -Differential Correction DN -Day Number EAROM -Electrically Alterable Read-Only Memory ECEF -Earth-Centered, Earth-Fixed ECI -Earth-Centered, Inertial EDC -Ephemeris Differential Correction EOE -Edge-of-Earth EOL -End of Life ERD -Estimated Range Deviation FEC -Forward Error Correction GGTO -GPS/GNSS Time Offset GNSS -Global Navigation Satellite System GPS -Global Positioning System HOW -Hand-Over Word ICC -Interface Control Contractor ID -Identification IERS -International Earth Rotation and Reference Systems Service IS-GPS-200D 7 Dec 2004 47 IODC -Issue of Data, Clock IODE -Issue of Data, Ephemeris IRM -IERS Reference Meridian IRP -IERS Reference Pole IS -Interface Specification ISC -Inter-Signal Correction LSB -Least Significant Bit LSF -Leap Seconds Future L2 C -L2 Civil Signal L2 CL -L2 Civil-Long Code L2 CM -L2 Civil-Moderate Code MCS -Master Control Station MSB -Most Significant Bit NAV -Navigation NDUS -Nudet Detection User Segment NMCT -Navigation Message Correction Table NSC -Non-Standard C/A-Code NSCL -Non-Standard L2 CL-Code NSCM -Non-Standard L2 CM-Code NSY -Non-Standard Y-code OBCP -On-Board Computer Program OCS -Operational Control System PRN -Pseudo-Random Noise RF -Radio Frequency RMS -Root Mean Square SA -Selective Availability SEP -Spherical Error Probable sps -symbols per second IS-GPS-200D 7 Dec 2004 48 SS -Space Segment SV -Space Vehicle SVN -Space Vehicle Number TBD -To Be Determined TBS -To Be Supplied TLM -Telemetry TOW -Time Of Week UE -User Equipment URA -User Range Accuracy URE -User Range Error US -User Segment USNO -U.S. Naval Observatory UTC -Coordinated Universal Time WGS 84 -World Geodetic System 1984 WN -Week Number WNe -Extended Week Number IS-GPS-200D 7 Dec 2004 49 (This page intentionally left blank.) IS-GPS-200D 7 Dec 2004 50 6.2 Definitions 6.2.1 User Range Accuracy. User range accuracy (URA) is a statistical indicator of the ranging accuracies obtainable with a specific SV. URA is a one-sigma estimate of the user range errors in the navigation data for the transmitting satellite. It includes all errors for which the Space and Control Segments are responsible. It does not include any errors introduced in the user set or the transmission media. While the URA may vary over a given subframe fit interval, the URA index (N) reported in the NAV message corresponds to the maximum value of URA anticipated over the fit interval. 6.2.2 SV Block Definitions. The following block definitions are given to facilitate discussion regarding the capability of the various blocks of GPS satellites to support the SV-to-US interface. 6.2.2.1 Developmental SVs. The original concept validation satellites developed by Rockwell International and designated as satellite vehicle numbers (SVNs) 1-11 are termed "Block I" SVs. These SVs were designed to provide 3-4 days of positioning service without contact from the CS. These SVs transmitted a configuration code of 000 (reference paragraph 20.3.3.5.1.4). There are no longer any active Block I SVs in the GPS constellation. The last Block I SV was decommissioned in 1995. 6.2.2.2 Operational SVs. The operational satellites are designated Block II, Block IIA, Block IIR, Block IIR-M and Block IIF SVs. Characteristics of these SVs are provided below. Modes of operation for these SVs and accuracy of positioning services provided are described in paragraphs 6.3.2 through 6.3.4. These SVs transmit configuration codes as specified in paragraph 20.3.3.5.1.4. The navigation signal provides no direct indication of the type of the transmitting SV. 6.2.2.2.1 Block II SVs. The first block of full scale operational SVs developed by Rockwell International are designated as SVNs 13-21 and are termed "Block II" SVs. These SVs were designed to provide 14 days of positioning service without contact from the CS. 6.2.2.2.2 Block IIA SVs. The second block of full scale operational SVs developed by Rockwell International are designated as SVNs 22-40 and are termed "Block IIA" SVs. These SVs are capable of providing 60 days of positioning service without contact from the CS. IS-GPS-200D 7 Dec 2004 51 6.2.2.2.3 Block IIR SVs. The block of operational replenishment SVs developed by Lockheed Martin are designated as SVNs 41-61 and are termed "Block IIR" SVs. These SVs have the capability of storing at least 60 days of navigation data with current memory margins, while operating in a IIA mode, to provide positioning service without contact from the CS for that period. (Contractual requirements for these SVs specify transmission of correct data for only 14 days to support short-term extended operations while in IIA mode.) The IIR SV will provide a minimum of 60 days of positioning service without contact from the CS when operating in autonomous navigation (Autonav) mode. 6.2.2.2.4 Block IIR-M SVs. The subset of operational replenishment SVs developed by Lockheed Martin which are “Modernized” configuration of “Block IIR” SVs are termed “Block IIR-M”. 6.2.2.2.5 Block IIF SVs. The block of operational replenishment SVs developed by Boeing are designated as SVNs 62-73 and are termed “Block IIF” SVs. This is the first block of operational SVs that transmit the L5 Civil signal. These SVs will provide at least 60 days of positioning service without contact from the CS. 6.2.3 Operational Interval Definitions. The following three operational intervals have been defined. These labels will be used to refer to differences in the interface definition as time progresses from SV acceptance of the last navigation data upload. 6.2.3.1 Normal Operations. The SV is undergoing normal operations whenever the fit interval flag (reference paragraph 20.3.3.4.3.1) is zero. 6.2.3.2 Short-term Extended Operations. The SV is undergoing short-term extended operations whenever the fit interval flag is one and the IODE (reference paragraph 20.3.4.4) is less than 240. 6.2.3.3 Long-term Extended Operations. The SV is undergoing long-term extended operations whenever the fit interval flag is one and the IODE is in the range 240-255. IS-GPS-200D 7 Dec 2004 52 6.2.4 GPS Week Number. The GPS week numbering system is established with week number zero (0) being defined as that week which started with the X1 epoch occurring at midnight UTC(USNO) on the night of January 5, 1980/ morning of January 6, 1980. The GPS week number continuously increments by one (1) at each end/start of week epoch without ever resetting to zero. Users must recognize that the week number information contained in the Nav Message may not necessarily reflect the current full GPS week number (see paragraphs 20.3.3.3.1.1, 20.3.3.5.1.5, 20.3.3.5.2.4, and 30.3.3.1.1.1). 6.2.5 L5 Civil Signal. L5 is the GPS downlink signal at a nominal carrier frequency of 1176.45 MHz. The L5 signal is only available on Block IIF and subsequent blocks of SVs and the signal is specified/described in a separate and different interface control document. 6.3 Supporting Material 6.3.1 Received Signals. The guaranteed minimum user-received signal levels are defined in paragraph 3.3.1.6. As additional supporting material, Figure 6-1 illustrates an example variation in the minimum received power of the near-ground user-received L1 and L2 signals from Block II/IIA/IIR SVs as a function of SV elevation angle. Higher received signals levels can be caused by such factors as SV attitude errors, mechanical antenna alignment errors, transmitter power output variations due to temperature variations, voltage variations and power amplifier variations, and due to a variability in link atmospheric path loss. For Block II/IIA and IIR SVs, the maximum received signal levels as a result of these factors is not expected to exceed -155.5 dBW and -153.0 dBW, respectively, for the P(Y) and C/A components of the L1 channel, nor -158.0 dBW for either signal on the L2 channel. For Block IIR-M and IIF SVs, the maximum received signal levels as a result of these factors is not expected to exceed -155.5 dBW and -153.0 dBW, respectively, for the P(Y) and C/A components of the L1 channel and L2 channel. In addition, due to programmable power output capabilities of Block IIR-M and IIF SVs, under certain operational scenarios, individual signal components of Block IIR-M/IIF SVs may exceed the previously stated maximum but are not expected to exceed -150 dBW. IS-GPS-200D 7 Dec 2004 53 RECEIVED POWER AT 3dBi LINEARLY POLARIZED ANTENNA (dBW) -158.5 -161.5 -164.5 C/A - L1 P - L1 P - L2 or C/A - L2 0o 5o 20o 40o 60o 80o 90o 100o USER ELEVATION ANGLE (DEG) Figure 6-1. User Received Minimum Signal Level Variations (Example, Block II/IIA/IIR) IS-GPS-200D 7 Dec 2004 54 6.3.2 Extended Navigation Mode (Block II/IIA). The Block II and IIA SVs are capable of being uploaded by the CS with a minimum of 60 days of navigation data to support a 60 day positioning service. Due to memory retention limitations, the Block II SVs may not transmit correct data for the entire 60 days but are guaranteed to transmit correct data for at least 14 days to support short-term extended operations. Under normal conditions the CS will provide daily uploads to each SV, which will allow the SV to maintain normal operations as defined in paragraph 6.2.3.1 and described within this IS. During normal operations, the SVs will have a user range error that is at or below a level required to support a positioning accuracy of 16 meters spherical error probable (SEP). In addition, the almanac data, UTC parameters and ionospheric data will be maintained current to meet the accuracy specified in this IS. If the CS is unable to upload the SVs (the CS is unavailable or the SV is unable to accept and process the upload), each SV will individually transition to short-term extended operations and eventually to long-term extended operations (based on time from each SV's last upload) as defined in paragraphs 6.2.3.2 and 6.2.3.3, and as further described throughout this IS. As time from upload continues through these three operational intervals, the user range error of the SV will increase, causing a positioning service accuracy degradation. The rate of accuracy degradation is slow over the short-term extended operations interval, such that at the end of this interval (approximately 14 days after upload) the US will be able to achieve a positioning accuracy of 425 meters SEP. The rate of accuracy degradation increases in the long-term extended interval, such that by the 180th day after the last upload, the positioning errors will have grown to 10 kilometers SEP. During these intervals the URA will continue to provide the proper estimate of the user range errors. During short-term and long-term extended operations (approximately day 2 through day 62 after an upload), the almanac data, UTC parameters and ionospheric data will not be maintained current and will degrade in accuracy from the time of last upload. IS-GPS-200D 7 Dec 2004 55 6.3.3 Block IIA Mode (Block IIR/IIR-M). The Block IIR/IIR-M SVs, when operating in the Block IIA mode, will perform similarly to the Block IIA SVs and have the capability of storing at least 60 days of navigation data, with current memory margins, to provide positioning service without contact from the CS for that period (through short-term and long-term extended operations). (Contractual requirements for these SVs specify transmission of correct data for only 14 days to support short-term extended operations while in IIA mode.) Under normal conditions, the CS will provide daily uploads to each SV, which will allow the SV to maintain normal operations as defined in paragraph 6.2.3.1 and described within this IS. If the CS is unable to upload the SVs (the CS is unavailable or the SV is unable to accept and process the upload), each SV will individually transition to short-term extended operations and eventually to long-term extended operations (based on time from each SV’s last upload) as defined in paragraph 6.2.3.2 and 6.2.3.3, and as further described throughout this IS. As time from upload continues through these three operational intervals, the user range error (URE) of the SV will increase, causing a positioning service accuracy degradation. 6.3.4 Autonomous Navigation Mode. The Block IIR/IIR-M and Block IIF SV, in conjunction with a sufficient number of other Block IIR/IIR-M or Block IIF SVs, operates in an Autonav mode when commanded by the CS. Each Block IIR/IIR-M/IIF SV in the constellation determines its own ephemeris and clock correction parameters via SV-to-SV ranging, communication of data, and on-board data processing which updates data uploaded by the CS. In the Autonav mode the Block IIR/IIR-M/IIF SV will maintain normal operations as defined in paragraph 6.2.3.1 and as further described within this IS, and will have a URE of no larger than 6 meters, one sigma for Block IIR/IIRM. URE of 6 meters, one sigma, is expected to support 16 meter SEP accuracy under a nominal position dilution of precision. If the CS is unable to upload the SVs, the Block IIR/IIR-M/IIF SVs will maintain normal operations for period of at least 60 days after the last upload. In the Autonav mode, the almanac data, UTC parameters and ionospheric data are still calculated and maintained current by the CS and uploaded to the SV as required. If the CS is unable to upload the SVs, the almanac data, UTC parameters and ionospheric data will not be maintained current and will degrade in accuracy from the time of the last upload. IS-GPS-200D 7 Dec 2004 56 6.3.5 PRN Code sequences expansion. The additional PRN sequences provided in this section are for information only. The additional PRN sequences identified in this section are not applicable to Block II/IIA, IIR/IIR-M, IIF SVs. In addition, the current valid range for GPS PRN signal number for C/A-and P-code is 1 – 37 as specified in Table 3-I. The PRN sequences provided in this section are for other L1/L2 signal applications, such as Satellite Based Augmentation System (SBAS) satellite signals, and potential use in the future by GPS. 6.3.5.1 Additional C/A-code PRN sequences. The PRN C/A-code is described in Section 3.2.1.3 and 36 legacy C/A-code sequences are assigned by SV-ID number in Table 3-I. An additional set of 173 C/A-code PRN sequences are selected and assigned with PRN numbers in this section as shown in Table 6-I. Among the 173 additional sequences; PRN numbers 38 through 63 are reserved for future GPS SVs; PRN numbers 64 through 119 are reserved for future Ground Based Augmentation System (GBAS) and other augmentation systems; PRN numbers 120 through 158 are reserved for SBAS; and PRN numbers 159 through 210 are reserved for other Global Navigation Satellite System (GNSS) applications. For GPS application, the CNAV data, Dc(t), will be modulo-2 added to the C/A-code sequences of PRN numbers 38 through 63. Any assignment of a C/A-code PRN number and its code sequence for any additional SV and/or other L1/L2 signal applications, such as SBAS satellite signals, will be selected from the sequences of Table 6-I and will be approved, controlled, and managed by the GPS JPO. It should be noted that, in Table 6-I, the C/A-code sequences are identified by “G2 Delay” and “Initial G2 Setting” which is not as same as the method used in Table 3-I. The two-tap coder implementation method referenced and used in Table 3-I is not used in Table 6-I due to its limitation in generating C/A-code sequences. The “G2 Delay” specified in Table 6-I may be accomplished by using the “Initial G2 Setting” as the initialization vector for the G2 shift register of Figure 3-9. IRN-200D-001 IS-GPS-200D 7 Mar 2006 56a 6.3.5.2 Additional P-Code PRN sequences. The PRN P-code set of 37 mutually exclusive sequences are described in Section 3.2.1.1, and assignment of these code segments by SV-ID number is given in Table 3-I. An additional set of 173 P-code PRN sequences are described in this section. Among the 173 additional sequences; PRN numbers 38 through 63 are reserved for future GPS SVs; PRN numbers 64 through 119 are reserved for future GBAS and other augmentation systems; and PRN numbers 120 through 210 are reserved for other future applications. For GPS application, the CNAV data, Dc(t), which may include additional future military message types, will be modulo-2 added to the P-code sequences of PRN numbers 38 through 63. The P-code PRN numbers and their code sequences defined in Table 6-I are not for general use and will be approved, controlled, and managed by the GPS JPO. 6.3.5.2.1 Additional P-code Generation. The generation of 37 mutually exclusive P-code PRN sequences are described in Section 3.3.2.2. The additional set of 173 P-code PRN sequences are generated by circularly shifting each of the original 37 sequences (over one week) by an amount corresponding to 1, 2, 3, 4, or 5 days. The additional sequences are therefore time shifted (i.e. offset) versions of the original 37 sequences. These offset P- code PRN sequences, Pi(t), are described as follows: Pi(t) = Pi-37x(t – xT), where i is an integer from 38 to 210, x is an integer portion of (i-1)/37, and T is defined to equal 24 hours. As an example, P-code sequence for PRN 38 would be the same sequence as PRN 1 shifted 24 hours into a week (i.e. 1st chip of PRN 38 at beginning of week is the same chip for PRN 1 at 24 hours after beginning of week). The complete list of the additional P-code PRN assignment is shown in Table 6-I. Any assignment of a P-code PRN number and its code sequence for any additional SV and/or other L1/L2 signal applications will be selected from the sequences of Table 6-I. IRN-200D-001 IS-GPS-200D 7 Mar 2006 56b Table 6-I Additional C/A-/P-Code Phase Assignments (sheet 1 of 6) PRN Signal No. * C/A P G2 Delay (Chips) Initial G2 Setting (Octal)** First 10 Chips (Octal)** X2 Delay (Chips) P-code Relative Delay (Hours) *** First 12 Chips (Octal) 38 67 0017 1760 1 P1(t-24) 3373 39 103 0541 1236 2 P2(t-24) 3757 40 91 1714 0063 3 P3(t-24) 3545 41 19 1151 0626 4 P4(t-24) 5440 42 679 1651 0126 5 P5(t-24) 4402 43 225 0103 1674 6 P6(t-24) 4023 44 625 0543 1234 7 P7(t-24) 4233 45 946 1506 0271 8 P8(t-24) 2337 46 638 1065 0712 9 P9(t-24) 3375 47 161 1564 0213 10 P10(t-24) 3754 48 10011365 0412 11 P11(t-24) 3544 49 554 1541 0236 12 P12(t-24) 3440 50 280 1327 0450 13 P13(t-24) 5402 51 710 1716 0061 14 P14(t-24) 2423 52 709 1635 0142 15 P15(t-24) 5033 53 775 1002 0775 16 P16(t-24) 2637 54 864 1015 0762 17 P17(t-24) 3135 55 558 1666 0111 18 P18(t-24) 5674 56 220 0177 1600 19 P19(t-24) 4514 57 397 1353 0424 20 P20(t-24) 2064 58 55 0426 1351 21 P21(t-24) 5210 59 898 0227 1550 22 P22(t-24) 2726 60 759 0506 1271 23 P23(t-24) 5171 61 367 0336 1441 24 P24(t-24) 2656 62 299 1333 0444 25 P25(t-24) 5105 63 1018 1745 0032 26 P26(t-24) 2660 * PRN sequences 38 through 63 are reserved for GPS. ** In the octal notation for the first 10 chips of the C/A-code or the initial settings as shown in this table, the first digit (1/0) represents a "1" or “0”, respectively, for the first chip and the last three digits are the conventional octal representation of the remaining 9 chips. (For example, the first 10 chips of the C/A code for PRN Signal Assembly No. 38 are: 1111110000). *** Pi(t-N): P-code sequence of PRN number i shifted by N hours. See Section 6.3.5.2.1. IRN-200D-001 IS-GPS-200D 7 Mar 2006 56c Table 6-I Additional C/A-/P-Code Phase Assignments (sheet 2 of 6) PRN Signal No. C/A P G2 Delay (Chips) Initial G2 Setting (Octal)** First 10 Chips (Octal)** X2 Delay (Chips) P-code Relative Delay (Hours) *** First 12 Chips (Octal) 64 729 0254 1523 27 P27(t-24) 5112 65 695 1602 0175 28 P28(t-24) 4667 66 780 1160 0617 29 P29(t-24) 2111 67 801 1114 0663 30 P30(t-24) 5266 68 788 1342 0435 31 P31(t-24) 4711 69 732 0025 1752 32 P32(t-24) 4166 70 34 1523 0254 33 P33(t-24) 2251 71 320 1046 0731 34 P34(t-24) 5306 72 327 0404 1373 35 P35(t-24) 4761 73 389 1445 0332 36 P36(t-24) 2152 74 407 1054 0723 37 P37(t-24) 5247 75 525 0072 1705 1 P1(t-48) 5736 76 405 0262 1515 2 P2(t-48) 2575 77 221 0077 1700 3 P3(t-48) 3054 78 761 0521 1256 4 P4(t-48) 3604 79 260 1400 0377 5 P5(t-48) 3520 80 326 1010 0767 6 P6(t-48) 5472 81 955 1441 0336 7 P7(t-48) 4417 82 653 0365 1412 8 P8(t-48) 2025 83 699 0270 1507 9 P9(t-48) 3230 84 422 0263 1514 10 P10(t-48) 5736 85 188 0613 1164 11 P11(t-48) 4575 86 438 0277 1500 12 P12(t-48) 2054 87 959 1562 0215 13 P13(t-48) 3204 88 539 1674 0103 14 P14(t-48) 3720 89 879 1113 0664 15 P15(t-48) 5572 90 677 1245 0532 16 P16(t-48) 4457 91 586 0606 1171 17 P17(t-48) 4005 92 153 0136 1641 18 P18(t-48) 2220 93 792 0256 1521 19 P19(t-48) 3332 94 814 1550 0227 20 P20(t-48) 3777 95 446 1234 0543 21 P21(t-48) 3555 ** In the octal notation for the first 10 chips of the C/A-code or the initial settings as shown in this table, the first digit (1/0) represents a "1" or “0”, respectively, for the first chip and the last three digits are the conventional octal representation of the remaining 9 chips. (For example, the first 10 chips of the C/A code for PRN Signal Assembly No. 38 are: 1111110000) *** Pi(t-N): P-code sequence of PRN number i shifted by N hours. See Section 6.3.5.2.1. IRN-200D-001 IS-GPS-200D 7 Mar 2006 56d Table 6-I Additional C/A-/P-Code Phase Assignments (sheet 3 of 6) PRN Signal No. C/A P G2 Delay (Chips) Initial G2 Setting (Octal)** First 10 Chips (Octal)** X2 Delay (Chips) P-code Relative Delay (Hours) *** First 12 Chips (Octal) 96 264 0260 1517 22 P22(t-48) 3444 97 10151455 0322 23 P23(t-48) 3400 98 278 1535 0242 24 P24(t-48) 5422 99 536 0746 1031 25 P25(t-48) 2433 100 819 1033 0744 26 P26(t-48) 3037 101 156 1213 0564 27 P27(t-48) 5635 102 957 0710 1067 28 P28(t-48) 2534 103 159 0721 1056 29 P29(t-48) 5074 104 712 1763 0014 30 P30(t-48) 4614 105 885 1751 0026 31 P31(t-48) 2124 106 461 0435 1342 32 P32(t-48) 5270 107 248 0735 1042 33 P33(t-48) 2716 108 713 0771 1006 34 P34(t-48) 5165 109 126 0140 1637 35 P35(t-48) 4650 110 807 0111 1666 36 P36(t-48) 2106 111 279 0656 1121 37 P37(t-48) 5261 112 122 1016 0761 1 P1(t-72) 2752 113 197 0462 1315 2 P2(t-72) 5147 114 693 1011 0766 3 P3(t-72) 4641 115 632 0552 1225 4 P4(t-72) 2102 116 771 0045 1732 5 P5(t-72) 5263 117 467 1104 0673 6 P6(t-72) 2713 118 647 0557 1220 7 P7(t-72) 3167 119 203 0364 1413 8 P8(t-72) 3651 120 145 1106 0671 9 P9(t-72) 3506 121 175 1241 0536 10 P10(t-72) 5461 122 52 0267 1510 11 P11(t-72) 4412 123 21 0232 1545 12 P12(t-72) 2027 124 237 1617 0160 13 P13(t-72) 5231 125 235 1076 0701 14 P14(t-72) 2736 ** In the octal notation for the first 10 chips of the C/A-code or the initial settings as shown in this table, the first digit (1/0) represents a "1" or “0”, respectively, for the first chip and the last three digits are the conventional octal representation of the remaining 9 chips. (For example, the first 10 chips of the C/A code for PRN Signal Assembly No. 38 are: 1111110000) *** Pi(t-N): P-code sequence of PRN number i shifted by N hours. See Section 6.3.5.2.1. IRN-200D-001 IS-GPS-200D 7 Mar 2006 56e Table 6-I Additional C/A-/P-Code Phase Assignments (sheet 4 of 6) PRN Signal No. C/A P G2 Delay (Chips) Initial G2 Setting (Octal)** First 10 Chips (Octal)** X2 Delay (Chips) P-code Relative Delay (Hours) *** First 12 Chips (Octal) 126 886 1764 0013 15 P15(t-72) 3175 127 657 0717 1060 16 P16(t-72) 5654 128 634 1532 0245 17 P17(t-72) 2504 129 762 1250 0527 18 P18(t-72) 5060 130 355 0341 1436 19 P19(t-72) 2612 131 10120551 1226 20 P20(t-72) 3127 132 176 0520 1257 21 P21(t-72) 5671 133 603 1731 0046 22 P22(t-72) 4516 134 130 0706 1071 23 P23(t-72) 4065 135 359 1216 0561 24 P24(t-72) 4210 136 595 0740 1037 25 P25(t-72) 4326 137 68 1007 0770 26 P26(t-72) 4371 138 386 0450 1327 27 P27(t-72) 2356 139 797 0305 1472 28 P28(t-72) 5345 140 456 1653 0124 29 P29(t-72) 4740 141 499 1411 0366 30 P30(t-72) 2142 142 883 1644 0133 31 P31(t-72) 5243 143 307 1312 0465 32 P32(t-72) 2703 144 127 1060 0717 33 P33(t-72) 5163 145 211 1560 0217 34 P34(t-72) 4653 146 121 0035 1742 35 P35(t-72) 4107 147 118 0355 1422 36 P36(t-72) 4261 148 163 0335 1442 37 P37(t-72) 4312 149 628 1254 0523 1 P1(t-96) 2525 150 853 1041 0736 2 P2(t-96) 3070 151 484 0142 1635 3 P3(t-96) 5616 152 289 1641 0136 4 P4(t-96) 2525 153 811 1504 0273 5 P5(t-96) 3070 154 202 0751 1026 6 P6(t-96) 3616 155 1021 1774 0003 7 P7(t-96) 3525 ** In the octal notation for the first 10 chips of the C/A-code or the initial settings as shown in this table, the first digit (1/0) represents a "1" or “0”, respectively, for the first chip and the last three digits are the conventional octal representation of the remaining 9 chips. (For example, the first 10 chips of the C/A code for PRN Signal Assembly No. 38 are: 1111110000) *** Pi(t-N): P-code sequence of PRN number i shifted by N hours. See Section 6.3.5.2.1. IRN-200D-001 IS-GPS-200D 7 Mar 2006 56f Table 6-I Additional C/A-/P-Code Phase Assignments (sheet 5 of 6) PRN Signal No. C/A P G2 Delay (Chips) Initial G2 Setting (Octal)** First 10 Chips (Octal)** X2 Delay (Chips) P-code Relative Delay (Hours) *** First 12 Chips (Octal) 156 463 0107 1670 8 P8(t-96) 5470 157 568 1153 0624 9 P9(t-96) 4416 158 904 1542 0235 10 P10(t-96) 4025 159 670 1223 0554 11 P11(t-96) 4230 160 230 1702 0075 12 P12(t-96) 4336 161 911 0436 1341 13 P13(t-96) 2375 162 684 1735 0042 14 P14(t-96) 5354 163 309 1662 0115 15 P15(t-96) 2744 164 644 1570 0207 16 P16(t-96) 5140 165 932 1573 0204 17 P17(t-96) 4642 166 12 0201 1576 18 P18(t-96) 4103 167 314 0635 1142 19 P19(t-96) 2263 168 891 1737 0040 20 P20(t-96) 5313 169 212 1670 0107 21 P21(t-96) 2767 170 185 0134 1643 22 P22(t-96) 5151 171 675 1224 0553 23 P23(t-96) 2646 172 503 1460 0317 24 P24(t-96) 3101 173 150 1362 0415 25 P25(t-96) 5662 174 395 1654 0123 26 P26(t-96) 4513 175 345 0510 1267 27 P27(t-96) 2067 176 846 0242 1535 28 P28(t-96) 3211 177 798 1142 0635 29 P29(t-96) 3726 178 992 1017 0760 30 P30(t-96) 3571 179 357 1070 0707 31 P31(t-96) 3456 180 995 0501 1276 32 P32(t-96) 3405 181 877 0455 1322 33 P33(t-96) 3420 182 112 1566 0211 34 P34(t-96) 5432 183 144 0215 1562 35 P35(t-96) 4437 184 476 1003 0774 36 P36(t-96) 2035 185 193 1454 0323 37 P37(t-96) 5234 ** In the octal notation for the first 10 chips of the C/A-code or the initial settings as shown in this table, the first digit (1/0) represents a "1" or “0”, respectively, for the first chip and the last three digits are the conventional octal representation of the remaining 9 chips. (For example, the first 10 chips of the C/A code for PRN Signal Assembly No. 38 are: 1111110000) *** Pi(t-N): P-code sequence of PRN number i shifted by N hours. See Section 6.3.5.2.1. IRN-200D-001 IS-GPS-200D 7 Mar 2006 56g Table 6-I Additional C/A-/P-Code Phase Assignments (sheet 6 of 6) PRN Signal No. C/A P G2 Delay (Chips) Initial G2 Setting (Octal)** First 10 Chips (Octal)** X2 Delay (Chips) P-code Relative Delay (Hours) *** First 12 Chips (Octal) 186 109 1665 0112 1 P1(t-120) 5067 187 445 0471 1306 2 P2(t-120) 2611 188 291 1750 0027 3 P3(t-120) 5126 189 87 0307 1470 4 P4(t-120) 4671 190 399 0272 1505 5 P5(t-120) 4116 191 292 0764 1013 6 P6(t-120) 2265 192 901 1422 0355 7 P7(t-120) 5310 193 339 1050 0727 8 P8(t-120) 2766 194 208 1607 0170 9 P9(t-120) 5151 195 711 1747 0030 10 P10(t-120) 2646 196 189 1305 0472 11 P11(t-120) 3101 197 263 0540 1237 12 P12(t-120) 3662 198 537 1363 0414 13 P13(t-120) 5513 199 663 0727 1050 14 P14(t-120) 4467 200 942 0147 1630 15 P15(t-120) 4011 201 173 1206 0571 16 P16(t-120) 4226 202 900 1045 0732 17 P17(t-120) 4331 203 30 0476 1301 18 P18(t-120) 4376 204 500 0604 1173 19 P19(t-120) 2355 205 935 1757 0020 20 P20(t-120) 5344 206 556 1330 0447 21 P21(t-120) 4740 207 373 0663 1114 22 P22(t-120) 2142 208 85 1436 0341 23 P23(t-120) 5243 209 652 0753 1024 24 P24(t-120) 2703 210 310 0731 1046 25 P25(t-120) 5163 ** In the octal notation for the first 10 chips of the C/A-code or the initial settings as shown in this table, the first digit (1/0) represents a "1" or “0”, respectively, for the first chip and the last three digits are the conventional octal representation of the remaining 9 chips. (For example, the first 10 chips of the C/A code for PRN Signal Assembly No. 38 are: 1111110000) *** Pi(t-N): P-code sequence of PRN number i shifted by N hours. See Section 6.3.5.2.1. IRN-200D-001 IS-GPS-200D 7 Mar 2006 56h 6.3.5.3 Additional L2 CM-/L2 CL-Code PRN sequences. The PRN L2 CM-code and L2 CL-code are described in Sections 3.2.1.4 and 3.2.1.5, respectively, and 37 L2 CM-/L2 CL-code sequence pairs are assigned by SV-ID number in Table 3-II. An additional set of 80 L2 CM-/L2 CL-code PRN sequence pairs are selected and assigned with PRN numbers in this section as shown in Table 6-II. Among the 80 additional sequences, PRN numbers 38 through 63 are reserved for future GPS SVs, and PRN numbers 159 through 210 are reserved for other GNSS applications. PRN allocations do not exist for numbers 64 through 158 for L2 CM-/L2 CL-code. Any assignment of a L2 CM-/L2 CL-code PRN number and its code sequence pair for any additional SV and/or other L2 signal applications will be selected from the sequences of Table 6-II and will be approved, controlled, and managed by the GPS JPO. Table 6-II. Additional L2 CM-/L2 CL-Code Phase Assignments (sheet 1 of 3) PRN Signal No. *** Initial Shift Register State (Octal) End Shift Register State (Octal) L2 CM L2 CL L2 CM * L2 CL ** 38 771353753 101232630 453413162 463624741 39 226107701 132525726 637760505 673421367 40 022025110 315216367 612775765 703006075 41 402466344 377046065 136315217 746566507 42 752566114 655351360 264252240 444022714 43 702011164 435776513 113027466 136645570 44 041216771 744242321 774524245 645752300 45 047457275 024346717 161633757 656113341 46 266333164 562646415 603442167 015705106 47 713167356 731455342 213146546 002757466 48 060546335 723352536 721323277 100273370 49 355173035 000013134 207073253 304463615 50 617201036 011566642 130632332 054341657 51 157465571 475432222 606370621 333276704 52 767360553 463506741 330610170 750231416 53 023127030 617127534 744312067 541445326 54 431343777 026050332 154235152 316216573 55 747317317 733774235 525024652 007360406 56 045706125 751477772 535207413 112114774 57 002744276 417631550 655375733 042303316 58 060036467 052247456 316666241 353150521 59 217744147 560404163 525453337 044511154 60 603340174 417751005 114323414 244410144 61 326616775 004302173 755234667 562324657 62 063240065 715005045 526032633 027501534 63 111460621 001154457 602375063 521240373 * Short cycled period = 10230 ** Short cycled period = 767250 *** PRN sequences 38 through 63 are reserved for GPS. IRN-200D-001 IS-GPS-200D 7 Mar 2006 56i Table 6-II. Additional L2 CM-/L2 CL-Code Phase Assignments (sheet 2 of 3) PRN Signal No. Initial Shift Register State (Octal) End Shift Register State (Octal) L2 CM L2 CL L2 CM * L2 CL ** 159 604055104 605253024 425373114 044547544 160 157065232 063314262 427153064 707116115 161 013305707 066073422 310366577 412264037 162 603552017 737276117 623710414 223755032 163 230461355 737243704 252761705 403114174 164 603653437 067557532 050174703 671505575 165 652346475 227354537 050301454 606261015 166 743107103 704765502 416652040 223023120 167 401521277 044746712 050301251 370035547 168 167335110 720535263 744136527 516101304 169 014013575 733541364 633772375 044115766 170 362051132 270060042 007131446 704125517 171 617753265 737176640 142007172 406332330 172 216363634 133776704 655543571 506446631 173 755561123 005645427 031272346 743702511 174 365304033 704321074 203260313 022623276 175 625025543 137740372 226613112 704221045 176 054420334 056375464 736560607 372577721 177 415473671 704374004 011741374 105175230 178 662364360 216320123 765056120 760701311 179 373446602 011322115 262725266 737141001 180 417564100 761050112 013051476 227627616 181 000526452 725304036 144541215 245154134 182 226631300 721320336 534125243 040015760 183 113752074 443462103 250001521 002154472 184 706134401 510466244 276000566 301767766 185 041352546 745522652 447447071 226475246 186 664630154 373417061 000202044 733673015 187 276524255 225526762 751430577 602507667 188 714720530 047614504 136741270 753362551 189 714051771 034730440 257252440 746265601 190 044526647 453073141 757666513 036253206 * Short cycled period = 10230 ** Short cycled period = 767250 IRN-200D-001 IS-GPS-200D 7 Mar 2006 56j Table 6-II. Additional L2 CM-/L2 CL-Code Phase Assignments (sheet 3 of 3) PRN Signal No. Initial Shift Register State (Octal) End Shift Register State (Octal) L2 CM L2 CL L2 CM * L2 CL ** 191 207164322 533654510 606512137 202512772 192 262120161 377016461 734247645 701234023 193 204244652 235525312 415505547 722043377 194 202133131 507056307 705146647 240751052 195 714351204 221720061 006215430 375674043 196 657127260 520470122 371216176 166677056 197 130567507 603764120 645502771 123055362 198 670517677 145604016 455175106 707017665 199 607275514 051237167 127161032 437503241 200 045413633 033326347 470332401 275605155 201 212645405 534627074 252026355 376333266 202 613700455 645230164 113771472 467523556 203 706202440 000171400 754447142 144132537 204 705056276 022715417 627405712 451024205 205 020373522 135471311 325721745 722446427 206 746013617 137422057 056714616 412376261 207 132720621 714426456 706035241 441570172 208 434015513 640724672 173076740 063217710 209 566721727 501254540 145721746 110320656 210 140633660 513322453 465052527 113765506 * Short cycled period = 10230 ** Short cycled period = 767250 IRN-200D-001 IS-GPS-200D 7 Mar 2006 56k (This page intentionally left blank.) IRN-200D-001 IS-GPS-200D 7 Mar 2006 56l 10. APPENDIX I. LETTERS OF EXCEPTION 10.1 Scope. Approval of this document, as well as approval of any subsequent changes to the document, can be contingent upon a "letter of exception". This appendix depicts such "letters of exception" when authorized by the GPS JPO. 10.2 Applicable Documents. The documents listed in Section 2.0 shall be applicable to this appendix. 10.3 Letters of Exception. Any letter of exception which is in force for the revision of the IS is depicted in Figure 10-1. IS-GPS-200D 7 Dec 2004 57 (This page intentionally left blank.) IS-GPS-200D 7 Dec 2004 58 Lockheed Martin Space Systems Company Space & Strategic Missiles Valley Forge Operations P.O. Box 8555 Philadelphia, PA 19101 26 May 2003 GPS IIR-CM-MOD-147 SMC/CZK 2420 VELA WAY, SUITE 1467 LOS ANGELES AFB CA 90245-4659 Attention: Mr. David Smith Subject: GPS Block IIR Modernization Contract F04701-00-C-0006 Review and approval of ICD-GPS-PIRN-200C-007B, dated 08 November 2003, post 9 April 2003 CCB (L2C = -160). Reference: 1) PCOL# 03-012, dated 22 May 03; F04701-00-C-0006; REQUEST FOR IMPACTS DUE TO IMPLEMENTING PROPOSED CHANGES TO PIRN-200C-007 REVISION B Dear Mr. Smith: Lockheed Martin Space Systems Company has been asked to review and comment on changes made to ICD-GPS-PIRN-200C-007B at the JPO CCB boarded on or about 09 April 2003. It is our understanding that the ONLY change made to the 08 November 2002 of the subject ICD is L2C for IIR-M SVs changed from –161.4 dBW to –160.0 dBW. Based on that change, Lockheed Martin takes exception to IIR-M L2 C signal power specified in Table 3 III. Per Lockheed Martin contract requirements as specified in SS-SS-500, Rev. A, dated 14 May 2001, LMSSC calculates links using: • 0-dBi circularly polarized user receiving antenna (located) near ground when the SV is above a 5° elevation angle • Atmospheric loss of 0.5 dB at edge of earth • Assumes SV antenna gains are averaged about azimuth Using the assumptions as specified in paragraph 3.3.1.6 of PIRN-200C-007B, the GPS IIRM SVs provide a minimum receive signal of -161.4 dBW for L2 C signal. Lockheed Martin therefore takes exception to 160 dBW for L2C of PIRN-200c-007B. Formal request for cost and schedule impacts should come through the JPO Contracting Officer. To change from -161.4 dBW to -160.0 dBW would have to be analyzed and coordinated between Lockheed Martin and ITT. If such a change were technically possible, there would be impacts to L-Band level testing, SV level testing, test scripts, Specs, OOH, and various ICDs. These impacts would be in both cost and schedule. Figure 10-1. Letters of Exception. IS-GPS-200D 7 Dec 2004 59 GPS IIR-CM-MOD-147 Page 2 Currently, there is an ongoing effort between Lockheed Martin, Boeing, Arinc, Aerospace, and the JPO concerning signal flexibility under the ConOps study. Lockheed Martin recommends, based on the outcome and direction of this effort, that an impact to the ICD-200 change be included in the resulting request for ROMs for Flex Power implementation. Note that if Lockheed Martin has taken earlier exception to a change in any requirements in a previous revision of this document, Lockheed Martin continues to take exception to that change. A letter explicitly stating that the exception is no longer valid will accomplish the retraction of an exception. Should you have any questions, please contact Martin O’Connor at (610) 354-7866 for technical concerns, or the undersigned at (610) 354-7989 for contractual matters. Very truly yours, LOCKHEED MARTIN CORPORATION Signature on file Brent B. Achee II GPS Block IIR Deputy Program Director xc: Capt. K. Eggehorn Mary Guyes Soon Yi, ARINC J. Windfelder, DCMC Figure 10-1. Letters of Exception (continued). IS-GPS-200D 7 Dec 2004 60 Lockheed Martin Space Systems Company Space & Strategic Missiles Valley Forge Operations P.O. Box 8555 Philadelphia, PA 19101 27 September 2004 GPS IIR-CM-3023, Rev A ARINC 2250 E. Imperial Highway, Suite 450 El Segundo, CA 90245-3546 Attention: Mr. Soon K. Yi Subject: Review of IS-GPS-200 Rev D Reference: 1) Contract F04701-89-C-0073 2) IS-GPS-200D, dated 09 July 2004 Dear Mr. Yi: Lockheed Martin Space Systems Company has reviewed the subject version of IS-GPS-200D, dated 09 July 2004. It is Lockheed Martin’s understanding that the JPO and ARINC are in the process of incorporating major changes to ICD-200C, eliminating multiple Letters of Exception, and change the Interface Control Document to an Interface Specification (IS). With this in mind, Lockheed Martin is rescinding all previous letters of exception: 1. GPS IIR-CM-1046, dated 17 August 1994 2. GPS IIR-CM-MOD-0097, dated 08 May 2002 3. GPS IIR-CM-2837, dated 26 May 2003 4. GPS IIR-CM-MOD-0177, dated 16 March 2004 Lockheed Martin would like to establish this correspondence for the review of IS-GPS-200 as the baseline letter of exception. Lockheed Martin is taking exception to: 1. L2CNAV 2. IIR-M L2C Signal Power, as defined in Table 3-V The original Letter of Exception, dated 09 September 2004 listed IODC as an exception. Lockheed Martin has been able to verify this exception no longer exists. This revision to the LOE should therefore be used in it’s place. Specific reasoning for these exceptions are documented in the attached table. Lockheed Martin is also submitting technical comments identified herein. If this document is approved at JPO CCB, LMSSC will expect a letter from JPO requesting cost and schedule impacts to implement these out-of-scope requirements on the IIR and IIR-M contracts. Per discussions with ARINC, telecons with the JPO, and the IS-200D review directions, it is Lockheed Martin’s understanding that the once this document is Configuration Controlled by the JPO, ICD-200 will be removed from Lockheed Martin’s contract with the government and replace with IS-200. The approved IS-200 will contain this LOE and Lockheed Martin will be notified in writing as to changes that occurred as part of the CCB process for concurrence to said changes Figure 10-1. Letters of Exception (continued). IS-GPS-200D 7 Dec 2004 61 Should you have any questions, please contact Marty O’Connor at (610) 354-7866 for technical concerns, or the undersigned at (610) 354-2569 for contractual matters. Very truly yours, LOCKHEED MARTIN CORPORATION Signature on file Paul E. Ruffo, CPCM Manager of Contracts GPS Block IIR, IIR-M, III xc: Mary Guyes A. Trader J. Windfelder, DCMA Capt. Brian Knight Figure 10-1. Letters of Exception (continued). IS-GPS-200D 7 Dec 2004 62 Figure 10-1. Letters of Exception (continued). IS-GPS-200D 7 Dec 2004 63 Figure 10-1. Letters of Exception (continued). IS-GPS-200D 7 Dec 2004 64 20. APPENDIX II. GPS NAVIGATION DATA STRUCTURE FOR DATA, D(t) 20.1 Scope. This appendix describes the specific GPS navigation (NAV) data structure denoted as D(t). When transmitted as part of the NAV data, D(t), the specific data structure of D(t) shall be denoted by data ID number 2, represented by the two-bit binary notation as 01. 20.2 Applicable Documents. 20.2.1 Government Documents. In addition to the documents listed in paragraph 2.1, the following documents of the issue specified contribute to the definition of the NAV data related interfaces and form a part of this Appendix to the extent specified herein. Specifications None Standards None Other Publications None 20.2.2 Non-Government Documents. In addition to the documents listed in paragraph 2.2, the following documents of the issue specified contribute to the definition of the NAV data related interfaces and form a part of this Appendix to the extent specified herein. Specifications None Other Publications None IS-GPS-200D 7 Dec 2004 65 (This page intentionally left blank.) IS-GPS-200D 7 Dec 2004 66 20.3 Requirements 20.3.1 Data Characteristics. The data stream shall be transmitted by the SV on the L1 and L2 channels at a rate of 50 bps. In addition, upon ground command, the data stream shall be transmitted by the Block IIR-M SV on the L2 CM channel at a rate of 25 bps using FEC encoding resulting in 50 sps. 20.3.2 Message Structure. As shown in Figure 20-1, the message structure shall utilize a basic format of a 1500 bit long frame made up of five subframes, each subframe being 300 bits long. Subframes 4 and 5 shall be subcommutated 25 times each, so that a complete data message shall require the transmission of 25 full frames. The 25 versions of subframes 4 and 5 shall be referred to herein as pages 1 through 25 of each subframe. Each subframe shall consist of ten words, each 30 bits long; the MSB of all words shall be transmitted first. Each subframe and/or page of a subframe shall contain a telemetry (TLM) word and a handover word (HOW), both generated by the SV, and shall start with the TLM/HOW pair. The TLM word shall be transmitted first, immediately followed by the HOW. The latter shall be followed by eight data words. Each word in each frame shall contain parity (reference Section 20.3.5). Block II and IIA SVs are designed with sufficient memory capacity for storing at least 60 days of uploaded NAV data. However, the memory retention of these SVs will determine the duration of data transmission. Block IIR SVs have the capability, with current memory margin, to store at least 60 days of uploaded NAV data in the Block IIA mode and to store at least 60 days of CS data needed to generate NAV data on-board in the Autonav mode. Alternating ones and zeros will be transmitted in words 3 through 10 in place of the normal NAV data whenever the SV cannot locate the requisite valid control or data element in its on-board computer memory. The following specifics apply to this default action: (a) the parity of the affected words will be invalid, (b) the two trailing bits of word 10 will be zeros (to allow the parity of subsequent subframes to be valid -- reference paragraph 20.3.5), (c) if the problem is the lack of a data element, only the directly related subframe(s) will be treated in this manner, (d) if a control element cannot be located, this default action will be applied to all subframes and all subframes will indicate ID = 1 (Block II/IIA only) (i.e., an ID-code of 001) in the HOW (reference paragraph 20.3.3.2) (Block IIR/IIR-M and IIF SVs indicate the proper subframe ID for all subframes). Certain failures of control elements which may occur in the SV memory or during an upload will cause the SV to transmit in non-standard codes (NSC and NSY) which would preclude normal use by the US. Normal NAV data transmission will be resumed by the SV whenever a valid set of elements becomes available. IS-GPS-200D 7 Dec 2004 67 Block II/IIA SVs are uploaded with a minimum of 60 days of NAV data. However, the EAROM retentivity for Block II SVs is designed and guaranteed for only 14 days. Therefore, Block II SV memory is most likely to fail sometime during long-term extended operations after repeated write operations. In the case of memory failure, the SV will transmit alternating ones and zeros in word 3-10 as specified in the above paragraph. The EAROM retentivity for Block IIA SVs is designed and guaranteed for at least 60 days. The memory retentivity for the Block IIR/IIR-M/IIF SVs is designed and guaranteed for at least 60 days. Although the data content of the SVs will be temporarily reduced during the upload process, the transmission of valid NAV data will be continuous. The data capacity of specific operational SVs may be reduced to accommodate partial memory failures. IS-GPS-200D 7 Dec 2004 68 DIRECTION OF DATA FLOW FROM SV MSB FIRST 150 BITS 3 SECONDS WORD 1 WORD 2 WORD 3 WORD 4 WORD 5 DIRECTION OF DATA FLOW FROM SV MSB FIRST 150 BITS 3 SECONDS WORD 1 WORD 2 WORD 3 WORD 4 WORD 5 71 SUBFRAME PAGE NO. NO. 1 31 61 73 77 83 91 121 1 N/A L2 P DATA FLAG - 1 BIT URA INDEX -4 BITS SV HEALTH -6 BITS C/A OR P ON L2 -2 BITS TLM 22 BITS C HOW 22 BITS tP WN 10 BITS 2 MSBs IODC - 10 BITS TOTAL 23 BITS*** P 24 BITS*** PP P DIRECTION OF DATA FLOW FROM SV MSB FIRST 150 BITS 3 SECONDS WORD 6 WORD 7 WORD 8 WORD 9 WORD 10 151 181 197 211 219 241 271 1 N/A IODC - 10 BITS TOTAL 24 BITS*** P P P P P16 BITS*** TGD 8 BITS 8 LSBs toc 16 BITS af2 8 BITS af1 16 BITS af0 22 BITS t *** RESERVED P = 6 PARITY BITS t = 2 NONINFORMATION BEARING BITS USED FOR PARITY COMPUTATION (SEE PARAGRAPH 20.3.5) C = TLM BITS 23 AND 24 WHICH ARE RESERVED Figure 20-1. Data Format (sheet 1 of 11) IS-GPS-200D 7 Dec 2004 69 DIRECTION OF DATA FLOW FROM SV MSB FIRST 150 BITS 3 SECONDS WORD 1 WORD 2 WORD 3 WORD 4 WORD 5 DIRECTION OF DATA FLOW FROM SV MSB FIRST 150 BITS 3 SECONDS WORD 1 WORD 2 WORD 3 WORD 4 WORD 5 SUBFRAME PAGE NO. NO. 31 61 69 91 107 121 2 N/A M0 -32 BITS TOTAL P P P P P TLM 22 BITS HOW 22 BITS C t IODE 8 BITS Crs 16 BITS Δn 16 BITS 8 BITS 24 BITS MSBs LSBs DIRECTION OF DATA FLOW FROM SV MSB FIRST 150 BITS 3 SECONDS WORD 6 WORD 7 WORD 8 WORD 9 WORD 10 151 167 181 211 227 241 271 287 2 N/A e - 32 BITS TOTAL A - 32 BITS TOTAL FIT INTERVAL FLAG - 1 BIT AODO -5 BITS P P P P P MSBs CUC 16 BITS 8 BITS 24 BITS CUS 16 BITS 8 BITS LSBs MSBs LSBs 24 BITS toe 16 BITS t P = 6 PARITY BITS t = 2 NONINFORMATION BEARING BITS USED FOR PARITY COMPUTATION (SEE PARAGRAPH 20.3.5) C = TLM BITS 23 AND 24 WHICH ARE RESERVED Figure 20-1. Data Format (sheet 2 of 11) IS-GPS-200D 7 Dec 2004 70 DIRECTION OF DATA FLOW FROM SV MSB FIRST 150 BITS 3 SECONDS WORD 1 WORD 2 WORD 3 WORD 4 WORD 5 DIRECTION OF DATA FLOW FROM SV MSB FIRST 150 BITS 3 SECONDS WORD 1 WORD 2 WORD 3 WORD 4 WORD 5 SUBFRAME PAGE NO. NO. 31 61 77 91 121 3 N/A i0 -32 BITS TOTAL Ω0 -32 BITS TOTAL P P P P PC TLM 22 BITS t HOW 22 BITS Cic 16 BITS 8 BITS 24 BITS Cis 16 BITS 8 BITS MSBs LSBs DIRECTION OF DATA FLOW FROM SV MSB FIRST 150 BITS 3 SECONDS WORD 6 WORD 7 WORD 8 WORD 9 WORD 10 151 181 211 241 271 279 3 N/A ω - 32 BITS TOTAL i0 - 32 BITS TOTAL P = 6 PARITY BITS t = 2 NONINFORMATION BEARING BITS USED FOR PARITY COMPUTATION (SEE PARAGRAPH 20.3.5) C = TLM BITS 23 AND 24 WHICH ARE RESERVED •ΩP P P P P24 BITS Crc 16 BITS 8 BITS 24 BITS 24 BITS t IODE 8 BITS IDOT 14 BITS LSBs MSBs LSBs Figure 20-1. Data Format (sheet 3 of 11) IS-GPS-200D 7 Dec 2004 71 DIRECTION OF DATA FLOW FROM SV MSB FIRST 150 BITS 3 SECONDS WORD 1 WORD 2 WORD 3 WORD 4 WORD 5 DIRECTION OF DATA FLOW FROM SV MSB FIRST 150 BITS 3 SECONDS WORD 1 WORD 2 WORD 3 WORD 4 WORD 5 61 SUBFRAME PAGE NO. NO. 31 63 69 91 99 121 1 THRU 24 P P P P PC TLM 22 BITS t HOW 22 BITS e 16 BITS toa 8 BITS δi 16 BITS •Ω16 BITS 8 BITS DATA ID -2 BITS SV ID - 6 BITS SV HEALTH DIRECTION OF DATA FLOW FROM SV MSB FIRST 150 BITS 3 SECONDS WORD 6 WORD 7 WORD 8 WORD 9 WORD 10 151 181 211 241 271 279 290 1 THRU 24 af0 - 11 BITS TOTAL af1 - 11 BITS TOTAL P = 6 PARITY BITS t = 2 NONINFORMATION BEARING BITS USED FOR PARITY COMPUTATION (SEE PARAGRAPH 20.3.5) C = TLM BITS 23 AND 24 WHICH ARE RESERVED NOTE: PAGES 2, 3, 4, 5, 7, 8, 9 & 10 OF SUBFRAME 4 HAVE THE SAME FORMAT AS PAGES 1 THROUGH 24 OF SUBFRAME 5 P P P P PA 24 BITS Ω0 24 BITS ω24 BITS M0 24 BITS t 8 MSBs 3 LSBs Figure 20-1. Data Format (sheet 4 of 11) IS-GPS-200D 7 Dec 2004 72 DIRECTION OF DATA FLOW FROM SV MSB FIRST 150 BITS 3 SECONDS WORD 1 WORD 2 WORD 3 WORD 4 WORD 5 DIRECTION OF DATA FLOW FROM SV MSB FIRST 150 BITS 3 SECONDS WORD 1 WORD 2 WORD 3 WORD 4 WORD 5 61 SUBFRAME PAGE NO. NO. 31 63 69 91 121 5 25 P P P P P TLM 22 BITS C HOW 22 BITS t toa 8 BITS WNa 8 BITS SV HEALTH 6 BITS/SV SV 1 SV 2 SV 3 SV 4 SV HEALTH 6 BITS/SV SV 5 SV 6 SV 7 SV 8 DATA ID -2 BITS SV (PAGE) ID -6 BITS DIRECTION OF DATA FLOW FROM SV MSB FIRST 150 BITS 3 SECONDS WORD 6 WORD 7 WORD 8 WORD 9 WORD 10 151 5 25 ** RESERVED FOR SYSTEM USE *** RESERVED P = 6 PARITY BITS 181 211 271 241 277 P P P P P SV HEALTH 6 BITS/SV SV 9 SV 10 SV 11 SV 12 SV HEALTH 6 BITS/SV SV 13 SV 14 SV 15 SV 16 SV HEALTH 6 BITS/SV SV 17 SV 18 SV 19 SV 20 SV HEALTH 6 BITS/SV SV 21 SV 22 SV 23 SV 24 16 BITS ** t 6 BITS *** t = 2 NONINFORMATION BEARING BITS USED FOR PARITY COMPUTATION (SEE PARAGRAPH 20.3.5) C = TLM BITS 23 AND 24 WHICH ARE RESERVED Figure 20-1. Data Format (sheet 5 of 11) IS-GPS-200D 7 Dec 2004 73 DIRECTION OF DATA FLOW FROM SV MSB FIRST 150 BITS 3 SECONDS WORD 1 WORD 2 WORD 3 WORD 4 WORD 5 DIRECTION OF DATA FLOW FROM SV MSB FIRST 150 BITS 3 SECONDS WORD 1 WORD 2 WORD 3 WORD 4 WORD 5 61 SUBFRAME PAGE NO. NO. 31 63 69 91 121 1, 6, 11, 16 & 21 P P P P PC TLM 22 BITS HOW 22 BITS t 16 BITS*** 24 BITS*** 24 BITS*** DATA ID - 2 BITS SV (PAGE) ID - 6 BITS DIRECTION OF DATA FLOW FROM SV MSB FIRST 150 BITS 3 SECONDS WORD 6 WORD 7 WORD 8 WORD 9 WORD 10 1, 6, 11, 16 & 21 151 181 211 241 249 271 P P P P P24 BITS*** 24 BITS*** 24 BITS*** 8*** BITS 16 BITS*** t22 BITS** ** RESERVED FOR SYSTEM USE *** RESERVED P = 6 PARITY BITS t = 2 NONINFORMATION BEARING BITS USED FOR PARITY COMPUTATION (SEE PARAGRAPH 20.3.5) C = TLM BITS 23 AND 24 WHICH ARE RESERVED Figure 20-1. Data Format (sheet 6 of 11) IS-GPS-200D 7 Dec 2004 74 DIRECTION OF DATA FLOW FROM SV MSB FIRST 150 BITS 3 SECONDS WORD 1 WORD 2 WORD 3 WORD 4 WORD 5 DIRECTION OF DATA FLOW FROM SV MSB FIRST 150 BITS 3 SECONDS WORD 1 WORD 2 WORD 3 WORD 4 WORD 5 61 SUBFRAME PAGE NO. NO. 31 63 69 91 121 12, 19, 20, 22, 23 & 24 P P P P PC TLM 22 BITS HOW 22 BITS t 16 BITS*** 24 BITS*** 24 BITS*** DATA ID - 2 BITS SV (PAGE) ID - 6 BITS DIRECTION OF DATA FLOW FROM SV MSB FIRST 150 BITS 3 SECONDS WORD 6 WORD 7 WORD 8 WORD 9 WORD 10 12, 19, 20, 22, 23 & 24 151 181 211 241 249 271 P P P P P24 BITS*** 24 BITS*** 24 BITS*** 8*** BITS 16 BITS** t22 BITS** ** RESERVED FOR SYSTEM USE *** RESERVED P = 6 PARITY BITS t = 2 NONINFORMATION BEARING BITS USED FOR PARITY COMPUTATION (SEE PARAGRAPH 20.3.5) C = TLM BITS 23 AND 24 WHICH ARE RESERVED Figure 20-1. Data Format (sheet 7 of 11) IS-GPS-200D 7 Dec 2004 75 DIRECTION OF DATA FLOW FROM SV MSB FIRST 150 BITS 3 SECONDS WORD 1 WORD 2 WORD 3 WORD 4 WORD 5 DIRECTION OF DATA FLOW FROM SV MSB FIRST 150 BITS 3 SECONDS WORD 1 WORD 2 WORD 3 WORD 4 WORD 5 61 SUBFRAME PAGE NO. NO. 31 63 69 4 18 P P P P P TLM 22 BITS HOW 22 BITS C t α0 8 BITS α1 8 BITS α2 8 BITS α3 8 BITS β0 8 BITS β1 8 BITS β2 8 BITS β3 8 BITS DATA ID - 2 BITS SV (PAGE) ID - 6 BITS 77 91 99 107 121 129 137 DIRECTION OF DATA FLOW FROM SV MSB FIRST 150 BITS 3 SECONDS WORD 6 WORD 7 WORD 8 WORD 9 WORD 10 151 181 211 219 227 241 249 257 271 279 4 18 A0 - 32 BITS TOTAL WNLSF P P P P P A1 24 BITS 24 BITS 8 BITS tot 8 BITS WNt 8 BITS ΔtLS 8 BITS 8 BITS DN 8 BITS ΔtLSF 8 BITS t14 BITS** MSBs LSBs ** RESERVED FOR SYSTEM USE P = 6 PARITY BITS t = 2 NONINFORMATION BEARING BITS USED FOR PARITY COMPUTATION (SEE PARAGRAPH 20.3.5) C = TLM BITS 23 AND 24 WHICH ARE RESERVED Figure 20-1. Data Format (sheet 8 of 11) IS-GPS-200D 7 Dec 2004 76 DIRECTION OF DATA FLOW FROM SV MSB FIRST 150 BITS 3 SECONDS WORD 1 WORD 2 WORD 3 WORD 4 WORD 5 DIRECTION OF DATA FLOW FROM SV MSB FIRST 150 BITS 3 SECONDS WORD 1 WORD 2 WORD 3 WORD 4 WORD 5 SUBFRAME PAGE NO. NO. 31 91 121 4 25 P P P P P TLM 22 BITS HOW 22 BITS C t A-SPOOF & SV CONFIG SV 1 SV 2 SV 3 SV 4 SV 10 SV 5 SV 6 SV 7 SV 8 SV 9 A- SPOOF & SV CONFIG SV 16 SV 11 SV 12 SV 13 SV 14 SV 15 A- SPOOF & SV CONFIG DATA ID - 2 BITS SV (PAGE) ID - 6 BITS 61 63 69 DIRECTION OF DATA FLOW FROM SV MSB FIRST 150 BITS 3 SECONDS WORD 6 WORD 7 WORD 8 WORD 9 WORD 10 227 151 181 211 241 271 4 25 P P P P P SV 22 SV 17 SV 18 SV 19 SV 20 SV 21 A- SPOOF & SV CONFIG SV 28 SV 23 SV 24 SV 25 SV 26 SV 27 A- SPOOF & SV CONFIG A-SPOOF & SV CONFIG SV 29 SV 30 SV 31 SV 32 SV 25 SV 26 SV 27 SV 28 SV 29 SV HEALTH 6 BITS/SV t SV HEALTH 6 BITS/SV SV 30 SV 31 SV 32 ** RESERVED FOR SYSTEM USE P = 6 PARITY BITS 2 BITS ** 229 SV HEALTH -6 BITS 4 BITS ** t = 2 NONINFORMATION BEARING BITS USED FOR PARITY COMPUTATION (SEE PARAGRAPH 20.3.5) C = TLM BITS 23 AND 24 WHICH ARE RESERVED Figure 20-1. Data Format (sheet 9 of 11) IS-GPS-200D 7 Dec 2004 77 DIRECTION OF DATA FLOW FROM SV MSB FIRST 150 BITS 3 SECONDS WORD 1 WORD 2 WORD 3 WORD 4 WORD 5 DIRECTION OF DATA FLOW FROM SV MSB FIRST 150 BITS 3 SECONDS WORD 1 WORD 2 WORD 3 WORD 4 WORD 5 SUB FRAME PAGE NO. NO. 61 69 131 91 121 PPPPPC TLM 22 BITS HOW 22 BITS t 63 DATA ID - 2 BITS SV (PAGE) ID - 6 BITS 71 E R D 3 4 L S B S E R D 4 6 B I T S AVAILABILITY INDICATOR - 2 BITS E R D 5 6 B I T S E R D 6 6 B I T S E R D 7 2 M S B S E R D 7 4 L S B S E R D 8 6 B I T S E R D 9 6 B I T S E R D 1 0 6 B I T S E R D 1 1 2 M S B S E R D 1 6 B I T S E R D 2 6 B I T S E R D 3 2 M S B S 44 13 DIRECTION OF DATA FLOW FROM SV MSB FIRST 150 BITS 3 SECONDS WORD 6 WORD 7 WORD 8 WORD 9 WORD 10 Pt ERD30 6BI TS I ERD29 6B TS I ERD28 6B TS ERD27 4LSBS P ERD27 2MSBS I ERD26 6B TS I ERD25 6B TS ERD24 6BI TS ERD23 4LSBS P ERD23 2MSBS ERD22 6BI TS I ERD21 6B TS ERD20 6BITS ERD19 4LSBS P ERD19 2MSBS I ERD18 6B TS 151 181 211 241 271 I ERD17 6B TS I ERD16 6B TS ERD15 4LSBS P ERD15 2MSBS ERD14 6B TS I I ERD13 6B TS I ERD12 6B TS ERD11 4LSBS 13 P = 6 PARITY BITS t = 2 NONINFORMATION BEARING BITS USED FOR PARITY COMPUTATION (SEE PARAGRAPH 20.3.5) C = TLM BITS 23 AND 24 WHICH ARE RESERVED Figure 20-1. Data Format (sheet 10 of 11) IS-GPS-200D 7 Dec 2004 78 DIRECTION OF DATA FLOW FROM SV MSB FIRST 150 BITS 3 SECONDS WORD 1 WORD 2 WORD 3 WORD 4 WORD 5 DIRECTION OF DATA FLOW FROM SV MSB FIRST 150 BITS 3 SECONDS WORD 1 WORD 2 WORD 3 WORD 4 WORD 5 61 SUBFRAME PAGE NO. NO. 31 63 69 91 121 14, 15 & 17** P P P P PC TLM 22 BITS HOW 22 BITS t 16 BITS** 24 BITS** 24 BITS** DATA ID - 2 BITS SV (PAGE) ID - 6 BITS DIRECTION OF DATA FLOW FROM SV MSB FIRST 150 BITS 3 SECONDS WORD 6 WORD 7 WORD 8 WORD 9 WORD 10 151 181 211 241 271 14, 15 & 17** P P P P P24 BITS** 24 BITS** 24 BITS** 24 BITS** 22 BITS** t ** THE INDICATED PORTIONS OF WORDS 3 THROUGH 10 OF PAGES 14 AND 15 ARE RESERVED FOR SYSTEM USE, WHILE THOSE OF PAGE 17 ARE RESERVED FOR SPECIAL MESSAGES PER PARAGRAPH 20.3.3.5.1.10 P = 6 PARITY BITS t = 2 NONINFORMATION BEARING BITS USED FOR PARITY COMPUTATION (SEE PARAGRAPH 20.3.5) C = TLM BITS 23 AND 24 WHICH ARE RESERVED Figure 20-1. Data Format (sheet 11 of 11) IS-GPS-200D 7 Dec 2004 79 (This page intentionally left blank.) IS-GPS-200D 7 Dec 2004 80 20.3.3 Message Content. The format and contents of the TLM word and the HOW, as well as those of words three through ten of each subframe/page, are described in the following subparagraphs. The timing of the subframes and pages is covered in Section 20.3.4. 20.3.3.1 Telemetry Word. Each TLM word is 30 bits long, occurs every six seconds in the data frame, and is the first word in each subframe/page. The format shall be as shown in Figure 20-2. Bit 1 is transmitted first. Each TLM word shall begin with a preamble, followed by the TLM message, two reserved bits, and six parity bits. The TLM message contains information needed by the authorized user and by the CS, as described in the related SS/CS interface documentation. 20.3.3.2 Handover Word (HOW). The HOW shall be 30 bits long and shall be the second word in each subframe/page, immediately following the TLM word. A HOW occurs every 6 seconds in the data frame. The format and content of the HOW shall be as shown in Figure 20-2. The MSB is transmitted first. The HOW begins with the 17 MSBs of the time-of-week (TOW) count. (The full TOW count consists of the 19 LSBs of the 29-bit Z- count). These 17 bits correspond to the TOW-count at the X1 epoch which occurs at the start (leading edge) of the next following subframe (reference paragraph 3.3.4). Bit 18 is an "alert" flag. When this flag is raised (bit 18 = "1"), it shall indicate to the unauthorized user that the SV URA may be worse than indicated in subframe 1 and that he shall use that SV at his own risk. Bit 19 is an anti-spoof (A-S) flag. A "1" in bit-position 19 indicates that the A-S mode is ON in that SV. Bits 20, 21, and 22 of the HOW provide the ID of the subframe in which that particular HOW is the second word; the ID code shall be as follows: Subframe ID Code 1 001 2 010 3 011 4 100 5 101 IS-GPS-200D 7 Dec 2004 81 TLM Word MSB LSB Parity = Reserved Bits1 1 Preamble 1 0 0 0 1 0 1 1 TLM Message 1 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 HOW Solved for bits to preserveparity check with zeros in bits 29 and 30 Anti-Spoof Flag “Alert” Flag Parity MSB LSB TOW-Count Message (Truncated) Sub- frame ID 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 Figure 20-2. TLM and HOW Formats IS-GPS-200D 7 Dec 2004 82 20.3.3.3 Subframe 1. The content of words three through ten of subframe 1 are defined below, followed by related algorithms and material pertinent to use of the data. 20.3.3.3.1 Subframe 1 Content. The third through tenth words of subframe 1 shall each contain six parity bits as their LSBs; in addition, two non-information bearing bits shall be provided as bits 23 and 24 of word ten for parity computation purposes. The remaining 190 bits of words three through ten shall contain the clock parameters and other data described in the following. The clock parameters describe the SV time scale during the period of validity. The parameters in a data set shall be valid during the interval of time in which they are transmitted and shall remain valid for an additional period of time after transmission of the next data set has started. The timing information for subframes, pages, and data sets is covered in Section 20.3.4. 20.3.3.3.1.1 Transmission Week Number. The ten MSBs of word three shall contain the ten MSBs of the 29-bit Z- count as qualified herein. These ten bits shall be a modulo 1024 binary representation of the current GPS week number at the start of the data set transmission interval (see paragraph 3.3.4(b)). The GPS week number increments at each end/start of week epoch. For Block II SVs in long-term extended operations, beginning approximately 28 days after upload, the transmission week number may not correspond to the actual GPS week number due to curve fit intervals that cross week boundaries. 20.3.3.3.1.2 Code(s) on L2 Channel. Bits 11 and 12 of word three shall indicate which code(s) is (are) commanded ON for the L2 channel, as follows: 00 = Reserved, 01 = P code ON, 10 = C/A code ON. IS-GPS-200D 7 Dec 2004 83 20.3.3.3.1.3 SV Accuracy. Bits 13 through 16 of word three shall give the URA index of the SV (reference paragraph 6.2.1) for the unauthorized user. Except for Block IIR/IIR-M SVs in the Autonav mode, the URA index (N) is an integer in the range of 0 through 15 and has the following relationship to the URA of the SV: URA INDEX URA (meters) 0 0.00 < URA ≤ 2.40 1 2.40 < URA ≤ 3.40 2 3.40 < URA ≤ 4.85 3 4.85 < URA ≤ 6.85 4 6.85 < URA ≤ 9.65 5 9.65 < URA ≤ 13.65 6 13.65 < URA ≤ 24.00 7 24.00 < URA ≤ 48.00 8 48.00 < URA ≤ 96.00 9 96.00 < URA ≤ 192.00 10 192.00 < URA ≤ 384.00 11 384.00 < URA ≤ 768.00 12 768.00 < URA ≤ 1536.00 13 1536.00 < URA ≤ 3072.00 14 3072.00 < URA ≤ 6144.00 15 6144.00 < URA (or no accuracy prediction is available - unauthorized users are advised to use the SV at their own risk.) For each URA index (N), users may compute a nominal URA value (X) as given by: • If the value of N is 6 or less, X = 2(1 + N/2), • If the value of N is 6 or more, but less than 15, X = 2(N - 2), • N = 15 shall indicate the absence of an accuracy prediction and shall advise the unauthorized user to use that SV at his own risk. For N = 1, 3, and 5, X should be rounded to 2.8, 5.7, and 11.3 meters, respectively. For Block IIR/IIR-M SVs in the Autonav mode, the URA shall be defined to mean “no better than X meters”, with “X” as defined above for each URA index IS-GPS-200D 7 Dec 2004 84 20.3.3.3.1.4 SV Health. The six-bit health indication given by bits 17 through 22 of word three refers to the transmitting SV. The MSB shall indicate a summary of the health of the NAV data, where 0 = all NAV data are OK, 1 = some or all NAV data are bad. The five LSBs shall indicate the health of the signal components in accordance with the codes given in paragraph 20.3.3.5.1.3. The health indication shall be given relative to the "as designed" capabilities of each SV (as designated by the configuration code - see paragraph 20.3.3.5.1.4). Accordingly, any SV which does not have a certain capability will be indicated as "healthy" if the lack of this capability is inherent in its design or if it has been configured into a mode which is normal from a user standpoint and does not require that capability. Additional SV health data are given in subframes 4 and 5. The data given in subframe 1 may differ from that shown in subframes 4 and/or 5 of other SVs since the latter may be updated at a different time. 20.3.3.3.1.5 Issue of Data, Clock (IODC). Bits 23 and 24 of word three in subframe 1 shall be the two MSBs of the ten-bit IODC term; bits one through eight of word eight in subframe 1 shall contain the eight LSBs of the IODC. The IODC indicates the issue number of the data set and thereby provides the user with a convenient means of detecting any change in the correction parameters. Constraints on the IODC as well as the relationship between the IODC and the IODE (issue of data, ephemeris) terms are defined in paragraph 20.3.4.4. Short-term and Long-term Extended Operations. Whenever the fit interval flag indicates a fit interval greater than 4 hours, the IODC can be used to determine the actual fit interval of the data set (reference section 20.3.4.4). 20.3.3.3.1.6 Data Flag for L2 P-Code. When bit 1 of word four is a "1", it shall indicate that the NAV data stream was commanded OFF on the P-code of the L2 channel. IS-GPS-200D 7 Dec 2004 85 20.3.3.3.1.7 Estimated Group Delay Differential. Bits 17 through 24 of word seven contain the L1-L2 correction term, TGD, for the benefit of "L1 only" or "L2 only" users; the related user algorithm is given in paragraph 20.3.3.3.3. 20.3.3.3.1.8 SV Clock Correction. Bits nine through 24 of word eight, bits one through 24 of word nine, and bits one through 22 of word ten contain the parameters needed by the users for apparent SV clock correction (toc, af2, af1, af0). The related algorithm is given in paragraph 20.3.3.3.3. 20.3.3.3.2 Subframe 1 Parameter Characteristics. For those parameters whose characteristics are not fully defined in Section 20.3.3.3.1, the number of bits, the scale factor of the LSB (which shall be the last bit received), the range, and the units shall be as specified in Table 20-I. 20.3.3.3.3 User Algorithms for Subframe 1 Data. The algorithms defined below (a) allow all users to correct the code phase time received from the SV with respect to both SV code phase offset and relativistic effects, (b) permit the "single frequency" (L1 or L2) user to compensate for the effects of SV group delay differential (the user who utilizes both frequencies does not require this correction, since the clock parameters account for the induced effects), and (c) allow the "two frequency" (L1 and L2) user to correct for the group propagation delay due to ionospheric effects (the single frequency user may correct for ionospheric effects as described in paragraph 20.3.3.5.2.5). IS-GPS-200D 7 Dec 2004 86 Table 20-I. Subframe 1 Parameters Parameter No. of Bits** Scale Factor (LSB) Effective Range*** Units Code on L2 2 1 discretes Week No. 10 1 week L2 P data flag 1 1 discrete SV accuracy 4 (see text) SV health 6 1 discretes TGD 8* 2-31 seconds IODC 10 (see text) toc 16 24 604,784 seconds af2 8* 2-55 sec/sec2 af1 16* 2-43 sec/sec af0 22* 2-31 seconds * Parameters so indicated shall be two's complement, with the sign bit (+ or -) occupying the MSB; ** See Figure 20-1 for complete bit allocation in subframe; *** Unless otherwise indicated in this column, effective range is the maximum range attainable with indicated bit allocation and scale factor. IS-GPS-200D 7 Dec 2004 87 20.3.3.3.3.1 User Algorithm for SV Clock Correction. The polynomial defined in the following allows the user to determine the effective SV PRN code phase offset referenced to the phase center of the antennas (Δtsv) with respect to GPS system time (t) at the time of data transmission. The coefficients transmitted in subframe 1 describe the offset apparent to the two-frequency user for the interval of time in which the parameters are transmitted. This estimated correction accounts for the deterministic SV clock error characteristics of bias, drift and aging, as well as for the SV implementation characteristics of group delay bias and mean differential group delay. Since these coefficients do not include corrections for relativistic effects, the user's equipment must determine the requisite relativistic correction. Accordingly, the offset given below includes a term to perform this function. The user shall correct the time received from the SV with the equation (in seconds) t = tsv -Δtsv (1) where t = GPS system time (seconds), tsv = effective SV PRN code phase time at message transmission time (seconds), Δtsv = SV PRN code phase time offset (seconds). The SV PRN code phase offset is given by Δtsv = af0 + af1(t - toc) + af2(t - toc)2 + Δtr (2) where af0, af1 and af2 are the polynomial coefficients given in subframe 1, toc is the clock data reference time in seconds (reference paragraph 20.3.4.5), and Δtr is the relativistic correction term (seconds) which is given by Δtr = F e A sin Ek. The orbit parameters (e, A , Ek) used here are described in discussions of data contained in subframes 2 and 3, while F is a constant whose value is −2 µ sec F = = - 4.442807633 (10)-10 , 2 c meter IS-GPS-200D 7 Dec 2004 88 where 3 meters μ = 3.986005 x 1014 = value of Earth's universal gravitational parameters 2 second meters c = 2.99792458 x 108 = speed of light. second Note that equations (1) and (2), as written, are coupled. While the coefficients af0, af1 and af2 are generated by using GPS time as indicated in equation (2), sensitivity of tsv to t is negligible. This negligible sensitivity will allow the user to approximate t by tSV in equation (2). The value of t must account for beginning or end of week crossovers. That is, if the quantity t - toc is greater than 302,400 seconds, subtract 604,800 seconds from t. If the quantity t - toc is less than -302,400 seconds, add 604,800 seconds to t. The control segment will utilize the following alternative but equivalent expression for the relativistic effect when estimating the NAV parameters: →→ RV 2 • Δtr = − 2 c where → R is the instantaneous position vector of the SV, → V is the instantaneous velocity vector of the SV, and c is the speed of light. (Reference paragraph 20.3.4.3). →→ It is immaterial whether the vectors R and V are expressed in earth-fixed, rotating coordinates or in earth-centered, inertial coordinates. IS-GPS-200D 7 Dec 2004 89 20.3.3.3.3.2 L1 - L2 Correction. The L1 and L2 correction term, TGD, is initially calculated by the CS to account for the effect of SV group delay differential between L1 P(Y) and L2 P(Y) based on measurements made by the SV contractor during SV manufacture. The value of TGD for each SV may be subsequently updated to reflect the actual on-orbit group delay differential. This correction term is only for the benefit of "single-frequency" (L1 P(Y) or L2 P(Y)) users; it is necessitated by the fact that the SV clock offset estimates reflected in the af0 clock correction coefficient (see paragraph 20.3.3.3.3.1) are based on the effective PRN code phase as apparent with two frequency (L1 P(Y) and L2 P(Y)) ionospheric corrections. Thus, the user who utilizes the L1 P(Y) signal only shall modify the code phase offset in accordance with paragraph 20.3.3.3.3.1 with the equation (ΔtSV)L1P(Y) = ΔtSV -TGD where TGD is provided to the user as subframe 1 data. For the user who utilizes L2 P(Y) only, the code phase modification is given by (ΔtSV)L2P(Y) = ΔtSV -γTGD where, denoting the nominal center frequencies of L1 and L2 as fL1 and fL2 respectively, γ = (fL1/fL2)2 = (1575.42/1227.6)2 = (77/60)2. The value of TGD is not equal to the mean SV group delay differential, but is a measured value that represents the mean group delay differential multiplied by 1/(1- γ). That is, 1 TGD = (tL1P(Y) -tL2P(Y)) 1-γ where tLiP(Y) is the GPS time the ith frequency P(Y) signal (a specific epoch of the signal) is transmitted from the SV antenna phase center. IS-GPS-200D 7 Dec 2004 90 20.3.3.3.3.3 Ionospheric Correction. The two frequency (L1 P(Y) and L2 P(Y)) user shall correct for the group delay due to ionospheric effects by applying the relationship: PR -γ PR L2P(Y) L1P(Y) PR = 1-γ where PR = pseudorange corrected for ionospheric effects, PRi = pseudorange measured on the channel indicated by the subscript. and γ is as defined in paragraph 20.3.3.3.3.2. The clock correction coefficients are based on "two frequency" measurements and therefore account for the effects of mean differential delay in SV instrumentation. 20.3.3.3.3.4 Example Application of Correction Parameters. A typical system application of the correction parameters for a user receiver is shown in Figure 20-3. The ionospheric model referred to in Figure 20-3 is discussed in paragraph 20.3.3.5.2.5 in conjunction with the related data contained in page 18 of subframe 4. The ERD term referred to in Figure 20-3 is discussed in paragraph 20.3.3.5.2.6 in conjunction with the related data contained in page 13 of subframe 4. IS-GPS-200D 7 Dec 2004 91 TGD* af0, af1, af2, toc Δ tr Δ tSV CODE PHASE OFFSET -TRUE SV CLOCK EFFECTS - EQUIPMENT GROUP DELAY DIFFERENTIAL EFFECTS -RELATIVISTIC EFFECTS PSEUDORANGE DIVIDED BY THE SPEED OF LIGHT GPS TIME PATH DELAY -GEOMETRIC -TROPOSHERIC - -IONOSPHERIC* OTHER SATELLITES -CALIBRATION DATA -AUXILIARY SENSOR USER CLOCK BIAS CLOCK CORRECTION POLYNOMIAL TROPOSPHERIC MODEL IONOSPHERIC MODEL* FILTER AND COORDINATE CONVERTER RANGE DATA FROM USER POSITION, VELOCITY, and TIME (CLOCK BIAS) ESTIMATE OF SV TRANSMISSION TIME Ttropo Tiono c ERD ** αn, βn GPS TIME * SINGLE FREQUENCY USER ONLY ** OPTIONAL Figure 20-3. Sample Application of Correction Parameters IS-GPS-200D 7 Dec 2004 92 20.3.3.4 Subframes 2 and 3. The contents of words three through ten of subframes 2 and 3 are defined below, followed by material pertinent to the use of the data. 20.3.3.4.1 Content of Subframes 2 and 3. The third through tenth words of subframes 2 and 3 shall each contain six parity bits as their LSBs; in addition, two non-information bearing bits shall be provided as bits 23 and 24 of word ten of each subframe for parity computation purposes. Bits 288 through 292 of subframe 2 shall contain the Age of Data Offset (AODO) term for the navigation message correction table (NMCT) contained in subframe 4 (reference paragraph 20.3.3.5.1.9). The remaining 375 bits of those two subframes shall contain the ephemeris representation parameters of the transmitting SV. The ephemeris parameters describe the orbit during the curve fit intervals described in section 20.3.4. Table 20-II gives the definition of the orbital parameters using terminology typical of Keplerian orbital parameters; it shall be noted, however, that the transmitted parameter values are such that they provide the best trajectory fit in Earth- Centered, Earth-Fixed (ECEF) coordinates for each specific fit interval. The user shall not interpret intermediate coordinate values as pertaining to any conventional coordinate system. The issue of ephemeris data (IODE) term shall provide the user with a convenient means for detecting any change in the ephemeris representation parameters. The IODE is provided in both subframes 2 and 3 for the purpose of comparison with the 8 LSBs of the IODC term in subframe 1. Whenever these three terms do not match, a data set cutover has occurred and new data must be collected. The timing of the IODE and constraints on the IODC and IODE are defined in paragraph 20.3.4.4. Any change in the subframe 2 and 3 data will be accomplished with a simultaneous change in both IODE words. The CS shall assure that the toe value, for at least the first data set transmitted by an SV after an upload, is different from that transmitted prior to the cutover (reference paragraph 20.3.4.5). A "fit interval" flag is provided in subframe 2 to indicate whether the ephemerides are based on a four-hour fit interval or a fit interval greater than four hours (reference paragraph 20.3.3.4.3.1). The AODO word is provided in subframe 2 to enable the user to determine the validity time for the NMCT data provided in subframe 4 of the transmitting SV. The related algorithm is given in paragraph 20.3.3.4.4. IS-GPS-200D 7 Dec 2004 93 Table 20-II. Ephemeris Data Definitions M0 Mean Anomaly at Reference Time Δn Mean Motion Difference From Computed Value e Eccentricity A Square Root of the Semi-Major Axis Ω0 Longitude of Ascending Node of Orbit Plane at Weekly Epoch i0 Inclination Angle at Reference Time ω Argument of Perigee Ω • Rate of Right Ascension IDOT Rate of Inclination Angle Cuc Amplitude of the Cosine Harmonic Correction Term to the Argument of Latitude Cus Amplitude of the Sine Harmonic Correction Term to the Argument of Latitude Crc Amplitude of the Cosine Harmonic Correction Term to the Orbit Radius Crs Amplitude of the Sine Harmonic Correction Term to the Orbit Radius Cic Amplitude of the Cosine Harmonic Correction Term to the Angle of Inclination Cis Amplitude of the Sine Harmonic Correction Term to the Angle of Inclination toe Reference Time Ephemeris (reference paragraph 20.3.4.5) IODE Issue of Data (Ephemeris) IS-GPS-200D 7 Dec 2004 94 20.3.3.4.2 Subframe 2 and 3 Parameter Characteristics. For each ephemeris parameter contained in subframes 2 and 3, the number of bits, the scale factor of the LSB (which shall be the last bit received), the range, and the units shall be as specified in Table 20-III. The AODO word (which is not an ephemeris parameter) is a five-bit unsigned term with an LSB scale factor of 900, a range from 0 to 31, and units of seconds. 20.3.3.4.3 User Algorithm for Ephemeris Determination. The user shall compute the ECEF coordinates of position for the phase center of the SVs’ antennas utilizing a variation of the equations shown in Table 20-IV. Subframes 2 and 3 parameters are Keplerian in appearance; the values of these parameters, however, are produced by the CS via a least squares curve fit of the predicted ephemeris of the phase center of the SVs’ antennas (time-position quadruples; t, x, y, z expressed in ECEF coordinates). Particulars concerning the periods of the curve fit, the resultant accuracy, and the applicable coordinate system are given in the following subparagraphs. 20.3.3.4.3.1 Curve Fit Intervals. Bit 17 in word 10 of subframe 2 is a "fit interval" flag which indicates the curve- fit interval used by the CS in determining the ephemeris parameters, as follows: 0 = 4 hours, 1 = greater than 4 hours. The relationship of the curve-fit interval to transmission time and the timing of the curve-fit intervals is covered in section 20.3.4. IS-GPS-200D 7 Dec 2004 95 Table 20-III. Ephemeris Parameters Parameter No. of Bits** Scale Factor (LSB) Effective Range*** Units IODE 8 (see text) Crs 16* 2-5 meters Δn 16* 2-43 semi-circles/sec M0 32* 2-31 semi-circles Cuc 16* 2-29 radians e 32 2-33 0.03 dimensionless Cus 16* 2-29 radians A 32 2-19 meters toe 16 24 604,784 seconds Cic 16* 2-29 radians Ω0 32* 2-31 semi-circles Cis 16* 2-29 radians i0 32* 2-31 semi-circles Crc 16* 2-5 meters ω 32* 2-31 semi-circles Ω • 24* 2-43 semi-circles/sec IDOT 14* 2-43 semi-circles/sec * Parameters so indicated shall be two's complement, with the sign bit (+ or -) occupying the MSB; ** See Figure 20-1 for complete bit allocation in subframe; *** Unless otherwise indicated in this column, effective range is the maximum range attainable with indicated bit allocation and scale factor. IS-GPS-200D 7 Dec 2004 96 Table 20-IV. Elements of Coordinate Systems (sheet 1 of 2) μ = 3.986005 x 1014 meters3/sec2 WGS 84 value of the earth's gravitational constant for GPS user • Ωe = 7.2921151467 x 10-5 rad/sec WGS 84 value of the earth's rotation rate A =()2A Semi-major axis n0 = A3 μComputed mean motion (rad/sec) tk = t - toe * Time from ephemeris reference epoch n = n0 + Δn Corrected mean motion Mk = M0 + ntk Mean anomaly Mk = Ek - e sin Ek Kepler's Equation for Eccentric Anomaly (may be solved by iteration) (radians) ⎫⎬ ⎭ ⎧⎨⎩ νν =ν− k k1 k cos sin tan True Anomaly () () ( ) ⎪⎭ ⎫⎪ ⎬ ⎪⎩ ⎧⎪⎨ −− −− =− kk kk 2 1 e cos Ee / 1cos E e cos E/ 1sin Ee1 tan * t is GPS system time at time of transmission, i.e., GPS time corrected for transit time (range/speed of light). Furthermore, tk shall be the actual total time difference between the time t and the epoch time toe, and must account for beginning or end of week crossovers. That is, if tk is greater than 302,400 seconds, subtract 604,800 seconds from tk. If tk is less than -302,400 seconds, add 604,800 seconds to tk. IS-GPS-200D 7 Dec 2004 97 Table 20-IV. Elements of Coordinate Systems (sheet 2 of 2) ⎭ ⎬ ⎫ ⎩ ⎨ ⎧ ν+ ν+ = − k k1 k e cos1 cose cosE Eccentric Anomaly Φk = νk + ω Argument of Latitude δuk = cussin2Φk + cuccos2Φk Argument of Latitude Correction δrk = crssin2Φk + crccos2Φk Radius Correction δik = cissin2Φk + ciccos2Φk Inclination Correction uk = Φk + δuk Corrected Argument of Latitude rk = A(1 - e cosEk) + δrk Corrected Radius ik = i0 + δik + (IDOT) tk Corrected Inclination xk ′ = rkcosuk yk ′ = rksinuk Ωk = Ω0 + ( Ω • -Ω • e ) tk Ω • e toe Corrected longitude of ascending node. xk = xk ′cosΩk - yk ′cosiksinΩk yk = xk ′sinΩk + yk ′cosikcosΩk zk = yk ′sinik }Second Harmonic Perturbations } Earth-fixed coordinates. } Positions in orbital plane. IS-GPS-200D 7 Dec 2004 98 20.3.3.4.3.2 Parameter Sensitivity. The sensitivity of the SV's antenna phase center position to small perturbations in most ephemeris parameters is extreme. The sensitivity of position to the parameters A , Crc and Crs is about one meter/meter. The sensitivity of position to the angular parameters is on the order of 108 meters/semicircle, and to the angular rate parameters is on the order of 1012 meters/semicircle/second. Because of this extreme sensitivity to angular perturbations, the value of π used in the curve fit is given here. π is a mathematical constant, the ratio of a circle's circumference to its diameter. Here π is taken as π = 3.1415926535898. 20.3.3.4.3.3 Coordinate Systems. 20.3.3.4.3.3.1 ECEF Coordinate System. The equations given in Table 20-IV provide the SV's antenna phase center position in the WGS 84 ECEF coordinate system defined as follows: Origin* = Earth's center of mass Z-Axis** = The direction of the IERS (International Earth Rotation and Reference Systems Service) Reference Pole (IRP) X-Axis = Intersection of the IERS Reference Meridian (IRM) and the plane passing through the origin and normal to the Z-axis Y-Axis = Completes a right-handed, Earth-Centered, Earth-Fixed orthogonal coordinate system * Geometric center of the WGS 84 Ellipsoid ** Rotational axis of the WGS 84 Ellipsoid IS-GPS-200D 7 Dec 2004 99 20.3.3.4.3.3.2 Earth-Centered, Inertial (ECI) Coordinate System. In an ECI coordinate system, GPS signals propagate in straight lines at the constant speed c* (reference paragraph 20.3.4.3). A stable ECI coordinate system of convenience may be defined as being coincident with the ECEF coordinate system at a given time t0. The x, y, z coordinates in the ECEF coordinate system at some other time t can be transformed to the x′, y′, z′ coordinates in the selected ECI coordinate system of convenience by the simple** rotation: x′ = x cos(θ) – y sin(θ) y′ = x sin(θ) + y cos(θ) z′ = z where • θ = Ωe (t – t0) * The propagation speed c is constant only in a vacuum. The gravitational potential also has a small effect on the propagation speed, but may be neglected by most users. ** Neglecting effects due to polar motion, nutation, and precession which may be neglected by most users for small values of (t – t0). 20.3.3.4.3.4 Geometric Range. The user shall account for the geometric range (D) from satellite to receiver in an ECI coordinate system. D may be expressed as, →→ D = | r (tR) -R (tT)| where tT and tR are the GPS system times of transmission and reception, respectively, and where, → R (tT) = position vector of the GPS satellite in the selected ECI coordinate system at time tT, → r (tR) = position vector of the receiver in the selected ECI coordinate system at time tR. IS-GPS-200D 7 Dec 2004 100 20.3.3.4.4 NMCT Validity Time. Users desiring to take advantage of the NMCT data provided in page 13 of subframe 4 shall first examine the AODO term currently provided in subframe 2 of the NAV data from the transmitting SV. If the AODO term is 27900 seconds (i.e., binary 11111), then the NMCT currently available from the transmitting SV is invalid and shall not be used. If the AODO term is less than 27900 seconds, then the user shall compute the validity time for that NMCT (tnmct) using the ephemeris toe parameter and the AODO term from the current subframe 2 as follows: OFFSET = toe [modulo 7200] if OFFSET = 0, then tnmct = toe - AODO if OFFSET > 0, then tnmct = toe - OFFSET + 7200 -AODO Note that the foregoing computation of tnmct must account for any beginning or end of week crossovers; for example, if t* - tnmct > 302,400 then tnmct = tnmct + 604,800 if t* - tnmct < -302,400 then tnmct = tnmct - 604,800 * t is GPS system time at time of transmission. Users are advised that different SVs will transmit NMCTs with different tnmct and that the best performance will generally be obtained by applying data from the NMCT with the latest (largest) tnmct. As a result, users should compute and examine the tnmct values for all visible and available SVs in order to find and use the NMCT with the latest tnmct. If the same latest (largest) tnmct is provided by two or more visible and available SVs, then the NMCT from any SV with the latest tnmct may be selected and used; however, the estimated range deviation (ERD) value provided by the selected NMCT for the other SVs with the same tnmct shall be set to zero if those SVs are used in the positioning solution. It should be noted that the intended positioning solution accuracy improvement will not be obtained if the data from two different NMCTs are applied simultaneously or if the data from a given NMCT is applied to just a subset of the SVs used in the positioning solution (i.e., mixed mode operation results in potentially degraded solution accuracy). It should be noted that the NMCT information shall be supported by the Block IIR SV only when operating in the IIA like mode of operation including the Autonav Test mode. IS-GPS-200D 7 Dec 2004 101 20.3.3.5 Subframes 4 and 5. Both subframe 4 and 5 are subcommutated 25 times each; the 25 versions of these subframes are referred to as pages 1 through 25 of each subframe. With the possible exception of "reserved for system use" pages and explicit repeats, each page contains different specific data in words three through ten. As shown in Figure 20-1, the pages of subframe 4 utilize seven different formats, while those of subframe 5 use two. The content of words three through ten of each page is described below, followed by algorithms and material pertinent to the use of the data. 20.3.3.5.1 Content of Subframes 4 and 5. Words three through ten of each page contain six parity bits as their LSBs; in addition, two non-information bearing bits are provided as bits 23 and 24 of word ten in each page for parity computation purposes. The data contained in the remaining bits of words three through ten of the various pages in subframes 4 and 5 are described in the following subparagraphs. IS-GPS-200D 7 Dec 2004 102 A brief summary of the various data contained in each page of subframes 4 and 5 is as follows: a. Subframe 4: • Pages 1, 6, 11, 16 and 21: (reserved); • Pages 2, 3, 4, 5, 7, 8, 9 and 10: almanac data for SV 25 through 32 respectively; • Pages 12, 19, 20, 22, 23 and 24: (reserved); • Page 13: NMCT; • Pages 14 and 15: reserved for system use; • Page 17: special messages; • Page 18: ionospheric and UTC data; • Page 25: A-S flags/SV configurations for 32 SVs, plus SV health for SV 25 through 32. b. Subframe 5: • Pages 1 through 24: almanac data for SV 1 through 24; • Page 25: SV health data for SV 1 through 24, the almanac reference time, the almanac reference week number. IS-GPS-200D 7 Dec 2004 103 20.3.3.5.1.1 Data ID and SV ID. The two MSBs of word three in each page shall contain data ID. Data ID number two (denoted by binary code 01) denotes the NAV data structure of D(t) which is described in this Appendix. Future data IDs will be defined as necessary. As shown in Table 20-V, the data ID is utilized to provide one of two indications: (a) for those pages which are assigned to contain the almanac data of one specific SV, the data ID defines the data structure utilized by that SV whose almanac data are contained in that page; and (b) for all other pages, the data ID denotes the data structure of the transmitting SV. The SV ID is given by bits three through eight of word three in each page as shown in Table 20-V. Specific IDs are reserved for each page of subframes 4 and 5. The SV IDs are utilized in two different ways: (a) for those pages which contain the almanac data of a given SV, the SV ID is the same number that is assigned to the PRN code phase of that SV (reference Table 3-I), and (b) for all other pages the SV ID assigned in accordance with Table 20-V serves as the "page ID". IDs 1 through 32 are assigned to those pages which contain the almanac data of specific SVs (pages 1-24 of subframe 5 and pages 2-5 and 7-10 of subframe 4). The "0" ID (binary all zeros) is assigned to indicate a dummy SV, while IDs 51 through 63 are utilized for pages containing other than almanac data of a specific SV. The remaining IDs (33 through 50) are unassigned. Pages which carry the same SV ID (e.g., in subframe 4, pages 1, 6, 11, 16 and 21 carry an ID of 57, while pages 12 and 24 are designated by an ID of 62) may not be considered to contain identical data. The data in the pages with the same SV ID can be different. IS-GPS-200D 7 Dec 2004 104 Table 20-V. Data IDs and SV IDs in Subframes 4 and 5 Page Subframe 4 Subframe 5 Data ID SV ID* Data ID SV ID* 1 Note(2) 57 Note(1) 1 2 Note(1) 25 Note(1) 2 3 Note(1) 26 Note(1) 3 4 Note(1) 27 Note(1) 4 5 Note(1) 28 Note(1) 5 6 Note(2) 57 Note(1) 6 7 Note(1) 29 Note(1) 7 8 Note(1) 30 Note(1) 8 9 Note(1) 31 Note(1) 9 10 Note(1) 32 Note(1) 10 11 Note(2) 57 Note(1) 11 12 Note(2) 62 Note(1) 12 13 Note(2) 52 Note(1) 13 14 Note(2) 53 Note(1) 14 15 Note(2) 54 Note(1) 15 16 Note(2) 57 Note(1) 16 17 Note(2) 55 Note(1) 17 18 Note(2) 56 Note(1) 18 19 Note(2) 58 Note(3) Note(1) 19 20 Note(2) 59 Note(3) Note(1) 20 21 Note(2) 57 Note(1) 21 22 Note(2) 60 Note(3) Note(1) 22 23 Note(2) 61 Note(3) Note(1) 23 24 Note(2) 62 Note(1) 24 25 Note(2) 63 Note(2) 51 * Use "0" to indicate "dummy" SV. When using "0" to indicate dummy SV, use the data ID of the transmitting SV. Note 1: Data ID of that SV whose SV ID appears in that page. Note 2: Data ID of transmitting SV. Note 3: SV ID may vary (except for IIR/IIR-M/IIF SVs). IS-GPS-200D 7 Dec 2004 105 20.3.3.5.1.2 Almanac Data. Pages 1 through 24 of subframe 5, as well as pages 2 through 5 and 7 through 10 of subframe 4 contain the almanac data and a SV health word for up to 32 SVs (the health word is discussed in paragraph 20.3.3.5.1.3). The almanac data are a reduced-precision subset of the clock and ephemeris parameters. The data occupy all bits of words three through ten of each page except the eight MSBs of word three (data ID and SV ID), bits 17 through 24 of word five (SV health), and the 50 bits devoted to parity. The number of bits, the scale factor (LSB), the range, and the units of the almanac parameters are given in Table 20-VI. The algorithms and other material related to the use of the almanac data are given in paragraph 20.3.3.5.2. The almanac message for any dummy SVs shall contain alternating ones and zeros with valid parity. The almanac parameters shall be updated by the CS at least once every 6 days while the CS is able to upload the SVs. If the CS is unable to upload the SVs, the accuracy of the almanac parameters transmitted by the SVs will degrade over time. For Block II and IIA SVs, three sets of almanac shall be used to span at least 60 days. The first and second sets will be transmitted for up to six days each; the third set is intended to be transmitted for the remainder of the 60 days minimum, but the actual duration of transmission will depend on the individual SV's capability to retain data in memory. All three sets are based on six-day curve fits that correspond to the first six days of the transmission interval. For Block IIR/IIR-M and IIF SVs, multiple sets of almanac parameters shall be uploaded to span at least 60 days. IS-GPS-200D 7 Dec 2004 106 Table 20-VI. Almanac Parameters Parameter No. of Bits** Scale Factor (LSB) Effective Range*** Units e 16 2-21 dimensionless toa 8 212 602,112 seconds δi **** 16* 2-19 semi-circles Ω • 16* 2-38 semi-circles/sec A 24 2-11 meters Ω0 24* 2-23 semi-circles ω 24* 2-23 semi-circles M0 24* 2-23 semi-circles af0 11* 2-20 seconds af1 11* 2-38 sec/sec * Parameters so indicated shall be two's complement with the sign bit (+ or -) occupying the MSB; ** See Figure 20-1 for complete bit allocation in subframe; *** Unless otherwise indicated in this column, effective range is the maximum range attainable with indicated bit allocation and scale factor; **** Relative to i0 = 0.30 semi-circles. IS-GPS-200D 7 Dec 2004 107 20.3.3.5.1.3 SV Health. Subframes 4 and 5 contain two types of SV health data: (a) each of the 32 pages which contain the clock/ephemeris related almanac data provide an eight-bit SV health status word regarding the SV whose almanac data they carry, and (b) the 25th page of subframe 4 and of subframe 5 jointly contain six-bit health status data for up to 32 SVs. The three MSBs of the eight-bit health words indicate health of the NAV data in accordance with the code given in Table 20-VII. The six-bit words provide a one-bit summary of the NAV data's health status in the MSB position in accordance with paragraph 20.3.3.3.1.4. The five LSBs of both the eight-bit and the six-bit words provide the health status of the SV's signal components in accordance with the code given in Table 20-VIII. A special meaning is assigned, however, to the "6 ones" combination of the six-bit health words in the 25th page of subframes 4 and 5: it indicates that "the SV which has that ID is not available and there may be no data regarding that SV in that page of subframes 4 and 5 that is assigned to normally contain the almanac data of that SV" (NOTE: this special meaning applies to the 25th page of subframes 4 and 5 only). The health indication shall be given relative to the "as designed" capabilities of each SV (as designated by the configuration code -- see paragraph 20.3.3.5.1.4). Accordingly, any SV which does not have a certain capability will be indicated as "healthy" if the lack of this capability is inherent in its design or it has been configured into a mode which is normal from a user standpoint and does not require that capability. Additional SV health data are given in subframe 1. The data given in subframes 1, 4, and 5 of the other SVs may differ from that shown in subframes 4 and/or 5 since the latter may be updated at a different time. The eight-bit health status words shall occupy bits 17 through 24 of word five in those 32 pages which contain almanac data for individual SVs. The six-bit health status words shall occupy the 24 MSBs of words four through nine in page 25 of subframe 5 plus bits 19 through 24 of word 8, the 24 MSBs of word 9, and the 18 MSBs of word 10 in page 25 of subframe 4. The predicted health data will be updated at the time of upload when a new almanac has been built by the CS. The transmitted health data may not correspond to the actual health of the transmitting SV or other SVs in the constellation. IS-GPS-200D 7 Dec 2004 108 Table 20-VII. NAV Data Health Indications Bit Position in Page Indication 137 138 139 0 0 0 ALL DATA OK 0 0 1 PARITY FAILURE -- some or all parity bad 0 1 0 TLM/HOW FORMAT PROBLEM -- any departure from standard format (e.g., preamble misplaced and/or incorrect, etc.), except for incorrect Z-count, as reported in HOW 0 1 1 Z-COUNT IN HOW BAD -- any problem with Z-count value not reflecting actual code phase 1 0 0 SUBFRAMES 1, 2, 3 --one or more elements in words three through ten of one or more subframes are bad 1 0 1 SUBFRAMES 4, 5 --one or more elements in words three through ten of one or more subframes are bad 1 1 0 ALL UPLOADED DATA BAD -- one or more elements in words three through ten of any one (or more) subframes are bad 1 1 1 ALL DATA BAD -- TLM word and/or HOW and one or more elements in any one (or more) subframes are bad IS-GPS-200D 7 Dec 2004 109 Table 20-VIII. Codes for Health of SV Signal Components MSB LSB Definition 0 0 0 0 0 All Signals OK 0 0 0 0 1 All Signals Weak* 0 0 0 1 0 All Signals Dead 0 0 0 1 1 All Signals Have No Data Modulation 0 0 1 0 0 L1 P Signal Weak 0 0 1 0 1 L1 P Signal Dead 0 0 1 1 0 L1 P Signal Has No Data Modulation 0 0 1 1 1 L2 P Signal Weak 0 1 0 0 0 L2 P Signal Dead 0 1 0 0 1 L2 P Signal Has No Data Modulation 0 1 0 1 0 L1 C Signal Weak 0 1 0 1 1 L1 C Signal Dead 0 1 1 0 0 L1 C Signal Has No Data Modulation 0 1 1 0 1 L2 C Signal Weak 0 1 1 1 0 L2 C Signal Dead 0 1 1 1 1 L2 C Signal Has No Data Modulation 1 0 0 0 0 L1 & L2 P Signal Weak 1 0 0 0 1 L1 & L2 P Signal Dead 1 0 0 1 0 L1 & L2 P Signal Has No Data Modulation 1 0 0 1 1 L1 & L2 C Signal Weak 1 0 1 0 0 L1 & L2 C Signal Dead 1 0 1 0 1 L1 & L2 C Signal Has No Data Modulation 1 0 1 1 0 L1 Signal Weak* 1 0 1 1 1 L1 Signal Dead 1 1 0 0 0 L1 Signal Has No Data Modulation 1 1 0 0 1 L2 Signal Weak* 1 1 0 1 0 L2 Signal Dead 1 1 0 1 1 L2 Signal Has No Data Modulation 1 1 1 0 0 SV Is Temporarily Out (Do not use this SV during current pass)** 1 1 1 0 1 SV Will Be Temporarily Out (Use with caution)** 1 1 1 1 0 Spare 1 1 1 1 1 More Than One Combination Would Be Required To Describe Anomalies (Not including those marked with “**”) * ** 3 to 6 dB below specified power level due to reduced power output, excess phase noise, SV attitude, etc. See definition above for Health Code 11111. IS-GPS-200D 7 Dec 2004 110 20.3.3.5.1.4 Anti-Spoof (A-S) Flags and SV Configurations. Page 25 of subframe 4 shall contain a four-bit-long term for each of up to 32 SVs to indicate the A-S status and the configuration code of each SV. The MSB of each four-bit term shall be the A-S flag with a "1" indicating that A-S is ON. The three LSBs shall indicate the configuration of each SV using the following code: Code SV Configuration 001 “Block II/IIA/IIR” SV (A-S capability, plus flags for A-S and "alert" in HOW; memory capacity as described in paragraph 20.3.2). 010 “Block IIR-M” SV 011 “Block IIF” SV Additional codes will be assigned in the future, should the need arise. These four-bit terms shall occupy bits 9 through 24 of word three, the 24 MSBs of words four through seven, and the 16 MSBs of word eight, all in page 25 of subframe 4. Since the anti-spoof information is updated by the CS at the time of upload, the anti-spoof data may not correspond to the actual anti-spoof status of the transmitting SV or other SVs in the constellation. IS-GPS-200D 7 Dec 2004 111 20.3.3.5.1.5 Almanac Reference Week. Bits 17 through 24 of word three in page 25 of subframe 5 shall indicate the number of the week (WNa) to which the almanac reference time (toa) is referenced (see paragraphs 20.3.3.5.1.2 and 20.3.3.5.2.2). The WNa term consists of eight bits which shall be a modulo 256 binary representation of the GPS week number (see paragraph 6.2.4) to which the toa is referenced. Bits 9 through 16 of word three in page 25 of subframe 5 shall contain the value of toa which is referenced to this WNa. 20.3.3.5.1.6 Coordinated Universal Time (UTC) Parameters. The 24 MSBs of words six through nine plus the eight MSBs of word ten in page 18 of subframe 4 shall contain the parameters related to correlating UTC time with GPS time. The bit length, scale factors, ranges, and units of these parameters are given in Table 20-IX. The related algorithms are described in paragraph 20.3.3.5.2.4. The UTC parameters shall be updated by the CS at least once every six days while the CS is able to upload the SVs. If the CS is unable to upload the SVs, the accuracy of the UTC parameters transmitted by the SVs will degrade over time. 20.3.3.5.1.7 Ionospheric Data. The ionospheric parameters which allow the "L1 only" or "L2 only" user to utilize the ionospheric model (reference paragraph 20.3.3.5.2.5) for computation of the ionospheric delay are contained in page 18 of subframe 4. They occupy bits 9 through 24 of word three plus the 24 MSBs of words four and five. The bit lengths, scale factors, ranges, and units of these parameters are given in Table 20-X. The ionospheric data shall be updated by the CS at least once every six days while the CS is able to upload the SVs. If the CS is unable to upload the SVs, the ionospheric data transmitted by the SVs may not be accurate. IS-GPS-200D 7 Dec 2004 112 Table 20-IX. UTC Parameters Parameter No. of Bits** Scale Factor (LSB) Effective Range*** Units A0 32* 2-30 seconds A1 24* 2-50 sec/sec ∆ tLS 8* 1 seconds tot 8 212 602,112 seconds WNt 8 1 weeks WNLSF 8 1 weeks DN 8**** 1 7 days ∆ tLSF 8* 1 seconds * Parameters so indicated shall be two's complement with the sign bit (+ or -) occupying the MSB; ** See Figure 20-1 for complete bit allocation in subframe; *** Unless otherwise indicated in this column, effective range is the maximum range attainable with indicated bit allocation and scale factor; **** Right justified. IS-GPS-200D 7 Dec 2004 113 Table 20-X. Ionospheric Parameters Parameter No. of Bits** Scale Factor (LSB) Effective Range*** Units α 0 8* 2-30 seconds α 1 8* 2-27 sec/semi-circle α 2 8* 2-24 sec/(semi-circle)2 α 3 8* 2-24 sec/(semi-circle)3 β 0 8* 211 seconds β 1 8* 214 sec/semi-circle β 2 8* 216 sec/(semi-circle)2 β 3 8* 216sec/(semi-circle)3 * Parameters so indicated shall be two's complement with the sign bit (+ or -) occupying the MSB; ** See Figure 20-1 for complete bit allocation in subframe; *** Unless otherwise indicated in this column, effective range is the maximum range attainable with indicated bit allocation and scale factor. IS-GPS-200D 7 Dec 2004 114 20.3.3.5.1.8 Special Messages. Page 17 of subframe 4 shall be reserved for special messages with the specific contents at the discretion of the Operating Command. It shall accommodate the transmission of 22 eight-bit ASCII characters. The requisite 176 bits shall occupy bits 9 through 24 of word three, the 24 MSBs of words four through nine, plus the 16 MSBs of word ten. The eight MSBs of word three shall contain the data ID and SV ID, while bits 17 through 22 of word ten shall be reserved for system use. The remaining 50 bits of words three through ten are used for parity (six bits/word) and parity computation (two bits in word ten). The eight-bit ASCII characters shall be limited to the following set: Alphanumeric Character ASCII Character Code (Octal) A - Z A - Z 101 - 132 0 - 9 0 - 9 060 - 071 + + 053 --055 . (Decimal point) . 056 ' (Minute mark) ' 047 ° (Degree sign) ° 370 / / 057 Blank Space 040 : : 072 " (Second mark) " 042 IS-GPS-200D 7 Dec 2004 115 20.3.3.5.1.9 NMCT. Page 13 of subframe 4 shall contain the NMCT data appropriate to the transmitting SV. Each NMCT contains a two-bit availability indicator (AI) followed by 30 slots which may contain up to 30 valid six-bit ERD values. The layout of these 31 data items is as shown in Figure 20-1. The two-bit AI in bits 9 and 10 of word three of page 13 of subframe 4 provide the user with the following information. AI Navigation Message Correction Table Availability 00 The correction table is unencrypted and is available to both authorized and unauthorized users. 01 The correction table is encrypted and is available only to authorized users (normal mode). 10 No correction table available for either authorized or unauthorized users. 11 Reserved. Each one of the 30 six-bit ERD slots in bits 11 through 24 of word three, bits 1 through 24 of words four through nine, and bits 1 through 22 of word ten of page 13 of subframe 4 will correspond to an ERD value for a particular SV ID. There are 31 possible SV IDs that these ERD slots may correspond to, ranging from SV ID 1 to SV ID 31. SV ID 32 is not a valid SV ID for any of the slots in an NMCT. The correspondence between the 30 ERD slots and the 31 possible SV IDs depends on the SV ID of the particular transmitting SV in accordance with the following two rules: 1) the CS shall ensure via upload that no SV shall transmit an NMCT containing an ERD value which applies to its own SV ID, and 2) the CS shall ensure via upload that all ERD values shall be transmitted in ascending numerical slot order of the corresponding SV ID. To illustrate: the SV operating as SV ID 1 will transmit (in order) ERD values which correspond to SV ID 2 through SV ID 31 in ERD slots 1 through 30 respectively, while the SV operating as SV ID 31 will transmit ERD values which correspond to SV ID 1 through SV ID 30 in ERD slots 1 through 30 respectively. Each ERD value contained in an NMCT ERD slot shall be represented as a six-bit two’s complement field with the sign bit occupying the MSB and an LSB of 0.3 meters for an effective range of ±9.3 m. A binary value of “100000” shall indicate that no valid ERD for the corresponding SV ID is present in that slot. IS-GPS-200D 7 Dec 2004 116 20.3.3.5.2 Algorithms Related to Subframe 4 and 5 Data. The following algorithms shall apply when interpreting Almanac, Coordinated Universal Time, Ionospheric Model, and NMCT data in the NAV message. 20.3.3.5.2.1 Almanac. The almanac is a subset of the clock and ephemeris data, with reduced precision. The user algorithm is essentially the same as the user algorithm used for computing the precise ephemeris from the one subframe 1, 2, and 3 parameters (see Table 20-IV). The almanac content for one SV is given in Table 20-VI. A close inspection of Table 20-VI will reveal that a nominal inclination angle of 0.30 semicircles is implicit and that the parameter δ i (correction to inclination) is transmitted, as opposed to the value computed by the user. All other parameters appearing in the equations of Tables 20-IV, but not included in the content of the almanac, are set to zero for SV position determination. In these respects, the application of the Table 20-IV equations differs between the almanac and the ephemeris computations. The user is cautioned that the sensitivity to small perturbations in the parameters is even greater for the almanac than for the ephemeris, with the sensitivity of the angular rate terms over the interval of applicability on the order of 1014 meters/(semicircle/second). An indication of the URE provided by a given almanac during each of the operational intervals is as follows: Almanac Ephemeris URE (estimated by analysis) Operational Interval 1 sigma (meters) Normal 900*,† Short-term Extended 900 - 3,600* Long-term Extended 3600 - 300,000* * URE values generally tend to degrade quadratically over time. Larger errors may be encountered during eclipse seasons and whenever a propulsive event has occurred. † After the CS is unable to upload the SVs, URE values for the SVs operating in the Autonav mode tend to degrade quadratically such that the URE will approach 300,000 meters 1 sigma at 180 days. IS-GPS-200D 7 Dec 2004 117 20.3.3.5.2.2 Almanac Reference Time. Within each upload, the CS shall ensure that all toa values in subframes 4 and 5 shall be the same for a given almanac data set and shall differ for successive data sets which contain changes in almanac parameters or SV health. In addition, the Block IIR/IIR-M SVs will also ensure that, based on a valid CS upload, all toa values in subframes 4 and 5 will be the same for a given almanac data set and will differ for successive data sets which contain changes in almanac parameters. Note that cutover to a new upload may continue to indicate the same toa values in subframes 4 and 5 as prior to the cutover but the new almanac data set may contain changes in almanac parameters or SV health. Note also that cutover to a new upload may occur between the almanac pages of interest and page 25 of subframe 5 (reference paragraph 20.3.4.1), and thus there may be a temporary inconsistency between toa, in the almanac page of interest, and in word 3 of page 25 of subframe 5. The toa mismatch signifies that this WNa may not apply to the almanac of interest and that the user must not apply almanac data until the pages with identical values of toa are obtained. Normal and Short-term Extended Operations. The almanac reference time, toa, is some multiple of 212 seconds occurring approximately 70 hours after the first valid transmission time for this almanac data set (reference 20.3.4.5). The almanac is updated often enough to ensure that GPS time, t, shall differ from toa by less than 3.5 days during the transmission period. The time from epoch tk shall be computed as described in Table 20-IV, except that toe shall be replaced with toa. Long-term Extended Operations. During long-term extended operations or if the user wishes to extend the use time of the almanac beyond the time span that it is being transmitted, one must account for crossovers into time spans where these computations of tk are not valid. This may be accomplished without time ambiguity by recognizing that the almanac reference time (toa) is referenced to the almanac reference week (WNa), both of which are given in word three of page 25 of subframe 5 (see paragraph 20.3.3.5.1.5). IS-GPS-200D 7 Dec 2004 118 20.3.3.5.2.3 Almanac Time Parameters. The almanac time parameters shall consist of an 11-bit constant term (af0) and an 11-bit first order term (af1). The applicable first order polynomial, which shall provide time to within 2 microseconds of GPS time (t) during the interval of applicability, is given by t = tsv -Δtsv where t = GPS system time (seconds), tsv = effective SV PRN code phase time at message transmission time (seconds), Δtsv = SV PRN code phase time offset (seconds). The SV PRN code phase offset is given by Δtsv = af0 + af1 tk where the computation of tk is described in paragraph 20.3.3.5.2.2, and the polynomial coefficients af0 and af1 are given in the almanac. Since the periodic relativistic effect is less than 25 meters, it need not be included in the time scale used for almanac evaluation. Over the span of applicability, it is expected that the almanac time parameters will provide a statistical URE component of less than 135 meters, one sigma. This is partially due to the fact that the error caused by the truncation of af0 and af1 may be as large as 150 meters plus 50 meters/day relative to the toa reference time. During extended operations (short-term and long-term) the almanac time parameter may not provide the specified time accuracy or URE component. 20.3.3.5.2.4 Coordinated Universal Time (UTC). Page 18 of subframe 4 includes: (1) the parameters needed to relate GPS time to UTC, and (2) notice to the user regarding the scheduled future or recent past (relative to NAV message upload) value of the delta time due to leap seconds (ΔtLSF), together with the week number (WNLSF) and the day number (DN) at the end of which the leap second becomes effective. "Day one" is the first day relative to the end/start of week and the WNLSF value consists of eight bits which shall be a modulo 256 binary representation of the GPS week number (see paragraph 6.2.4) to which the DN is referenced. The user must account for the truncated nature of this parameter as well as truncation of WN, WNt, and WNLSF due to rollover of full week number (see paragraph 3.3.4(b)). The CS shall manage these parameters such that, when ΔtLS and ΔtLSF differ, the absolute value of the difference between the untruncated WN and WNLSF values shall not exceed 127. IS-GPS-200D 7 Dec 2004 119 Depending upon the relationship of the effectivity date to the user's current GPS time, the following three different UTC/GPS-time relationships exist: a. Whenever the effectivity time indicated by the WNLSF and the DN values is not in the past (relative to the user's present time), and the user's present time does not fall in the time span which starts at six hours prior to the effectivity time and ends at six hours after the effectivity time, the UTC/GPS-time relationship is given by tUTC = (tE -ΔtUTC) [modulo 86400 seconds] where tUTC is in seconds and ΔtUTC = ΔtLS + A0 + A1 (tE - tot + 604800 (WN - WNt)), seconds; tE = GPS time as estimated by the user after correcting tSV for factors described in paragraph 20.3.3.3.3 as well as for selective availability (SA) (dither) effects; ΔtLS = delta time due to leap seconds; A0 and A1 = constant and first order terms of polynomial; tot = reference time for UTC data (reference 20.3.4.5); WN = current week number (derived from subframe 1); WNt = UTC reference week number. The estimated GPS time (tE) shall be in seconds relative to end/start of week. During the normal and short-term extended operations, the reference time for UTC data, tot, is some multiple of 212 seconds occurring approximately 70 hours after the first valid transmission time for this UTC data set (reference 20.3.4.5). The reference time for UTC data (tot) shall be referenced to the start of that week whose number (WNt) is given in word eight of page 18 in subframe 4. The WNt value consists of eight bits which shall be a modulo 256 binary representation of the GPS week number (see paragraph 6.2.4) to which the tot is referenced. The user must account for the truncated nature of this parameter as well as truncation of WN, WNt, and WNLSF due to rollover of full week number (see paragraph 3.3.4(b)). The CS shall manage these parameters such that the absolute value of the difference between the untruncated WN and WNt values shall not exceed 127. IS-GPS-200D 7 Dec 2004 120 b. Whenever the user's current time falls within the time span of six hours prior to the effectivity time to six hours after the effectivity time, proper accommodation of the leap second event with a possible week number transition is provided by the following expression for UTC: tUTC = W[modulo (86400 + ΔtLSF -ΔtLS)], seconds; where W = (tE -ΔtUTC - 43200)[modulo 86400] + 43200, seconds; and the definition of ΔtUTC (as given in 20.3.3.5.2.4a above) applies throughout the transition period. Note that when a leap second is added, unconventional time values of the form 23:59:60.xxx are encountered. Some user equipment may be designed to approximate UTC by decrementing the running count of time within several seconds after the event, thereby promptly returning to a proper time indication. Whenever a leap second event is encountered, the user equipment must consistently implement carries or borrows into any year/week/day counts. c. Whenever the effectivity time of the leap second event, as indicated by the WNLSF and DN values, is in the "past" (relative to the user's current time), and the user’s current time does not fall in the time span as given above in 20.3.3.5.2.4b, the relationship previously given for tUTC in 20.3.3.5.2.4a above is valid except that the value of ΔtLSF is substituted for ΔtLS. The CS will coordinate the update of UTC parameters at a future upload so as to maintain a proper continuity of the tUTC time scale. IS-GPS-200D 7 Dec 2004 121 20.3.3.5.2.5 Ionospheric Model. The "two frequency" (L1 and L2) user shall correct the time received from the SV for ionospheric effect by utilizing the time delay differential between L1 and L2 (reference paragraph 20.3.3.3.3.3). The "one frequency" user, however, may use the model given in Figure 20-4 to make this correction. It is estimated that the use of this model will provide at least a 50 percent reduction in the single - frequency user's RMS error due to ionospheric propagation effects. During extended operations, or for the SVs in the Autonav mode if the CS is unable to upload the SVs, the use of this model will yield unpredictable results. 20.3.3.5.2.6 NMCT Data. For each SV, the ERD value in the NMCT is an estimated pseudorange error. Each ERD value is computed by the CS and represents the radial component of the satellite ephemeris error minus the speed of light times the satellite clock error. The satellite ephemeris and clock errors are computed by subtracting the broadcast from current estimates. Therefore, the ERD value may be used as follows to correct the user's measured pseudorange: PRc = PR – ERD where, PRc = pseudorange corrected with the ERD value from the NMCT PR = measured pseudorange Note that as described above, the ERD values are actually error estimates rather than differential corrections and so are subtracted rather than added in the above equation. IS-GPS-200D 7 Dec 2004 122 The ionospheric correction model is given by 24 xx x 1.57 Tiono2 24 (sec) F 5.0 10 9 (AMP)1 , F 5.0 109 , x 1.57 ⎫ ⎪⎪⎬⎪⎪⎭ < ≥ where ⎤ ⎥ ⎥⎦ Tisreferredtothe L1 frequency; if the user is operating on the L2 frequency, the correction termmust iono ⎞⎟⎟⎠ be multiplied by γ (reference paragraph 20.3.3.3.3.2), + ⎫⎪⎬⎪⎭ − ⎛⎜⎜⎝ = ⎫ ⎪⎬⎪⎭ 3 n αφ AMP0, = nm ≥ (sec)AMP0n ≥ + ) ifAMP0AMP0, − − < ) ∗ ∗ < ⎡ ⎢ ⎢⎣ ( ∗ ∗ Σ= Σ= 2 t -50400 ⎪ ⎨ ⎧⎪⎨⎪⎩ ( ⎧ ⎪ ⎪⎪⎩ (radians) x ⎧⎪⎨⎪⎩ = PER = = = π 3 n βφ , PER 72,000 nm PER n 0 (sec) if PER 72,000, PER 72,000 F = 1.0 + 16.0 [0.53 - E]3 and αn and βn are the satellite transmitted data words with n = 0, 1, 2, and 3. Figure 20-4. Ionospheric Model (Sheet 1 of 3) IS-GPS-200D 7 Dec 2004 123 Other equations that must be solved are φm = φi + 0.064cos(λi - 1.617) (semi-circles) ψ sinA λi =λu + (semi-circles) cos φ i ⎧ φ+ψ cosA, φ ≤ 0.416 ⎫ u i ⎪ ⎪ φi=⎨ if φi >+0.416, then φi =+0.416⎬ (semi-circles) ⎪⎪ if φ <−0.416, then φ =−0.416 ii ⎩⎭ 0.0137 ψ=-0.022 (semi-circles) E + 0.11 t = 4.32 (104) λi + GPS time (sec) where 0 ≤ t < 86400: therefore, if t ≥ 86400 seconds, subtract 86400 seconds; if t < 0 seconds, add 86400 seconds. Figure 20-4. Ionospheric Model (Sheet 2 of 3) IS-GPS-200D 7 Dec 2004 124 The terms used in computation of ionospheric delay are as follows: • Satellite Transmitted Terms αn -the coefficients of a cubic equation representing the amplitude of the vertical delay (4 coefficients - 8 bits each) βn -the coefficients of a cubic equation representing the period of the model (4 coefficients - 8 bits each) • Receiver Generated Terms E -elevation angle between the user and satellite (semi-circles) A -azimuth angle between the user and satellite, measured clockwise positive from the true North (semi-circles) φu -user geodetic latitude (semi-circles) WGS-84 λu -user geodetic longitude (semi-circles) WGS-84 GPS time -receiver computed system time • Computed Terms X -phase (radians) F -obliquity factor (dimensionless) t -local time (sec) φm -geomagnetic latitude of the earth projection of the ionospheric intersection point (mean ionospheric height assumed 350 km) (semi-circles) λi -geodetic longitude of the earth projection of the ionospheric intersection point (semi-circles) φi -geodetic latitude of the earth projection of the ionospheric intersection point (semi-circles) ψ -earth's central angle between the user position and the earth projection of ionospheric intersection point (semi-circles) Figure 20-4. Ionospheric Model (Sheet 3 of 3) IS-GPS-200D 7 Dec 2004 125 20.3.4 Timing Relationships. The following conventions shall apply. 20.3.4.1 Paging and Cutovers. At end/start of week (a) the cyclic paging of subframes 1 through 5 shall restart with subframe 1 regardless of which subframe was last transmitted prior to end/start of week, and (b) the cycling of the 25 pages of subframes 4 and 5 shall restart with page 1 of each of the subframes, regardless of which page was the last to be transmitted prior to the end/start of week. Cutovers to newly updated data for subframes 1, 2, and 3 occur on frame boundaries (i.e., modulo 30 seconds relative to end/start of week). Newly updated data for subframes 4 and 5 may start to be transmitted with any of the 25 pages of these subframes. 20.3.4.2 SV Time vs. GPS Time. In controlling the SVs and uploading of data, the CS shall allow for the following timing relationships: a. Each SV operates on its own SV time; b. All time-related data in the TLM word and the HOW shall be in SV-time; c. All other data in the NAV message shall be relative to GPS time; d. The acts of transmitting the NAV message shall be executed by the SV on SV time. 20.3.4.3 Speed of Light. The speed of light used by the CS for generating the data described in the above paragraphs is c = 2.99792458 x 108 meters per second which is the official WGS-84 speed of light. The user shall use the same value for the speed of light in all computations. IS-GPS-200D 7 Dec 2004 126 20.3.4.4 Data Sets. The IODE is an 8 bit number equal to the 8 LSBs of the 10 bit IODC of the same data set. The following rules govern the transmission of IODC and IODE values in different data sets: (1) The transmitted IODC will be different from any value transmitted by the SV during the preceding seven days; (2) The transmitted IODE will be different from any value transmitted by the SV during the preceding six hours. The range of IODC will be as given in Table 20-XI for Block II/IIA SVs and Table 20-XII for Block IIR/IIR-M/IIF SVs. Cutovers to new data sets will occur only on hour boundaries except for the first data set of a new upload. The first data set may be cut-in (reference paragraph 20.3.4.1) at any time during the hour and therefore may be transmitted by the SV for less than one hour. During short-term operations, cutover to 4-hour sets and subsequent cutovers to succeeding 4-hour data sets will always occur modulo 4 hours relative to end/start of week. Cutover from 4-hour data sets to 6-hour data sets shall occur modulo 12 hours relative to end/start of week. Cutover from 12-hour data sets to 24-hour data sets shall occur modulo 24 hours relative to end/start of week. Cutover from a data set transmitted 24 hours or more occurs on a modulo 24-hour boundary relative to end/start of week. The start of the transmission interval for each data set corresponds to the beginning of the curve fit interval for the data set. Each data set remains valid for the duration of its curve fit interval. Normal Operations. The subframe 1, 2, and 3 data sets are transmitted by the SV for periods of two hours. The corresponding curve fit interval is four hours. SVs operating in the Autonav mode will deviate. They will transmit subframe 1, 2, and 3 data sets for periods of one hour. The corresponding curve-fit interval will be four hours. Short-term and Long-term Extended Operations. The transmission intervals and curve fit intervals with the applicable IODC ranges are given in Tables 20-XI and 20-XII. IS-GPS-200D 7 Dec 2004 127 Table 20-XI. IODC Values and Data Set Lengths (Block II/IIA) Days Spanned Transmission Interval (hours) (Note 4) Curve Fit Interval (hours) IODC Range (Note 1) 1 2 4 (Note 2) 2-14 4 6 (Note 2) 15-16 6 8 240-247 17-20 12 14 248-255, 496 (Note 3) 21-27 24 26 497-503 28-41 48 50 504-510 42-59 72 74 511, 752-756 60-63 96 98 757 Note 1: For transmission intervals of 6 hours or greater, the IODC values shown will be transmitted in increasing order. Note 2: IODC values for blocks with 2- or 4-hour transmission intervals (at least the first 14 days after upload) shall be any numbers in the range 0 to 1023 excluding those values of IODC that correspond to IODE values in the range 240-255, subject to the constraints on re-transmission given in paragraph 20.3.4.4. Note 3: The ninth 12-hour data set may not be transmitted. Note 4: The first data set of a new upload may be cut-in at any time and therefore the transmission interval may be less than the specified value. IS-GPS-200D 7 Dec 2004 128 Table 20-XII. IODC Values and Data Set Lengths (Block IIR/IIR-M/IIF) Days Spanned Transmission Interval (hours) (Note 5) Curve Fit Interval (hours) IODC Range 1 2 (Note 4) 4 (Note 2) 2-14 4 6 (Note 2) 15-16 6 8 240-247 (Note 1) 17-20 12 14 248-255, 496 (Note 1) (Note 3) 21-6224 26 497-503, 1021-1023 Note 1: For transmission intervals of 6 and 12 hours, the IODC values shown will be transmitted in increasing order. Note 2: IODC values for blocks with 1-, 2- or 4-hour transmission intervals (at least the first 14 days after upload) shall be any numbers in the range 0 to 1023 excluding those values of IODC that correspond to IODE values in the range 240-255, subject to the constraints on re-transmission given in paragraph 20.3.4.4. Note 3: The ninth 12-hour data set may not be transmitted. Note 4: SVs operating in the Autonav mode will have transmission intervals of 1 hour per paragraph 20.3.4.4. Note 5: The first data set of a new upload may be cut-in at any time and therefore the transmission interval may be less than the specified value. IS-GPS-200D 7 Dec 2004 129 20.3.4.5 Reference Times. Many of the parameters which describe the SV state vary with true time, and must therefore be expressed as time functions with coefficients provided by the Navigation Message to be evaluated by the user equipment. These include the following parameters as functions of GPS time: a. SV time, b. Mean anomaly, c. Longitude of ascending node, d. UTC, e. Inclination. Each of these parameters is formulated as a polynomial in time. The specific time scale of expansion can be arbitrary. Due to the short data field lengths available in the Navigation Message format, the nominal epoch of the polynomial is chosen near the midpoint of the expansion range so that quantization error is small. This results in time epoch values which can be different for each data set. Time epochs contained in the Navigation Message and the different algorithms which utilize them are related as follows: Epoch Application Algorithm Reference toc 20.3.3.3.3.1 toe 20.3.3.4.3 toa 20.3.3.5.2.2 and 20.3.3.5.2.3 tot 20.3.3.5.2.4 Table 20-XIII describes the nominal selection which will be expressed modulo 604,800 seconds in the Navigation Message. IS-GPS-200D 7 Dec 2004 130 The coefficients of expansion are obviously dependent upon choice of epoch, and thus the epoch time and expansion coefficients must be treated as an inseparable parameter set. Note that a user applying current navigation data will normally be working with negative values of (t-toc) and (t-toe) in evaluating the expansions. The CS shall assure that the toe value, for at least the first data set transmitted by an SV after a new upload, is different from that transmitted prior to the cutover (see paragraph 20.3.4.4). As such, when a new upload is cutover for transmission, the CS shall introduce a small deviation in the toe resulting in the toe value that is offset from the hour boundaries (see Table 20-XIII). This offset toe will be transmitted by an SV in the first data set after a new upload cutover and the second data set, following the first data set, may also continue to reflect the same offset in the toe. When the toe, immediately prior to a new upload cutover, already reflects a small deviation (i.e. a new upload cutover has occurred in the recent past), then the CS shall introduce an additional deviation to the toe when a new upload is cutover for transmission. A change from the broadcast reference time immediately prior to cutover is used to indicate a change of values in the data set. The user may use the following example algorithm to detect the occurrence of a new upload cutover: DEV = toe [modulo 3600] If DEV ≠ 0, then a new upload cutover has occurred within past 4 hours. IS-GPS-200D 7 Dec 2004 131 Table 20-XIII. Reference Times Fit Interval (hours) Transmission Interval (hours) Hours After First Valid Transmission Time toc (clock) toe (ephemeris) toa (almanac) tot (UTC) 4 2* 2 2 6 4 3 3 8 6 4 4 14 12 7 7 26 24 13 13 50 48 25 25 74 72 37 37 98 96 49 49 122 120 61 61 146 144 73 73 144 (6 days) 144 70 70 > 144 (6 days) > 144 70 70 * Some SVs will have transmission intervals of 1 hour per paragraph 20.3.4.4. IS-GPS-200D 7 Dec 2004 132 20.3.5 Data Frame Parity. The data signal shall contain parity coding according to the following conventions. 20.3.5.1 SV/CS Parity Algorithm. This algorithm links 30-bit words within and across subframes of ten words using the (32,26) Hamming Code described in Table 20-XIV. 20.3.5.2 User Parity Algorithm. As far as the user is concerned, several options are available for performing data decoding and error detection. Figure 20-5 presents an example flow chart that defines one way of recovering data (dn) and checking parity. The parity bit D30* is used for recovering raw data. The parity bits D29* and D30*, along with the recovered raw data (dn) are modulo-2 added in accordance with the equations appearing in Table 20-XIV for D25 . . . D30, which provide parity to compare with transmitted parity D25 . . . D30. IS-GPS-200D 7 Dec 2004 133 Table 20-XIV. Parity Encoding Equations D1 D2 D3 • • • • D24 D25 D26 D27 D28 D29 D30 Where = d1 ⊕ D30  = d2 ⊕ D30  = d3 ⊕ D30  • • • • = d24 ⊕ D30  = D29  ⊕ d1 ⊕ d2 ⊕ d3 ⊕ d5 ⊕ d6 ⊕ d10 ⊕ d11 ⊕ d12 ⊕ d13 ⊕ d14 ⊕ d17 ⊕ d18 ⊕ d20 ⊕ d23 = D30  ⊕ d2 ⊕ d3 ⊕ d4 ⊕ d6 ⊕ d7 ⊕ d11 ⊕ d12 ⊕ d13 ⊕ d14 ⊕ d15 ⊕ d18 ⊕ d19 ⊕ d21 ⊕ d24 = D29  ⊕ d1 ⊕ d3 ⊕ d4 ⊕ d5 ⊕ d7 ⊕ d8 ⊕ d12 ⊕ d13 ⊕ d14 ⊕ d15 ⊕ d16⊕ d19 ⊕ d20 ⊕ d22 = D30  ⊕ d2 ⊕ d4 ⊕ d5 ⊕ d6 ⊕ d8 ⊕ d9 ⊕ d13 ⊕ d14 ⊕ d15 ⊕ d16 ⊕ d17 ⊕ d20 ⊕ d21 ⊕ d23 = D30  ⊕ d1⊕ d3⊕ d5⊕ d6⊕ d7⊕ d9 ⊕ d10 ⊕ d14 ⊕ d15 ⊕ d16 ⊕ d17 ⊕ d18 ⊕ d21 ⊕ d22⊕ d24 = D29  ⊕ d3 ⊕ d5 ⊕ d6 ⊕ d8 ⊕ d9 ⊕ d10 ⊕ d11 ⊕ d13 ⊕ d15 ⊕ d19 ⊕ d22 ⊕ d23 ⊕ d24 d1, d2, ..., d24 are the source data bits; the symbol 􀂐 is used to identify the last 2 bits of the previous word of the subframe; D25, D26, ..., D30 are the computed parity bits; D1, D2, ..., D29, D30 are the bits transmitted by the SV; ⊕ is the "modulo-2" or "exclusive-or" operation. IS-GPS-200D 7 Dec 2004 134 ENTER IS D30 = 1?* COMPLEMENT D1 . . . D24 TO OBTAIN d1 . . . d24 DO NOT COMPLEMENT D1 . . . D24 TO OBTAIN d1 . . . d24 YES NO IS D30 = 1?* COMPLEMENT D1 . . . D24 TO OBTAIN d1 . . . d24 DO NOT COMPLEMENT D1 . . . D24 TO OBTAIN d1 . . . d24 YES NO FAIL EXIT PARITY CHECK PASSES PASS EXIT PARITY CHECK FAILS SUBSTITUTE d1 . . . d24, D29 & D30 INTO PARITY EQUATIONS (TABLE 20-XIV) * * ARE COMPUTED D25 . . . D30 NO YES EQUAL TO CORRESPONDING RECEIVED D25 . . . D30? Figure 20-5. Example Flow Chart for User Implementation of Parity Algorithm IS-GPS-200D 7 Dec 2004 135 (This page intentionally left blank.) IS-GPS-200D 7 Dec 2004 136 30. APPENDIX III. GPS NAVIGATION DATA STRUCTURE FOR CNAV DATA, DC(t) 30.1 Scope. This appendix describes the specific GPS CNAV data structure denoted as DC(t). 30.2 Applicable Documents. 30.2.1 Government Documents. In addition to the documents listed in paragraph 2.1, the following documents of the issue specified contribute to the definition of the CNAV data related interfaces and form a part of this Appendix to the extent specified herein. Specifications None Standards None Other Publications None 30.2.2 Non-Government Documents. In addition to the documents listed in paragraph 2.2, the following documents of the issue specified contribute to the definition of the CNAV data related interfaces and form a part of this Appendix to the extent specified herein. Specifications None Other Publications None IS-GPS-200D 7 Dec 2004 137 (This page intentionally left blank.) IS-GPS-200D 7 Dec 2004 138 30.3 Requirements. 30.3.1 Data Characteristics. The CNAV data, DC(t), is a higher precision representation and nominally contains more accurate data than the NAV data, D(t), described in Appendix II. Also, the CNAV data stream uses a different parity algorithm. Users are advised that the CNAV data, DC(t), described in this appendix and the NAV data, D(t), described in Appendix II, should not be mixed in any user algorithms or applications. Each of the two data sets should be treated as a set and used accordingly. 30.3.2 Message Structure. As shown in Figures 30-1 through 30-14, the CNAV message structure utilizes a basic format of twelve-second 300-bit long messages. Each message contains a Cyclic Redundancy Check (CRC) parity block consisting of 24 bits covering the entire twelve-second message (300 bits) (reference Section 30.3.5). Message type 0 (zero) is defined to be the default message. In the event of message generation failure, the SV shall replace each affected message type with the default message type. In the event that a particular message is not assigned (by the CS) a particular message type for broadcast, the SV shall generate and broadcast the default message type in that message slot. Currently undefined and unused message types are reserved for future use. 30.3.3 Message Content. Each message starts with an 8-bit preamble – 10001011, followed by a 6-bit PRN number of the transmitting SV, a 6-bit message type ID with a range of 0 (000000) to 63 (111111), and the 17-bit message time of week (TOW) count. When the value of the message TOW count is multiplied by 6, it represents SV time in seconds at the start of the next 12-second message. An “alert” flag, when raised (bit 38 = “1”), indicates to the user that the SV URA and/or the SV User Differential Range Accuracy (UDRA) may be worse than indicated in the respective message types, and the SV should be used at the user’s own risk. For each default message (Message Type 0), bits 39 through 276 shall be alternating ones and zeros and the message shall contain a proper CRC parity block. IS-GPS-200D 7 Dec 2004 139 (This page intentionally left blank.) IS-GPS-200D 7 Dec 2004 140 DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS 8 BITS MESSAGE TYPE ID 6 BITS PREAMBLE PRN 6 BITS MESSAGE TOW COUNT* 17 BITS "ALERT" FLAG - 1 BIT 9 2115 39 L5 HEALTH - 1 BIT 71 URAoe INDEX 55 66 DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS 1 38 52 toe 11 BITS L2 HEALTH - 1 BIT L1 HEALTH - 1 BIT 5 BITS top 11 BITS 82 19 MSBs ΔA 13BITS WNn 133 150 173 108 25 BITS 23 BITS M0-n 28 MSBs 17 BITS A • Δ n0 • Δ n0 7 LSBs ΔA MSB FIRST DIRECTION OF DATA FLOW FROM SV 100 BITS 4 SECONDS 277 272 206 239 CRC 24 BITS 33 BITS en ωn 33 BITS M0-n 5 LSBs RESERVED – 5 BITs * MESSAGE TOW COUNT = 17 MSB OF ACTUAL TOW COUNT AT START OF NEXT 12-SECOND MESSAGE Figure 30-1. Message Type 10 - Ephemeris 1 IS-GPS-200D 7 Dec 2004 141 8 BITS MESSAGE TYPE ID 6 BITS PREAMBLE PRN 6 BITS MESSAGE TOW COUNT* 17 BITS "ALERT" FLAG - 1 BIT 9 2115 39 DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS 1 38 50 toe 11 BITS 83 18 MSBs 33 BITS Ω0-n i0-n 8 BITS MESSAGE TYPE ID 6 BITS PREAMBLE PRN 6 BITS MESSAGE TOW COUNT* 17 BITS "ALERT" FLAG - 1 BIT 9 2115 39 DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS 1 38 50 toe 11 BITS 83 18 MSBs 33 BITS Ω0-n i0-n DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS 116 133 148 164 180 17 BITS 16 BITS crs-n 21 MSBs 15 BITS ΔΩ• i0-n - DOT cis-n 15 LSBs i0-n 16 BITS cic-n * MESSAGE TOW COUNT = 17 MSB OF ACTUAL TOW COUNT AT START OF NEXT 12-SECOND MESSAGE Figure 30-2. Message Type 11 - Ephemeris 2 IS-GPS-200D 7 Dec 2004 142 204 270 CRC 24 BITS 277 RESERVED 7 BITS 228 201 24 BITS crc-n crs-n - 3 LSBs DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS 249 21 BITS cus-n 21 BITS cuc-n 8 BITS MESSAGE TYPE ID 6 BITS PREAMBLE PRN 6 BITS MESSAGE TOW COUNT* 17 BITS "ALERT" FLAG - 1 BIT 9 2115 39 DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS 1 38 50 top 11 BITS 98 11 BITS toc af1-n – 3 MSBs 5 BITS 26 BITS af0-n 55 58 72 URAoc2 INDEX - 3 BITS 61 URAoc1 INDEX -3 BITS URAoc INDEX 8 BITS MESSAGE TYPE ID 6 BITS PREAMBLE PRN 6 BITS MESSAGE TOW COUNT* 17 BITS "ALERT" FLAG - 1 BIT 9 2115 39 DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS 1 38 50 top 11 BITS 98 11 BITS toc af1-n – 3 MSBs 5 BITS 26 BITS af0-n 55 58 72 URAoc2 INDEX - 3 BITS 61 URAoc1 INDEX -3 BITS URAoc INDEX DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS 128 141 154 167 180 193 118 10 BITS 13 BITS ISCL5Q5 13 BITS 13 BITS af2-n ISCL1C/A ISCL2C 17 LSBs af1-n 13 BITS ISCL5I5 α0 8BITS 13 BITS TGD MSB FIRST DIRECTION OF DATA FLOW FROM SV 100 BITS 4 SECONDS 277 257 209 217 225 233 241 249 CRC 24 BITS RESERVED 20 BITS 8 BITS α1 8 BITs α2 8 BITS α3 8 BITS β0 8 BITS β1 8 BITS β2 8 BITS β3 * MESSAGE TOW COUNT = 17 MSB OF ACTUAL TOW COUNT AT START OF NEXT 12-SECOND MESSAGE Figure 30-3. Message Type 30 -Clock, IONO & Group Delay IS-GPS-200D 7 Dec 2004 143 DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS 8 BITS MESSAGE TYPE ID 6 BITS PREAMBLE PRN 6 BITS MESSAGE TOW COUNT* 17 BITS "ALERT" FLAG - 1 BIT 9 2115 39 DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS 1 38 50 top 11 BITS 98 11 BITS toc af1-n – 3 MSBs 5 BITS 26 BITS af0-n 55 58 72 URAoc2 INDEX - 3 BITS 61 URAoc1 INDEX -3 BITS URAoc INDEX DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS 8 BITS MESSAGE TYPE ID 6 BITS PREAMBLE PRN 6 BITS MESSAGE TOW COUNT* 17 BITS "ALERT" FLAG - 1 BIT 9 2115 39 DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS 1 38 50 top 11 BITS 98 11 BITS toc af1-n – 3 MSBs 5 BITS 26 BITS af0-n 55 58 72 URAoc2 INDEX - 3 BITS 61 URAoc1 INDEX -3 BITS URAoc INDEX 128 141 149 180 118 10 BITS 31 BITS Reduced Almanac Packet 2 21 MSBs 8 BITS af2-n toa Reduced Almanac Packet 1 17 LSBs af1-n 13 BITS WNa-n MSB FIRST DIRECTION OF DATA FLOW FROM SV 100 BITS 4 SECONDS 277 211 273 242 CRC 24 BITS 4 BITS 10 LSBs 31 BITS Reduced Almanac Packet 3 31 BITS Reduced Almanac Packet 4 Reduced Almanac Packet 2 RESERVED * MESSAGE TOW COUNT = 17 MSB OF ACTUAL TOW COUNT AT START OF NEXT 12-SECOND MESSAGE Figure 30-4. Message Type 31 - Clock & Reduced Almanac IS-GPS-200D 7 Dec 2004 144 8 BITS MESSAGE TYPE ID 6 BITS PREAMBLE PRN 6 BITS MESSAGE TOW COUNT* 17 BITS "ALERT" FLAG - 1 BIT 9 2115 39 DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS 1 38 50 top 11 BITS 98 11 BITS toc af1-n – 3 MSBs 5 BITS 26 BITS af0-n 55 58 72 URAoc2 INDEX - 3 BITS 61 URAoc1 INDEX -3 BITS URAoc INDEX 8 BITS MESSAGE TYPE ID 6 BITS PREAMBLE PRN 6 BITS MESSAGE TOW COUNT* 17 BITS "ALERT" FLAG - 1 BIT 9 2115 39 DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS 1 38 50 top 11 BITS 98 11 BITS toc af1-n – 3 MSBs 5 BITS 26 BITS af0-n 55 58 72 URAoc2 INDEX - 3 BITS 61 URAoc1 INDEX -3 BITS URAoc INDEX DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS 144 128 165 180 118 10 BITS 21 BITS 15 BITS af2-n PM-X PM-Y 17 LSBs af1-n 21 BITS PM-X • 16 BITS tEOP MSB FIRST DIRECTION OF DATA FLOW FROM SV 100 BITS 4 SECONDS 277 266 247 216 PM-Y 15 BITS CRC 24 BITS RESERVED 11 BITS 31 BITS ΔUT1 19 BITS • ΔUT1 • * MESSAGE TOW COUNT = 17 MSB OF ACTUAL TOW COUNT AT START OF NEXT 12-SECOND MESSAGE Figure 30-5. Message Type 32 - Clock & EOP IS-GPS-200D 7 Dec 2004 145 8 BITS MESSAGE TYPE ID 6 BITS PREAMBLE PRN 6 BITS MESSAGE TOW COUNT* 17 BITS "ALERT" FLAG - 1 BIT 9 2115 39 DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS 1 38 50 top 11 BITS 98 11 BITS toc af1-n – 3 MSBs 5 BITS 26 BITS af0-n 55 58 72 URAoc2 INDEX - 3 BITS 61 URAoc1 INDEX -3 BITS URAoc INDEX 8 BITS MESSAGE TYPE ID 6 BITS PREAMBLE PRN 6 BITS MESSAGE TOW COUNT* 17 BITS "ALERT" FLAG - 1 BIT 9 2115 39 DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS 1 38 50 top 11 BITS 98 11 BITS toc af1-n – 3 MSBs 5 BITS 26 BITS af0-n 55 58 72 URAoc2 INDEX - 3 BITS 61 URAoc1 INDEX -3 BITS URAoc INDEX DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS 128 164 118 144 157 172 188 10 BITS 16 BITS WNot 13 BITS 8 BITS af2-n ΔtLS tot 17 LSBs af1-n 16 BITS A0-n 7BITS A2-n 13 BITS A1-n MSB FIRST DIRECTION OF DATA FLOW FROM SV 100 BITS 4 SECONDS 277 226 214 218 CRC 24 BITS RESERVED 51 BITS 13 BITS WNLSF 8 BITS ΔtLSF 4 BITS DN * MESSAGE TOW COUNT = 17 MSB OF ACTUAL TOW COUNT AT START OF NEXT 12-SECOND MESSAGE Figure 30-6. Message Type 33 - Clock & UTC IS-GPS-200D 7 Dec 2004 146 MSB FIRST DIRECTION OF DATA FLOW FROM SV 100 BITS 4 SECONDS 8 BITS 6 BITS PRN 6 BITS MESSAGE TOW COUNT* 17 BITS 9 2115 391 38 50 top 11 BITS 98 11 BITS toc 5 BITS 26 BITS af0-n 55 58 7261 URAoc2 INDEX - 3 BITS MESSAGE TYPE ID URAoc1 INDEX -3 BITS PREAMBLE "ALERT" FLAG - 1 BIT URAoc INDEX af1-n – 3 MSBs DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS 150 139 10 BITS EDC 16 MSBs 101 af2-n 128 17 LSBs 118 af1-n 185 11 BITS top-D 34 BITS CDC 151 11 BITS tOD DC DATA TYPE – 1 BIT DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS 277 CRC 24 BITS 76 LSBs EDC * MESSAGE TOW COUNT = 17 MSB OF ACTUAL TOW COUNT AT START OF NEXT 12-SECOND MESSAGE CDC = Clock Differential Correction EDC = Ephemeris Differential Correction Figure 30-7. Message Type 34 - Clock & Differential Correction IS-GPS-200D 7 Dec 2004 147 DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS 8 BITS MESSAGE TYPE ID 6 BITS PREAMBLE PRN 6 BITS MESSAGE TOW COUNT* 17 BITS "ALERT" FLAG - 1 BIT 9 2115 39 DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS 1 38 50 top 11 BITS 98 11 BITS toc af1-n – 3 MSBs 5 BITS 26 BITS af0-n 55 58 72 URAoc2 INDEX - 3 BITS 61 URAoc1 INDEX -3 BITS URAoc INDEX DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS 8 BITS MESSAGE TYPE ID 6 BITS PREAMBLE PRN 6 BITS MESSAGE TOW COUNT* 17 BITS "ALERT" FLAG - 1 BIT 9 2115 39 DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS 1 38 50 top 11 BITS 98 11 BITS toc af1-n – 3 MSBs 5 BITS 26 BITS af0-n 55 58 72 URAoc2 INDEX - 3 BITS 61 URAoc1 INDEX -3 BITS URAoc INDEX 128 118 187 142 155 158 174 194 10 BITS 13 BITS 7BITS 7 BITS af2-n A2GGTO A1GGTO 17 LSBs af1-n 14 BITS t0GGTO 16 BITS A0GGTO 13 BITS WNGGTO RESERVED GNSS ID – 3 BITS CRC 24 BITS 277 RESERVED 76 BITS 201 DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS * MESSAGE TOW COUNT = 17 MSB OF ACTUAL TOW COUNT AT START OF NEXT 12-SECOND MESSAGE Figure 30-8. Message Type 35 - Clock & GGTO IS-GPS-200D 7 Dec 2004 148 MSB FIRST DIRECTION OF DATA FLOW FROM SV 100 BITS 4 SECONDS 8 BITS MESSAGE TYPE ID 6 BITS PREAMBLE PRN 6 BITS MESSAGE TOW COUNT* 17 BITS "ALERT" FLAG - 1 BIT 9 2115 391 38 50 top 11 BITS 98 11 BITS toc af1-n – 3 MSBs 5 BITS 26 BITS af0-n 55 58 72 URAoc2 INDEX - 3 BITS 61 URAoc1 INDEX -3 BITS URAoc INDEX DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS 128 118 10 BITS af2-n 17 LSBs af1-n 73 MSBs TEXT MESSAGE (18 8-BIT CHAR) MSB FIRST DIRECTION OF DATA FLOW FROM SV 100 BITS 4 SECONDS 276 CRC 24 BITS 277 RESERVED – 1 BIT 201 71 LSBs TEXT PAGE TEXT MESSAGE (18 8-BIT CHAR) 272 4 BITS * MESSAGE TOW COUNT = 17 MSB OF ACTUAL TOW COUNT AT START OF NEXT 12-SECOND MESSAGE Figure 30-9. Message Type 36 - Clock & Text IS-GPS-200D 7 Dec 2004 149 DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS 8 BITS MESSAGE TYPE ID 6 BITS PREAMBLE PRN 6 BITS MESSAGE TOW COUNT* 17 BITS "ALERT" FLAG - 1 BIT 9 2115 39 DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS 1 38 50 top 11 BITS 98 11 BITS toc af1-n – 3 MSBs 5 BITS 26 BITS af0-n 55 58 72 URAoc2 INDEX - 3 BITS 61 URAoc1 INDEX -3 BITS URAoc INDEX DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS 8 BITS MESSAGE TYPE ID 6 BITS PREAMBLE PRN 6 BITS MESSAGE TOW COUNT* 17 BITS "ALERT" FLAG - 1 BIT 9 2115 39 DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS 1 38 50 top 11 BITS 98 11 BITS toc af1-n – 3 MSBs 5 BITS 26 BITS af0-n 55 58 72 URAoc2 INDEX - 3 BITS 61 URAoc1 INDEX -3 BITS URAoc INDEX 155 158 149 128 169 118 141 180 191 10 BITS 10 MSBs 11 BITS af2-n δi 17 LSBs DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS af1-n 11 BITS e 8 BITS WNa-n toa 13 BITS A 11 BITS Ω•L5 HEALTH – 1 BIT L2 HEALTH – 1 BIT L1 HEALTH – 1 BIT 6BITS PRNa 277 240 256 267 224 208 CRC 24 BITS 11 BITS 16 BITS Ω0 16 BITS ω7 LSBs M0A 16 BITS af0 10 BITS af1 * MESSAGE TOW COUNT = 17 MSB OF ACTUAL TOW COUNT AT START OF NEXT 12-SECOND MESSAGE Figure 30-10. Message Type 37 - Clock & Midi Almanac IS-GPS-200D 7 Dec 2004 150 8 BITS MESSAGE TYPE ID 6 BITS PREAMBLE PRN 6 BITS MESSAGE TOW COUNT* 17 BITS "ALERT" FLAG - 1 BIT 9 2115 39 DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS 1 38 WNa-n 13 BITS 91 10 MSBs 8 BITS toa 31 BITS Reduced Almanac Packet 1 6052 Reduced Almanac Packet 2 8 BITS MESSAGE TYPE ID 6 BITS PREAMBLE PRN 6 BITS MESSAGE TOW COUNT* 17 BITS "ALERT" FLAG - 1 BIT 9 2115 39 DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS 1 38 WNa-n 13 BITS 91 10 MSBs 8 BITS toa 31 BITS Reduced Almanac Packet 1 6052 Reduced Almanac Packet 2 DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS 122 153 184 31 BITS Reduced Almanac Packet 5 17 MSBs Reduced Almanac Packet 4 21 LSBs Reduced Almanac Packet 2 31 BITS Reduced Almanac Packet 3 MSB FIRST DIRECTION OF DATA FLOW FROM SV 100 BITS 4 SECONDS 277 215 246 CRC 24 BITS 31 BITS Reduced Almanac Packet 6 31 BITS Reduced Almanac Packet 7 14 LSBs Reduced Almanac Packet 5 * MESSAGE TOW COUNT = 17 MSB OF ACTUAL TOW COUNT AT START OF NEXT 12-SECOND MESSAGE Figure 30-11. Message Type 12 - Reduced Almanac IS-GPS-200D 7 Dec 2004 151 MSB FIRST DIRECTION OF DATA FLOW FROM SV 100 BITS 4 SECONDS 61 96 38 50 21 62 97 1 9 15 39 8 BITS MESSAGE TYPE ID 6 BITS PREAMBLE PRN 6 BITS MESSAGE TOW COUNT* 17 BITS "ALERT" FLAG - 1 BIT top-D 11 BITS 11 BITS tOD 34 BITS CDC CDC - 4 MSBs DC DATA TYPE – 1 BIT DC DATA TYPE – 1 BIT DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS 131 166 167 132 30 LSBs CDC 34 BITS CDC 34 BITS CDC DC DATA TYPE – 1 BIT DC DATA TYPE – 1 BIT MSB FIRST DIRECTION OF DATA FLOW FROM SV 100 BITS 4 SECONDS 201 236 271 277 202 237 CRC 24 BITS 6 BITS 34 BITS CDC 34 BITS CDC DC DATA TYPE – 1 BIT DC DATA TYPE – 1 BIT RESERVED * MESSAGE TOW COUNT = 17 MSB OF ACTUAL TOW COUNT AT START OF NEXT 12-SECOND MESSAGE CDC = Clock Differential Correction Figure 30-12. Message Type 13 – Clock Differential Correction IS-GPS-200D 7 Dec 2004 152 MSB FIRST DIRECTION OF DATA FLOW FROM SV 100 BITS 4 SECONDS 38 61 8 BITS MESSAGE TYPE ID 6 BITS PREAMBLE PRN 6 BITS MESSAGE TOW COUNT* 17 BITS "ALERT" FLAG - 1 BIT 9 2115 391 50 top-D 11 BITS 11 BITS tOD 39 MSBs EDC 62 DC DATA TYPE – 1 BIT DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS 154 101 53 LSBs EDC 46 MSBs EDC 155 DC DATA TYPE – 1 BIT MSB FIRST DIRECTION OF DATA FLOW FROM SV 100 BITS 4 SECONDS 247 277 CRC 24 BITS RESERVED 30 BITS 46 LSBs EDC * MESSAGE TOW COUNT = 17 MSB OF ACTUAL TOW COUNT AT START OF NEXT 12-SECOND MESSAGE EDC = Ephemeris Differential Correction Figure 30-13. Message Type 14 – Ephemeris Differential Correction IS-GPS-200D 7 Dec 2004 153 8 BITS MESSAGE TYPE ID 6 BITS PREAMBLE PRN 6 BITS MESSAGE TOW COUNT* 17 BITS "ALERT" FLAG - 1 BIT 9 2115 39 DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS 1 38 TEXT MESSAGE (29 8-BIT CHAR) 62 MSBs 8 BITS MESSAGE TYPE ID 6 BITS PREAMBLE PRN 6 BITS MESSAGE TOW COUNT* 17 BITS "ALERT" FLAG - 1 BIT 9 2115 39 DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS 1 38 TEXT MESSAGE (29 8-BIT CHAR) 62 MSBs DIRECTION OF DATA FLOW FROM SV MSB FIRST 100 BITS 4 SECONDS BITS 63-162 TEXT MESSAGE (29 8-BIT CHAR) MSB FIRST DIRECTION OF DATA FLOW FROM SV 100 BITS 4 SECONDS 275 271 277 CRC 24 BITS 70 LSBs TEXT MESSAGE (29 8-BIT CHAR) 4 BITS TEXT PAGE RESERVED – 2 BITS * MESSAGE TOW COUNT = 17 MSB OF ACTUAL TOW COUNT AT START OF NEXT 12-SECOND MESSAGE Figure 30-14. Message Type 15 - Text IS-GPS-200D 7 Dec 2004 154 30.3.3.1 Message Type 10 and 11 Ephemeris and Health Parameters. 30.3.3.1.1 Message Type 10 and 11 Ephemeris and Health Parameter Content. The contents of the SV health, ephemeris representation and accuracy parameters in message types 10 and 11 are defined below, followed by material pertinent to the use of the data. Message type 10 in conjunction with message type 11, provide users with the requisite data to calculate SV position. The general format of message types 10 and 11 consist of data fields for reference time tags, a set of gravitational harmonic correction terms, rates and rate corrections to quasi-Keplerian elements, and an accuracy indicator for ephemeris-related data. The ephemeris parameters in the message type 10 and type 11 describe the orbit of the transmitting SV during the curve fit interval of three hours. The nominal transmission interval is two hours, and shall coincide with the first two hours of the curve fit interval. The period of applicability for ephemeris data coincides with the entire three- hour curve fit interval. Table 30-I gives the definition of the orbital parameters using terminology typical of Keplerian orbital parameters; it is noted, however, that the transmitted parameter values are expressed such that they provide the best trajectory fit in Earth-Centered, Earth-Fixed (ECEF) coordinates for each specific fit interval. The user shall not interpret intermediate coordinate values as pertaining to any conventional coordinate system. Any change in the Message Type 10 and 11 ephemeris data will be accomplished with a simultaneous change in the toe value. The CS will assure that the toe value, for at least the first data set transmitted by an SV after an upload, is different from that transmitted prior to the cutover. See Section 20.3.4.5 for additional information regarding toe. 30.3.3.1.1.1 Transmission Week Number. Bits 39 through 51 of message type 10 shall contain 13 bits which are a modulo-8192 binary representation of the current GPS week number at the start of the data set transmission interval (see paragraph 6.2.4). These 13 bits are comprised of 10 LSBs that represent the ten MSBs of the 29-bit Z-count as qualified in paragraph 20.3.3.3.1.1, and 3 MSBs which are extra bits that extend the range of transmission week number from 10 to 13 bits. IS-GPS-200D 7 Dec 2004 155 30.3.3.1.1.2 Signal Health (L1/L2/L5). The three, one-bit, health indication in bits 52 through 54 of message type 10 refers to the L1, L2, and L5 signals of the transmitting SV. The health of each signal is indicated by: 0 = Signal OK, 1 = Signal bad or unavailable. The predicted health data will be updated at the time of upload when a new data set has been built by the CS. The transmitted health data may not correspond to the actual health of the transmitting SV. Additional SV health data are given in the almanac in messages types 12, 31, and 37. The data given in message type 10 may differ from that shown in other messages of the transmitting SV and/or other SVs since the latter may be updated at a different time. 30.3.3.1.1.3 Data Predict Time of Week. Bits 55 through 65 of message type 10 shall contain the data predict time of week (top). The top term provides the epoch time of week of the state estimate utilized for the prediction of satellite quasi-Keplerian ephemeris parameters. IS-GPS-200D 7 Dec 2004 156 30.3.3.1.1.4 SV Accuracy. Bits 66 through 70 of message type 10 shall contain the ephemeris User Range Accuracy (URAoe) index of the SV for the unauthorized (non-Precise Positioning Service) user. URAoe index shall provide the ephemeris-related user range accuracy index of the SV as a function of the current ephemeris message curve fit interval. While the ephemeris-related URA may vary over the ephemeris message curve fit interval, the URAoe index (N) in message type 10 shall correspond to the maximum URAoe expected over the entire curve fit interval. The URAoe index is a signed, two’s complement integer in the range of +15 to –16 and has the following relationship to the ephemeris URA: URAoe Index URAoe (meters) 15 6144.00 < URAoe 14 3072.00 < URAoe ≤ 6144.00 13 1536.00 < URAoe ≤ 3072.00 12 768.00 < URAoe ≤ 1536.00 11 384.00 < URAoe ≤ 768.00 10 192.00 < URAoe ≤ 384.00 9 96.00 < URAoe ≤ 192.00 8 48.00 < URAoe ≤ 96.00 7 24.00 < URAoe ≤ 48.00 6 13.65 < URAoe ≤ 24.00 5 9.65 < URAoe ≤ 13.65 4 6.85 < URAoe ≤ 9.65 3 4.85 < URAoe ≤ 6.85 2 3.40 < URAoe ≤ 4.85 1 2.40 < URAoe ≤ 3.40 0 1.70 < URAoe ≤ 2.40 -1 1.20 < URAoe ≤ 1.70 -2 0.85 < URAoe ≤ 1.20 -3 0.60 < URAoe ≤ 0.85 -4 0.43 < URAoe ≤ 0.60 -5 0.30 < URAoe ≤ 0.43 -6 0.21 < URAoe ≤ 0.30 -7 0.15 < URAoe ≤ 0.21 -8 0.11 < URAoe ≤ 0.15 -9 0.08 < URAoe ≤ 0.11 -10 0.06 < URAoe ≤ 0.08 -11 0.04 < URAoe ≤ 0.06 -12 0.03 < URAoe ≤ 0.04 -13 0.02 < URAoe ≤ 0.03 -14 0.01 < URAoe ≤ 0.02 -15 URAoe ≤ 0.01 -16 No accuracy prediction available—use at own risk IS-GPS-200D 7 Dec 2004 157 30.3.3.1.2 Message Type 10 and 11 Ephemeris Parameter Characteristics. For each ephemeris parameter contained in message types 10 and 11, the number of bits, the scale factor of the least significant bit (LSB) (which is the last bit received), the range, and the units are as specified in Table 30-I. See Figures 30-1 and 30-2 for complete bit allocation in message types 10 and 11. 30.3.3.1.3 User Algorithm for Determination of SV Position. The user shall compute the ECEF coordinates of position for the SV’s antenna phase center (APC) utilizing a variation of the equations shown in Table 30-II. The ephemeris parameters are Keplerian in appearance; however, the values of these parameters are produced by the CS via a least squares curve fit of the predicted ephemeris of the SV APC (time-position quadruples: t, x, y, z expressed in ECEF coordinates). Particulars concerning the applicable coordinate system are given in Sections 20.3.3.4.3.3 and 20.3.3.4.3.4. The sensitivity of the SV’s position to small perturbations in most ephemeris parameters is extreme. The sensitivity of position to the parameters A, Crc-n, and Crs-n is about one meter/meter. The sensitivity of position to the angular parameters is on the order of 108 meters/semi-circle, and to the angular rate parameters is on the order of 1012 meters/semi-circle/second. Because of this extreme sensitivity to angular perturbations, the value of π used in the curve fit is given here. π is a mathematical constant, the ratio of a circle’s circumference to its diameter. Here π is taken as 3.1415926535898. IS-GPS-200D 7 Dec 2004 158 Table 30-I. Message Types 10 and 11 Parameters (1 of 2) Parameter No. of Bits** Scale Factor (LSB) Effective Range*** Units Week No. 13 1 weeks SV accuracy 5* (see text) Signal health (L1/L2/L5) 3 1 (see text) top Data predict time of week 11 300 604,500 seconds ΔA **** Semi-major axis difference at reference time 26* 2-9 meters A• Change rate in semi-major axis 25* 2-21 meters/sec Δn0 Mean Motion difference from computed value at reference time 17* 2-44 semi-circles/sec Δn0 • Rate of mean motion difference from computed value 23* 2-57 semi-circles/sec2 M0-n Mean anomaly at reference time 33* 2-32 semi-circles en Eccentricity 33 2-34 0.03 dimensionless ωn Argument of perigee 33* 2-32 semi-circles * Parameters so indicated are two’s complement, with the sign bit (+ or -) occupying the MSB; ** See Figure 30-1 for complete bit allocation in Message Type 10; *** Unless otherwise indicated in this column, effective range is the maximum range attainable with indicated bit allocation and scale factor. **** Relative to AREF = 26,559,710 meters. IS-GPS-200D 7 Dec 2004 159 Table 30-I. Message Types 10 and 11 Parameters (2 of 2) Parameter No. of Bits** Scale Factor (LSB) Effective Range*** Units toe Ephemeris data reference time of week 11 300 604,500 seconds Ω0-n**** Reference right ascension angle 33* 2-32 semi-circles *****∆ Ω • Rate of right ascension difference 17* 2-44 semi-circles/sec i0-n Inclination angle at reference time 33* 2-32 semi-circles i0-n –DOT Rate of inclination angle 15* 2-44 semi-circles/sec Cis-n Amplitude of the sine harmonic correction term to the angle of inclination 16* 2-30 radians Cic-n Amplitude of the cosine harmonic correction term to the angle of inclination 16* 2-30 radians Crs-n Amplitude of the sine correction term to the orbit radius 24* 2-8 meters Crc-n Amplitude of the cosine correction term to the orbit radius 24* 2-8 meters Cus-n Amplitude of the sine harmonic correction term to the argument of latitude 21* 2-30 radians Cuc-n Amplitude of the sine harmonic correction term to the argument of latitude 21* 2-30 radians * Parameters so indicated are two’s complement, with the sign bit (+ or -) occupying the MSB; ** See Figure 30-1 and Figure 30-2 for complete bit allocation in Message Types 10 and 11; *** Unless otherwise indicated in this column, effective range is the maximum range attainable with indicated bit allocation and scale factor. **** Ω0-n is the right ascension angle at the weekly epoch (Ω0-w) propagated to the reference time at the rate of right ascension {ΩREF Table 30-II }. ***** Relative to ΩREF = -2.6 x 10-9 semi-circles/second. • • IS-GPS-200D 7 Dec 2004 160 Table 30-II. Elements of Coordinate System (part 1 of 2) Element/Equation Description µ = 3.986005 x 1014 meters3/sec2 Ωe = 7.2921151467 x 10-5 rad/sec A0 = AREF + ΔA * Ak = A0 + (A) tk n0 = 3A0 µ tk = t – toe ** ΔnA = Δn0 +½ Δn0 tk nA = n0 + ΔnA Mk = M0 + nA tk Mk = Ek – en sin Ek νk = tan-1 ⎭ ⎬ ⎫ ⎩ ⎨ ⎧ ν ν k k cos sin = tan-1 ( ) ( ) ( ) ⎪⎭ ⎪ ⎬ ⎫ ⎪⎩ ⎪ ⎨ ⎧ −− −− knnk knk 2 n cos Ee/ 1ecos E cos Ee/ 1sin Ee1 • • • WGS 84 value of the earth’s gravitational constant for GPS user WGS 84 value of the earth’s rotation rate Semi-Major Axis at reference time Semi-Major Axis Computed Mean Motion (rad/sec) Time from ephemeris reference time Mean motion difference from computed value Corrected Mean Motion Mean Anomaly Kepler’s equation for Eccentric Anomaly (radians) (may be solved by iteration) True Anomaly Ek = cos-1 ⎭ ⎬ ⎫ ⎩ ⎨ ⎧ ν+ ν+ kn kn cose1 cose Eccentric Anomaly * AREF = 26,559,710 meters ** t is GPS system time at time of transmission, i.e., GPS time corrected for transit time (range/speed of light). Furthermore, tk shall be the actual total difference between the time t and the epoch time toe, and must account for beginning or end of week crossovers. That is if tk is greater than 302,400 seconds, subtract 604,800 seconds from tk. If tk is less than -302,400 seconds, add 604,800 seconds to tk. IS-GPS-200D 7 Dec 2004 161 Table 30-II. Elements of Coordinate System (part 2 of 2) Element/Equation * Description Φk = νk + ωn Argument of Latitudeδuk = Cus-nsin2Φk + Cuc-ncos2Φk Argument of Latitude Correction Second Harmonic δrk = Crs-nsin2Φk + Crc-ncos2Φk Radial Correction Perturbations δik = Cis-nsin2Φk + Cic-ncos2Φk Inclination Correction uk = Φk + δuk Corrected Argument of Latitude rk = Ak(1 – en cos Ek) + δrk Corrected Radius ik = io-n + (io-n-DOT)tk + δikCorrected Inclination xk ' = rk cos uk Positions in orbital plane yk ' = rk sin uk Ω = ΩREF + ΔΩ *** • • • Rate of Right Ascension Ωk = Ω0-n + ( Ω − Ωe ) tk – Ωe toe• • • Corrected Longitude of Ascending Node xk = xk ' cos Ωk − yk ' cos ik sin Ωk yk = xk ' sin Ωk + yk ' cos ik cos Ωk Earth-fixed coordinates of SV antenna phase center zk = yk ' sin ik *** ΩREF = −2.6 x 10-9 semi-circles/second. • IS-GPS-200D 7 Dec 2004 162 30.3.3.2 Message Types 30 Through 37 SV Clock Correction Parameters. 30.3.3.2.1 Message Type 30 Through 37 SV Clock Correction Parameter Content. The clock parameters in any one of message types 30 through 37 describe the SV time scale during the period of validity. The clock parameters in a data set shall be valid during the interval of time in which they are transmitted and shall remain valid for an additional period of time after transmission of the next data set has started. The general format of message types 30 through 37 includes data fields for SV clock correction coefficients. Any one of message types 30 through 37 in conjunction with message types 10 and 11 provide users with the requisite data to correct SV time and to calculate SV position precisely. In general, any message type 30’s (i.e. 30-39) will provide SV clock correction parameters as described in this section. 30.3.3.2.1.1 SV Clock Correction. Any one of message types 30 through 37, Figure 30-3 through Figure 30-10, contains the parameters needed by the users for apparent SV clock correction. Bits 61 to 71 contain toc , clock data reference time of week. Bits 72 to 127 contain SV clock correction coefficients. The related algorithm is given in paragraph 20.3.3.3.3.1. 30.3.3.2.1.2 Data Predict Time of Week. Bits 39 through 49 of message types 30 through 37 shall contain the data predict time of week (top). The top term provides the epoch time of week of the state estimate utilized for the prediction of SV clock correction coefficients. 30.3.3.2.2 Clock Parameter Characteristics. The number of bits, the scale factor of the LSB (which is the last bit received), the range, and the units of clock correction parameters shall be as specified in Table 30-III. 30.3.3.2.3 User Algorithms for SV Clock Correction Data. The algorithms defined in paragraph 20.3.3.3.3.1 allow all users to correct the code phase time received from the SV with respect to both SV code phase offset and relativistic effects. However, since the SV clock corrections of equations in paragraph 20.3.3.3.3.1 are estimated by the CS using dual frequency L1 and L2 P(Y) code measurements, the single-frequency L1 or L2 user and the dual- frequency L1 C/A – L2 C users must apply additional terms to the SV clock correction equations. These terms are described in paragraph 30.3.3.3.1. IS-GPS-200D 7 Dec 2004 163 Table 30-III. Clock Correction and Accuracy Parameters Parameter No. of Bits** Scale Factor (LSB) Effective Range*** Units toc URAoc Index URAoc1 Index URAoc2 Index af2-n af1-n af0-n Clock Data Reference Time of Week SV Clock Accuracy Index SV Clock Accuracy Change Index SV Clock Accuracy Change Rate Index SV Clock Drift Rate Correction Coefficient SV Clock Drift Correction Coefficient SV Clock Bias Correction Coefficient 11 5* 3 3 10* 20* 26* 300 2-60 2-48 2-35 604,500 seconds (see text) (see text) (see text) sec/sec2 sec/sec seconds * Parameters so indicated are two’s complement, with the sign bit (+ or -) occupying the MSB; ** See Figure 30-3 through 30-10 for complete bit allocation in Message types 30 to 37; *** Unless otherwise indicated in this column, effective range is the maximum range attainable with indicated bit allocation and scale factor. IS-GPS-200D 7 Dec 2004 164 30.3.3.2.4 SV Clock Accuracy Estimates. Bits 50 through 54, and 55 through 57, and 58 through 60 of message types 30 through 37 shall contain the URAoc Index,URAoc1 Index, and URAoc2 Index, respectively, of the SV (reference paragraph 6.2.1) for the unauthorized user. The URAoc Index together with URAoc1 Index and URAoc2 Index shall give the clock-related user range accuracy of the SV as a function of time since the prediction (top) used to generate the uploaded clock correction polynomial terms. The user shall calculate the clock-related URA with the equation (in meters); URAoc = URAocb + URAoc1 (t – top) for t-top < 93,600 seconds URAoc = URAocb + URAoc1 (t – top) + URAoc2 (t – top – 93,600)2 for t-top > 93,600 seconds where t = GPS time (must account for beginning or end of week crossovers), top = time of week of the state estimate utilized for the prediction of satellite clock correction parameters. The CS shall derive URAocb at time top which, when used together with URAoc1 and URAoc2 in the above equations, results in the minimum URAoc that is greater than the predicted URAoc during the entire duration up to 14 days after top. IS-GPS-200D 7 Dec 2004 165 The user shall use the broadcast URAoc Index to derive URAocb. The index is a signed, two’s complement integer in the range of +15 to –16 and has the following relationship to the clock-related user derived URAocb: URAoc Index URAocb (meters) 15 6144.00 < URAocb 14 3072.00 < URAocb ≤ 6144.00 13 1536.00 < URAocb ≤ 3072.00 12 768.00 < URAocb ≤ 1536.00 11 384.00 < URAocb ≤ 768.00 10 192.00 < URAocb ≤ 384.00 9 96.00 < URAocb ≤ 192.00 8 48.00 < URAocb ≤ 96.00 7 24.00 < URAocb ≤ 48.00 6 13.65 < URAocb ≤ 24.00 5 9.65 < URAocb ≤ 13.65 4 6.85 < URAocb ≤ 9.65 3 4.85 < URAocb ≤ 6.85 2 3.40 < URAocb ≤ 4.85 1 2.40 < URAocb ≤ 3.40 0 1.70 < URAocb ≤ 2.40 -1 1.20 < URAocb ≤ 1.70 -2 0.85 < URAocb ≤ 1.20 -3 0.60 < URAocb ≤ 0.85 -4 0.43 < URAocb ≤ 0.60 -5 0.30 < URAocb ≤ 0.43 -6 0.21 < URAocb ≤ 0.30 -7 0.15 < URAocb ≤ 0.21 -8 0.11 < URAocb ≤ 0.15 -9 0.08 < URAocb ≤ 0.11 -10 0.06 < URAocb ≤ 0.08 -11 0.04 < URAocb ≤ 0.06 -12 0.03 < URAocb ≤ 0.04 -13 0.02 < URAocb ≤ 0.03 -14 0.01 < URAocb ≤ 0.02 -15 URAocb ≤ 0.01 -16 No accuracy prediction available—use at own risk The user may use the upper bound value in the URAocb range corresponding to the broadcast index, thereby calculating the maximum URAoc that is equal to or greater than the CS predicted URAoc, or the user may use the lower bound value in the range which will provide the minimum URAoc that is equal to or less than the CS predicted URAoc. IS-GPS-200D 7 Dec 2004 166 The transmitted URAoc1 Index is an integer value in the range 0 to 7. URAoc1 Index has the following relationship to the URAoc1: 1 URAoc1 = (meters/second) 2N where N = 4 + URAoc1 Index The transmitted URAoc2 Index is an integer value in the range 0 to 7. URAoc2 Index has the following relationship to the URAoc2: 1 URAoc2 = (meters/second2) 2N where N = 25 + URAoc2 Index IS-GPS-200D 7 Dec 2004 167 30.3.3.3 Message Type 30 Ionospheric and Group Delay Correction Parameters. 30.3.3.3.1 Message Type 30 Ionospheric and Group Delay Correction Parameter Content. Message type 30 provides SV clock correction parameters (ref. Section 30.3.3.2) and ionospheric and group delay correction parameters. Bits 128 through 192 of message type 30 provide the group delay differential correction terms for L1, L2, and L5 signal users. Bits 193 through 256 provide the ionospheric correction parameters for single frequency user. The following algorithms shall apply when interpreting the correction parameters in the message. 30.3.3.3.1.1 Estimated L1-L2 Group Delay Differential. The group delay differential correction terms, TGD, ISCL1C/A, ISCL2C for the benefit of single frequency L1 P, L1 C/A, L2 P, L2 C users and dual frequency L1/L2 users are contained in bits 128 through 166 of message type 30 (see Figure 30-3 for complete bit allocation). The bit length, scale factors, ranges, and units of these parameters are given in Table 30-IV. The bit string of “1000000000000” shall indicate that the group delay value is not available. The related algorithm is given in paragraphs 30.3.3.3.1.1.1 and 30.3.3.3.1.1.2. Table 30-IV. Group Delay Differential Parameters **** Parameter No. of Bits** Scale Factor (LSB) Effective Range*** Units TGD ISCL1C/A ISCL2C 13* 13* 13* 2-35 2-35 2-35 seconds seconds seconds * Parameters so indicated are two’s complement with the sign bit (+ or -) occupying the MSB; ** See Figure 30-3 for complete bit allocation in Message type 30; *** Effective range is the maximum range attainable with indicated bit allocation and scale factor; **** The bit string of “1000000000000” will indicate that the group delay value is not available. IS-GPS-200D 7 Dec 2004 168 30.3.3.3.1.1.1 Inter-Signal Group Delay Differential Correction. The correction terms, TGD, ISCL1C/A and ISCL2C, are initially provided by the CS to account for the effect of SV group delay differential between L1 P(Y) and L2 P(Y), L1 P(Y) and L1 C/A, and between L1 P(Y) and L2 C, respectively, based on measurements made by the SV contractor during SV manufacture. The values of TGD and ISCs for each SV may be subsequently updated to reflect the actual on-orbit group delay differential. For maximum accuracy, the single frequency L1 C/A user must use the correction terms to make further modifications to the code phase offset in paragraph 20.3.3.3.3.1 with the equation: (ΔtSV)L1C/A = ΔtSV - TGD + ISCL1C/A where TGD (see paragraph 20.3.3.3.3.2) and ISCL1C/A are provided to the user as Message Type 30 data, described in paragraph 30.3.3.3.1.1. For the single frequency L2 C user, the code phase offset modification is given by: (ΔtSV)L2C = ΔtSV - TGD + ISCL2C where, ISCL2C is provided to the user as Message Type 30 data. The values of ISCL1C/A and ISCL2C are measured values that represent the mean SV group delay differential between the L1 P(Y)-code and the L1 C/A- or L2 C-codes respectively as follows, ISCL1C/A = tL1P(Y) -tL1C/A ISCL2C = tL1P(Y) -tL2C. where, tLix is the GPS time the ith frequency x signal (a specific epoch of the signal) is transmitted from the SV antenna phase center. IS-GPS-200D 7 Dec 2004 169 30.3.3.3.1.1.2 L1 /L2 Ionospheric Correction. The two frequency (L1 C/A and L2 C) user shall correct for the group delay and ionospheric effects by applying the relationship: (PR L2C −γ12PRL1C/ A) + c (ISCL2C −γ12ISCL1C / A ) PR =− cT GD 1 −γ 12 where, PR = pseudorange corrected for ionospheric effects, PRi = pseudorange measured on the channel indicated by the subscript, ISCi = inter-signal correction for the channel indicated by the subscript (see paragraph 30.3.3.3.1.1), TGD = see paragraph 20.3.3.3.3.2, c = speed of light, and where, denoting the nominal center frequencies of L1 and L2 as fL1 and fL2 respectively, γ12 = (fL1/fL2)2 = (1575.42/1227.6)2 = (77/60)2. 30.3.3.3.1.2 Ionospheric Data. The ionospheric parameters which allow the “L1 only” or “L2 only” user to utilize the ionospheric model for computation of the ionospheric delay are contained in Message Type 30. The “one frequency” user should use the model given in paragraph 20.3.3.5.2.5 to make this correction for the ionospheric effects. The bit lengths, scale factors, ranges, and units of these parameters are given in Table 20-X. The ionospheric data shall be updated by the CS at least once every six days while the CS is able to upload the SVs. If the CS is unable to upload the SVs, the ionospheric data transmitted by the SVs may not be accurate. IS-GPS-200D 7 Dec 2004 170 30.3.3.4 Message Types 31, 12, and 37 Almanac Parameters. The almanac parameters are provided in any one of message types 31, 37, and 12. Message type 37 provides Midi almanac parameters and the reduced almanac parameters are provided in either message type 31 or type 12. The SV shall broadcast both message types 31 (and/or 12) and 37. However, the reduced almanac parameters (i.e. message types 31 and/or 12) for the complete set of SVs in the constellation will be broadcast by a SV using shorter duration of time compared to the broadcast of the complete set of Midi almanac parameters (i.e. message type 37). The parameters are defined below, followed by material pertinent to the use of the data. 30.3.3.4.1 Almanac Reference Week. Bits 39 through 51 of message type 12, and bits 128 through 140 of message types 31 and 37 shall indicate the number of the week (WNa-n) to which the almanac reference time (toa) is referenced (see paragraph 20.3.3.5.2.2). The WNa-n term consists of 13 bits which shall be a modulo-8192 binary representation of the GPS week number (see paragraph 6.2.4) to which the toa is referenced. Bits 52 through 59 of message type 12, and bits 141 to 148 of message types 31 and 37 shall contain the value of toa, which is referenced to this WNa-n. 30.3.3.4.2 Almanac Reference Time. See paragraph 20.3.3.5.2.2. 30.3.3.4.3 SV PRN Number. Bits 149 through 154 of message type 37 and bits 1 through 6 in each packet of reduced almanac shall specify PRN number of the SV whose almanac or reduced almanac, respectively, is provided in the message or in the packet. 30.3.3.4.4 Signal Health (L1/L2/L5). The three, one-bit, health indication in bits 155, 156, and 157 of message type 37 and bits 29,30 and 31 of each packet of reduced almanac refers to the L1, L2, and L5 signals of the SV whose PRN number is specified in the message or in the packet. For each health indicator, a “0” signifies that all navigation data are okay and “1” signifies that some or all navigation data are bad. The predicted health data will be updated at the time of upload when a new reduced almanac has been built by the CS. The transmitted health data may not correspond to the actual health of the transmitting SV or other SVs in the constellation. IS-GPS-200D 7 Dec 2004 171 30.3.3.4.5 Midi Almanac Parameter Content. Message type 37, Figure 30-10, provides Midi almanac data for a SV whose PRN number is specified in the message. The number of bits, the scale factor (LSB), the range, and the units of the almanac parameters are given in Table 30-V. The user algorithm is essentially the same as the user algorithm used for computing the precise ephemeris as specified in Table 20-IV. Other parameters appearing in the equations of Table 20-IV, but not provided by the Midi almanac with the reference values, are set to zero for SV position determination. See paragraph 20.3.3.5.2.3 for almanac time parameters. 30.3.3.4.6 Reduced Almanac Parameter Content. Message type 31, Figure 30-4, provides SV clock correction parameters (ref. Section 30.3.3.2) and reduced almanac data packets for 4 SVs. Message type 12, Figure 30-11, contains reduced almanac data packets for 7 SVs. 30.3.3.4.6.1 Reduced Almanac Data. Message type 31 or 12 contains reduced almanac data and SV health words for SVs in the constellation. The reduced almanac data of a SV is broadcast in a packet of 31 bits long, as described in Figure 30-15. The reduced almanac data are a subset of the almanac data which provide less precise ephemeris. The reduced almanac data values are provided relative to pre-specified reference values. The number of bits, the scale factor (LSB), the range, and the units of the reduced almanac parameters are given in Table 30-VI. The algorithms and other material related to the use of the reduced almanac data are given in Section 30.3.3.4.6.2. The reduced almanac parameters shall be updated by the CS at least once every 3 days while the CS is able to upload the SVs. If the CS is unable to upload the SVs, the accuracy of the reduced almanac parameters transmitted by the SVs will degrade over time. 30.3.3.4.6.2 Reduced Almanac Packet. The following shall apply when interpreting the data provided in each packet of reduced almanac (see Figure 30-15). 30.3.3.4.6.2.1 Reduced Almanac. The reduced almanac data is provided in bits 7 through 28 of each packet. The data from a packet along with the reference values (see Table 30-VI) provide ephemeris with further reduced precision. The user algorithm is essentially the same as the user algorithm used for computing the precise ephemeris from the parameters of the message types 10 and 11 (see paragraph 30.3.3.1.3 and Table 30-II). Other parameters appearing in the equations of Table 30-II, but not provided by the reduced almanac with the reference values, are set to zero for SV position determination. IS-GPS-200D 7 Dec 2004 172 Table 30-V. Midi Almanac Parameters Parameter No. of Bits** Scale Factor (LSB) Effective Range*** Units toa 8 212 602,112 seconds e 11 2-16 dimensionless δi **** 11* 2-14 semi-circles Ω • 11* 2-33 semi-circles/sec A 17 2-4 meters Ω0 16* 2-15 semi-circles ω 16* 2-15 semi-circles M0 16* 2-15 semi-circles af0 11* 2-20 seconds af1 10* 2-37 sec/sec * Parameters so indicated shall be two's complement with the sign bit (+ or -) occupying the MSB; ** See Figure 30-10 for complete bit allocation in message type 37; *** Unless otherwise indicated in this column, effective range is the maximum range attainable with indicated bit allocation and scale factor; **** Relative to i0 = 0.30 semi-circles. IS-GPS-200D 7 Dec 2004 173 31 BITS 1 7 15 22 29 30 31 PRNa δA Ω0 Φ0 8 BITS 7 BITS 6 BITS 7 BITS L1 HEALTH L2 HEALTH * See Figures 30-4 and 30-11 for complete bit allocation in the L5 HEALTH respective messages. Figure 30-15. Reduced Almanac Packet Content Table 30-VI. Reduced Almanac Parameters ***** Parameter No. of Bits Scale Factor (LSB) Effective Range ** Units δA *** Ω0 Φ0 **** 8 * 7 * 7 * 2+9 2-6 2-6 ** ** ** meters semi-circles semi-circles * Parameters so indicated shall be two’s complement with the sign bit (+ or -) occupying the MSB; ** Effective range is the maximum range attainable with indicated bit allocation and scale factor; *** Relative to Aref = 26,559,710 meters; **** Φ0 = Argument of Latitude at Reference Time = M0 + ω; ***** Relative to following reference values: e = 0 δi = +0.0056 semi-circles (i = 55 degrees) Ω = -2.6 x 10-9 semi-circles/second. • IS-GPS-200D 7 Dec 2004 174 30.3.3.5 Message Type 32 Earth Orientation Parameters (EOP). The earth orientation parameters are provided in message type 32. The parameters are defined below, followed by material pertinent to the use of the data. 30.3.3.5.1 EOP Content. Message type 32, Figure 30-5, provides SV clock correction parameters (ref. Section 30.3.3.2) and earth orientation parameters. The EOP message provides users with parameters to construct the ECEF and ECI coordinate transformation (a simple transformation method is defined in Section 20.3.3.4.3.3.2). The number of bits, scale factors (LSBs), the range, and the units of all EOP fields of message type 32 are given in Table 30-VII. 30.3.3.5.1.1 User Algorithm for Application of the EOP. The EOP fields in the message type 32 contain the EOP needed to construct the ECEF-to-ECI coordinate transformation. The user computes the ECEF position of the SV antenna phase center using the equations shown in Table 30-II. The coordinate transformation, for translating to the corresponding ECI SV antenna phase center position, is derived using the equations shown in Table 30-VIII. The coordinate systems are defined in Section 20.3.3.4.3.3 An ECI postion, Reci , is related to an ECEF position, Recef , by a series of rotation matrices as following: Recef = [A][B][C][D] Reci where the rotation matrices, A, B, C, and D, represent the effects of Polar Motion, Earth Rotation, Nutation and Precession, respectively. The message type 32 specifies the EOP parameters used in the construction of the Polar Motion, A, and Earth Rotation, B, matrices. The rotation matrices, A, B, C and D are specified in terms of elementary rotation matrices, Ri(α), where αis a positive rotation about the ith-axis ordinate, as follows: ⎡1 0 0 ⎤⎡cos() α 0 −α sin() ⎤ ⎢ ⎥⎢⎥ 1 ()α= α 2 () R ⎢0 cos( ) sin( ) ⎥ ,R ⎢ 01 0 ⎥ ⎢ ⎥⎢⎥ ⎢0 −sin() α cos( ) ⎥ ⎢α 0 cos() ⎥ α α= α sin() α ⎣ ⎦⎣⎦ αα 0⎤ ⎡cos( ) sin( ) ⎢⎥ R3 ()α=− ⎢ α cos( ) sin( ) α 0⎥ ⎢⎥ ⎢ 0 01⎥ ⎣ ⎦ The user shall compute the Inertial-to-Geodetic rotation matrix, ABCD using the equations shown in Table 30-VIII. IS-GPS-200D 7 Dec 2004 175 Table 30-VII. Earth Orientation Parameters Parameter No. of Bits** Scale Factor (LSB) Effective Range*** Units tEOP EOP Data Reference Time 16 24 604,784 seconds PM_X † X-Axis Polar Motion Value at Reference Time. 21* 2-20 1 arc-seconds PM_X• X-Axis Polar Motion Drift at Reference Time. 15* 2-21 7.8125 x 10-3 arc-seconds/day PM_Y †† Y-Axis Polar Motion Value at Reference Time. 21* 2-20 1 arc-seconds PM_Y • Y-Axis Polar Motion Drift at Reference Time. 15* 2-21 7.8125 x 10-3 arc-seconds/day ΔUT1 ††† UT1-UTC Difference at Reference Time. 31* 2-24 64 seconds ΔUT1 †††• Rate of UT1-UTC Difference at Reference Time 19* 2-25 7.8125 x 10-3 seconds/day * Parameters so indicated are two’s complement, with the sign bit (+ or -) occupying the MSB; ** See Figure 30-5 for complete bit allocation in Message type 32; *** Unless otherwise indicated in this column, effective range is the maximum range attainable with indicated bit allocation and scale factor. † Represents the predicted angular displacement of instantaneous Celestial Ephemeris Pole with respect to semi-minor axis of the reference ellipsoid along Greenwich meridian. †† Represents the predicted angular displacement of instantaneous Celestial Ephemeris Pole with respect to semi-minor axis of the reference ellipsoid on a line directed 90° west of Greenwich meridian. ††† With zonal tides restored. IS-GPS-200D 7 Dec 2004 176 Table 30-VIII. Application of EOP Parameters (Part 1 of 2) Element/Equation Description TDT = t + s51 .184 J.E.D. = TDT expressed in days of 86400 sec −π ⎡ ⎤° ° = +⎢ ⎥° ⎣ ⎦ J.E.D. 2451545 g 357.528 35999. 05 180 36525 ( )s0.001658sin g 0.0167sin gJ.B.D. J.E.D. 86400s + = + − = J.B.D. 2451545T 36525 ′′ ′′ ′′ζ= + +2 32306 .2181 T 0 .30188 T 0 .017998 T ′′ ′′ ′′= + +2 3z 2306 . 2181 T 1 . 09468 T 0 . 018203 T ′′ ′′ ′′θ= − 2 − 32004 . 3109 T 0 . 42665 T 0 . 041833 T ( ) () ( )= − − θ − ζo oDR 90 z R R 903 1 3 ° ′′′ ′ ′′′ ′′ε= − − ′′+ 223 26 21 .448 46 . 815 T 0.00059 T 0.001813 T3 ( ) ( ) ()= − ε + Δε −Δψ εCR ( ) R R1 3 1 Compute Terrestrial Dynamical Time relative to GPS Time t Compute Julian Ephemeris Date Compute Mean Anomaly of Earth in its orbit, g Compute Julian Date in Barycentric Dynamical Time Compute time from J2000 Julian Epoch in Julian Centuries Compute Precession Fundamental Angles at time t Calculate Precession Matrix at time, t Compute Mean Obliquity, ε , at time t Compute Nutation Matrix at time, t IS-GPS-200D 7 Dec 2004 177 Table 30-VIII. Application of EOP Parameters (Part 2 of 2) Element/Equation Description 106 5 1 1 sini j j i j a e Eψ = = ⎛ ⎞ Δ= ⎜ ⎟ ⎝ ⎠ ∑ ∑ †† 64 5 1 1 cos i j j i j b e Eε = = ⎛ ⎞ Δ= ⎜ ⎟ ⎝ ⎠ ∑ ∑ †† UT1 = UTC + ΔUT1 + ΔUT1 (t – tEOP) − = = J.D. 2451545 TU 36525 where J.D. UT1 expressed in days of 86400 sec ⎛ ⎞+⎜ ⎟ π⎜ ⎟α= +⎜ ⎟ ⎜ ⎟−⎜ ⎟+ − ×⎝ ⎠ hm sUT1 6 41 50 .54841 2 s8640184.812866TU24h s 2 s 6 30 .093104T 6 . 2 10 TU U • Nutation in Longitude Nutation in Obliquity Compute Universal Time at time t Compute Universal Time from J2000 Julian Epoch in Julian Centuries Compute Mean Greenwich Hour Angle α=α+Δψ ε+Δεcos( ) ()= αBR3 A = R2 (-xp) R1 (-yp) where xp = PM_X + PM_X (t – tEOP) yp = PM_Y + PM_Y (t – tEOP) =⎡⎤⎡⎤⎡ ⎤⎡ ⎤⎣⎦⎣⎦⎣ ⎦⎣ ⎦ ABCD A B C D • • Compute True Greenwich Hour Angle Compute Rotation Matrix at time, t Compute Polar Motion Matrix at time, t Compute Inertial-to-Geodetic Rotation matrix, ABCD t is GPS system time at time of transmission, i.e., GPS time corrected for transit time (range/speed of light). †† The Nutation in Longitude and the Nutation in Obliquity are as described in The Astronomical Almanac (1983), pp. S23-S26, evaluated at time T. IS-GPS-200D 7 Dec 2004 178 30.3.3.6 Message Type 33 Coordinated Universal Time (UTC) Parameters. Message type 33, Figure 30-6, contains the UTC parameters. The contents of message type 33 are defined below, followed by material pertinent to the use of the UTC data. 30.3.3.6.1 UTC Parameter Content. Message type 33 provides SV clock correction parameters (ref. Section 30.3.3.2) and also, shall contain the parameters related to correlating UTC (USNO) time with GPS Time. The bit lengths, scale factors, ranges, and units of these parameters are given in Table 30-IX. See Figure 30-6 for complete bit allocation in message type 33. The parameters relating GPS time to UTC (USNO) shall be updated by the CS at least once every three days while the CS is able to upload the SVs. If the CS is unable to upload the SVs, the accuracy of the UTC parameters transmitted by the SVs will degrade over time. 30.3.3.6.2 UTC and GPS Time. Message type 33 includes: (1) the parameters needed to relate GPS Time to UTC(USNO), and (2) notice to the user regarding the scheduled future or recent past (relative to Nav message upload) value of the delta time due to leap seconds (ΔtLSF), together with the week number (WNLSF) and the day number (DN) at the end of which the leap second becomes effective. Information required to use these parameters to calculate tUTC is in paragraph 20.3.3.5.2.4 except the following definition of ΔtUTC shall be used. ΔtUTC = ΔtLS + A0-n + A1-n (tE – tot + 604800 (WN – WNot)) + A2-n (tE – tot + 604800 (WN – WNot))2 seconds IS-GPS-200D 7 Dec 2004 179 Table 30-IX. UTC Parameters Parameter No. of Bits** Scale Factor (LSB) Effective Range*** Units A0-n Bias coefficient of GPS time scale relative to UTC time scale 16* 2-35 Seconds A1-n Drift coefficient of GPS time scale relative to UTC time scale 13* 2-51 sec/sec A2-n Drift rate correction coefficient of GPS time scale relative to UTC time scale 7* 2-68 sec/sec2 ΔtLS Current or past leap second count 8* 1 seconds tot Time data reference Time of Week 16 24 604,784 seconds WNot Time data reference Week Number 13 1 weeks WNLSF Leap second reference Week Number 8 1 weeks DN Leap second reference Day Number 4**** 1 days ΔtLSF Current or future leap second count 8* 1 seconds * Parameters so indicated shall be two's complement with the sign bit (+ or -) occupying the MSB; ** See Figure 30-6 for complete bit allocation; *** Unless otherwise indicated in this column, effective range is the maximum range attainable with indicated bit allocation and scale factor; **** Right justified. IS-GPS-200D 7 Dec 2004 180 30.3.3.7 Message Types 34, 13, and 14 Differential Correction Parameters. Differential Correction (DC) parameters are provided either in message types 34 or in types 13 and 14. These parameters provide users with sets of correction terms that apply to the clock and ephemeris data transmitted by other SVs. DC parameters are grouped in packets, as described in the next sections. The availability of these message types is subject to the control and determination of the CS. 30.3.3.7.1 Differential Correction Parameters Content. Message type 34 provides SV clock correction parameters (ref. Section 30.3.3.2) and also, shall contain DC parameters that apply to the clock and ephemeris data transmitted by another SV. One message type 34, Figure 30-7, shall contain 34 bits of clock differential correction (CDC) parameters and 92 bits of ephemeris differential correction (EDC) parameters for one SV other than the transmitting SV. Bit 150 of message type 34 shall be a DC Data Type indicator that indicates the data type for which the DC parameters apply. Zero (0) signifies that the corrections apply to CNAV data, Dc(t), and one (1) signifies that the corrections apply to NAV data, D(t). Message types 13 and 14 together also provide DC parameters. Message type 13, Figure 30-12, shall contain CDC parameters applicable to 6 SVs and message type 14, Figure 30-13, shall contain EDC parameters applicable to 2 SVs. There shall be a DC Data Type indicator preceding each CDC or EDC packet. The content of an individual data packet is depicted in Figure 30-16. The number of bits, scale factors (LSB), the range, and the units of all fields in the DC packet are given in Table 30-X. 30.3.3.7.2 DC Data Packet. Each DC data packet contains: corrections to SV clock polynomial coefficients provided in any one of the message types 30 to 37 of the corresponding SV; corrections to quasi-Keplerian elements • referenced to tOD of the corresponding SV; and User Differential Range Accuracy (UDRA) and UDRA indices that enable users to estimate the accuracy obtained after corrections are applied. Each DC packet is made up of two different segments. The first segment contains 34 bits for the CDC parameters and the second segment contains 92 bits of EDC parameters totaling 126 bits. The CDC and EDC parameters form an indivisible pair and users must utilize CDC and EDC as a pair. Users must utilize CDC and EDC data pair of same top-D and of same tOD. 30.3.3.7.2.1 Differential Correction Data Predict Time of Week. The DC data predict time of week (top-D) provides the epoch time of week, in increments of 300 seconds (i.e. five minutes), at which the prediction for the associated DC data was performed. IS-GPS-200D 7 Dec 2004 181 30.3.3.7.2.2 Time of Differential Correction Data. The time of DC data, tOD, specifies the reference time of week, in increments of 300 seconds (i.e., five minutes) relative to the GPS week, for the associated CDC and EDC data. 30.3.3.7.2.3 SV PRN Identification. The PRN ID of both CDC and EDC of Figure 30-16 identifies the satellite to which the subject 126-bit differential correction packet data applies (by PRN code assignment). A value of all ones “11111111” in any PRN ID field shall indicate that no DC data is contained in the remainder of the data block. In this event, the remainder of the data block shall be filler bits, i.e., alternating ones and zeros beginning with one. CDC = Clock Differential Correction MSB LSB 1 9 22 3430 PRN ID 8 BITS δaf0 13 BITS δaf1 8 BITS UDRA 5 BITS EDC = Ephemeris Differential Correction MSB LSB 1 9 23 PRN ID 8 BITS Δα14 BITS 36 Δβ14 BITS MSB LSB Δi 12 BITS 63 Δγ15 BITS 37 52 MSB LSB 88 92 76 64 • ΔA ΔΩ UDRA 12 BITS 5 BITS 12 BITS Figure 30-16. Differential Correction Data Packet IS-GPS-200D 7 Dec 2004 182 Table 30-X. Differential Correction Parameters Parameter No. of Bits** Scale Factor (LSB) Effective Range*** Units PRN ID 8 see text δaf0 SV Clock Bias Correction 13* 2-35 seconds δaf1 SV Clock Drift Correction 8* 2-51 seconds/second UDRA User Differential Range Accuracy Index 5* see text Δα Alpha Correction to Ephemeris Parameters 14* 2-34 dimensionless Δβ Beta Correction to Ephemeris Parameters 14* 2-34 dimensionless Δγ Gamma Correction to Ephemeris Parameters 15* 2-32 semi-circles Δi Angle of Inclination Correction 12* 2-32 semi-circles ΔΩ Angle of Right Ascension Correction 12* 2-32 semi-circles ΔA Semi-Major Correction 12* 2-9 meters UDRA • Change Rate of User Differential Range Accuracy Index. 5* see text * Parameters so indicated are two’s complement, with the sign bit (+ or -) occupying the MSB; ** See Figure 30-7, 11 and 12 for complete bit allocation in Message types 34, 13 and 14; *** Unless otherwise indicated in this column, effective range is the maximum range attainable with indicated bit allocation and scale factor. IS-GPS-200D 7 Dec 2004 183 30.3.3.7.3 Application of Clock-Related DC Data. The SV PRN code phase offset, uncorrected by clock correction coefficient updates, is given by equation 2 in paragraph 20.3.3.3.3.1 (see para. 30.3.3.2.3). If the matched pair of DC data for the subject SV is available, the user may apply clock correction coefficient update values by; Δtsv = (af0 + δaf0) + (af1 + δaf1)(t − toc) + af2(t − toc )2 + Δtr, where δaf0 and δaf1, (see Table 30-X), are given in message types 34 or 13, and all other terms are as stated in paragraph 20.3.3.3.3.1. Clock-related DC data shall not be applied to any SV transmitting clock correction parameters message(s) containing a top value greater than the top-D value of messages types 34 or 13 containing the clock-related DC data. 30.3.3.7.4 Application of Orbit-Related DC Data. The DC data packet includes corrections to parameters that correct the state estimates for ephemeris parameters transmitted in the message types 10 and 11 (broadcast by the SV to which the DC data packet applies). The user will update the ephemeris parameters utilizing a variation of the algorithm expressed in the following equations. The user will then incorporate the updated quasi-Keplerian element set in all further calculations of SV position, as represented by the equations in Table 30-II (see para. 30.3.3.1.3). Ephemeris-related DC data shall not be applied to any SV transmitting message types 10 and 11 containing a top value greater than the top-D value of message types 34 or 14 containing the ephemeris-related DC data. The user will construct a set of initial (uncorrected) elements by: Ai = A0 ei = en ii = i0-n Ωi = Ω0-n αi = en cos(ωn) βi = en sin(ωn) γi = M0-n + ωn IS-GPS-200D 7 Dec 2004 184 where A0, en, i0-n, Ω0-n, ωn and M0-n are obtained from the applicable SV’s message types 10 and 11 data. The terms αi, βi, and γi form a subset of stabilized ephemeris elements which are subsequently corrected by Δα, Δβ and Δγ⎯the values of which are supplied in the message types 34 or 14 — as follows: αc = αi + Δα βc = βi + Δβ γc = γi + Δγ The quasi-Keplerian elements are then corrected by Ac = Ai + ΔA 2)1/2 ec = (αc2 + βc ic = ii + Δi Ωc = Ωi + ΔΩ ωc = tan-1 (βc/αc) M0-c = γc −ωc + ΔM0 where ΔA, Δi and ΔΩ are provided in the EDC data packet of the message type 34 or 14 and ΔM0 is obtained from ΔM0 = −3 μ2 [(toe) − (tOD )]. Ac The corrected quasi-Keplerian elements above are applied to the user algorithm for determination of antenna phase center position in Section 30.3.3.1.3, Table 30-II. IS-GPS-200D 7 Dec 2004 185 • 30.3.3.7.5 SV Differential Range Accuracy Estimates. The UDRAop-D and UDRA shall give the differential user range accuracy for the SV. It must be noted that the two parameters provide estimated accuracy after both clock and • ephemeris DC are applied. The UDRA and UDRA indices are signed, two’s complement integers in the range of +15 to –16 and has the following relationship: • Index Value UDRAop-D (meters) UDRA (10-6 m/sec) 15 6144.00 < UDRAop-D 6144.00 14 3072.00 < UDRAop-D ≤ 6144.00 3072.00 13 1536.00 < UDRAop-D ≤ 3072.00 1536.00 12 768.00 < UDRAop-D ≤ 1536.00 768.00 11 384.00 < UDRAop-D ≤ 768.00 384.00 10 192.00 < UDRAop-D ≤ 384.00 192.00 9 96.00 < UDRAop-D ≤ 192.00 96.00 8 48.00 < UDRAop-D ≤ 96.00 48.00 7 24.00 < UDRAop-D ≤ 48.00 24.00 6 13.65 < UDRAop-D ≤ 24.00 13.65 5 9.65 < UDRAop-D ≤ 13.65 9.65 4 6.85 < UDRAop-D ≤ 9.65 6.85 3 4.85 < UDRAop-D ≤ 6.85 4.85 2 3.40 < UDRAop-D ≤ 4.85 3.40 1 2.40 < UDRAop-D ≤ 3.40 2.40 0 1.70 < UDRAop-D ≤ 2.40 1.70 -1 1.20 < UDRAop-D ≤ 1.70 1.20 -2 0.85 < UDRAop-D ≤ 1.20 0.85 -3 0.60 < UDRAop-D ≤ 0.85 0.60 -4 0.43 < UDRAop-D ≤ 0.60 0.43 -5 0.30 < UDRAop-D ≤ 0.43 0.30 -6 0.21 < UDRAop-D ≤ 0.30 0.21 -7 0.15 < UDRAop-D ≤ 0.21 0.15 -8 0.11 < UDRAop-D ≤ 0.15 0.11 -9 0.08 < UDRAop-D ≤ 0.11 0.08 -10 0.06 < UDRAop-D ≤ 0.08 0.06 -11 0.04 < UDRAop-D ≤ 0.06 0.04 -12 0.03 < UDRAop-D ≤ 0.04 0.03 -13 0.02 < UDRAop-D ≤ 0.03 0.02 -14 0.01 < UDRAop-D ≤ 0.02 0.01 -15 UDRAop-D ≤ 0.01 0.005 -16 No accuracy prediction available—use at own risk For any time, tk, other than top-D, UDRA is found by, • UDRA = UDRAop-D + UDRA (tk – top-D) IS-GPS-200D 7 Dec 2004 186 30.3.3.8 Message Type 35 GPS/GNSS Time Offset. Message type 35, Figure 30-8, contains the GPS/Global Navigation Satellite System (GNSS) Time Offset (GGTO) parameters. The contents of message type 35 are defined below. The validity period of the GGTO shall be 1 day as a minimum. 30.3.3.8.1 GPS/GNSS Time Offset Parameter Content. Message Type 35 provides SV clock correction parameters (ref. Section 30.3.3.2) and also, shall contain the parameters related to correlating GPS time with other GNSS time. Bits 155 through 157 of message type 35 shall identify the other GPS like navigation system to which the offset data applies. The three bits are defined as follows; 000 = no data available, 001 = Galileo, 010 = GLONASS, 011 through 111 = reserved for other systems. The number of bits, the scales factor (LSB), the range, and the units of the GGTO parameters are given in Table 30 XI. See Figure 30-8 for complete bit allocation in message type 35. 30.3.3.8.2 GPS and GNSS Time. The GPS/GNSS-time relationship is given by, tGNSS = tE – (A0GGTO + A1GGTO (tE – totGGTO + 604800 (WN – WNotGGTO) + A2GGTO (tE – totGGTO + 604800 (WN – WNotGGTO))2) where tGNSS is in seconds, tE and WN are as defined in Section 20.3.3.5.2.4, and the remaining parameters are as defined in Table 30-XI. IS-GPS-200D 7 Dec 2004 187 Table 30-XI. GPS/GNSS Time Offset Parameters Parameter No. of Bits** Scale Factor (LSB) Effective Range*** Units A0GGTO Bias coefficient of GPS time scale relative to GNSS time scale 16* 2-35 seconds A1GGTO Drift coefficient of GPS time scale relative to GNSS time scale 13* 2-51 sec/sec A2GGTO Drift rate correction coefficient of GPS time scale relative to GNSS time scale 7* 2-68 sec/sec2 totGGTO Time data reference Time of Week 16 24 604,784 seconds WNotGGTO Time data reference Week Number 13 20 weeks GNSS ID GNSS Type ID 3 see text * Parameters so indicated shall be two's complement with the sign bit (+ or -) occupying the MSB; ** See Figure 30-8 for complete bit allocation; *** Unless otherwise indicated in this column, effective range is the maximum range attainable with indicated bit allocation and scale factor. 30.3.3.9 Message Types 36 and 15 Text Messages. Text messages are provided either in message type 36, Figure 30-9, or type 15, Figure 30-14. The specific contents of text message will be at the discretion of the Operating Command. Message type 36 can accommodate the transmission of 18 eight-bit ASCII characters. Message type 15 can accommodate the transmission of 29 eight-bit ASCII characters. The requisite bits shall occupy bits 39 through 270 of message type 15 and bits 128 through 275 of message type 36. The eight-bit ASCII characters shall be limited to the set described in paragraph 20.3.3.5.1.8. IS-GPS-200D 7 Dec 2004 188 30.3.4 Timing Relationships. The following conventions shall apply. 30.3.4.1 Paging and Cutovers. Broadcast system of messages is completely arbitrary, but sequenced to provide optimum user performance. Message types 10 and 11 shall be broadcast at least once every 48 seconds. All other messages shall be broadcast in-between, not exceeding the maximum broadcast interval in Table 30-XII. Message type 15 will be broadcast as needed, but will not reduce the maximum broadcast interval of the other messages. Type 15 messages that are longer than one page will not necessarily be broadcast consecutively. Table 30-XII. Message Broadcast Intervals Message Data Message Type Number Maximum Broadcast Intervals † Ephemeris 10 & 11 48 sec Clock Type 30’s 48 sec ISC, IONO 30 * 288 sec Reduced Almanac 31* or 12 20 min** Midi Almanac 37 120 min** EOP 32* 30 min UTC 33* 288 sec Diff Correction 34* or 13 & 14 30 min*** GGTO 35* 288 sec Text 36* or 15 As needed * Also contains SV clock correction parameters. ** Complete set of SVs in the constellation. *** When Differential Corrections are available. † The intervals specified are maximum. As such, the broadcast intervals may be shorter than the specified value. IS-GPS-200D 7 Dec 2004 189 30.3.4.2 SV Time vs. GPS Time. In controlling the SVs and uploading of data, the CS shall allow for the following timing relationships: a. Each SV operates on its own SV time; b. All time-related data (TOW) in the messages shall be in SV-time; c. All other data in the Nav message shall be relative to GPS time; d. The acts of transmitting the Nav messages shall be executed by the SV on SV time. 30.3.4.3 Speed of Light. The speed of light used by the CS for generating the data described in the above paragraphs is c = 2.99792458 x 108 meters per second which is the official WGS-84 speed of light. The user shall use the same value for the speed of light in all computations. IS-GPS-200D 7 Dec 2004 190 30.3.5 Data Frame Parity. The data signal contains parity coding according to the following conventions. 30.3.5.1 Parity Algorithm. Twenty-four bits of CRC parity will provide protection against burst as well as random errors with a probability of undetected error ≤2-24 = 5.96×10-8 for all channel bit error probabilities ≤ 0.5. The CRC word is calculated in the forward direction on a given message using a seed of 0. The sequence of 24 bits (p1,p2,...,p24) is generated from the sequence of information bits (m1,m2,...,m276) in a given message. This is done by means of a code that is generated by the polynomial 24 i g()X =ΣgiX i=0 where gi =1 for i =0,1,3,4,5,6,7,10,11,14,17,18,23,24 =0 otherwise This code is called CRC-24Q. The generator polynomial of this code is in the following form (using binary polynomial algebra): g() ( X =1 )() +XpX where p(X) is the primitive and irreducible polynomial 23171312119 8 7 5 3 ()=X +X +X +X +X +X +X +X +X +X +1 pX When, by the application of binary polynomial algebra, the above g(X) is divided into m(X)X24, where the information sequence m(X) is expressed as 2k −1 mX =mX +mX ⋅⋅+mX () k +mk −1k −2 +⋅ 1 IS-GPS-200D 7 Dec 2004 191 The result is a quotient and a remainder R(X) of degree < 24. The bit sequence formed by this remainder represents the parity check sequence. Parity bit pi, for any i from 1 to 24, is the coefficient of X24-i in R(X). This code has the following characteristics: 1) It detects all single bit errors per code word. 2) It detects all double bit error combinations in a codeword because the generator polynomial g(X) has a factor of at least three terms. 3) It detects any odd number of errors because g(X) contains a factor 1+X. 4) It detects any burst error for which the length of the burst is ≤ 24 bits. 5) It detects most large error bursts with length greater than the parity length r = 24 bits. The fraction of error bursts of length b > 24 that are undetected is: a) 2-24 = 5.96 × 10-8, if b > 25 bits. b) 2-23 = 1.19 × 10-7, if b = 25 bits. IS-GPS-200D 7 Dec 2004 192 (This page intentionally left blank.) IS-GPS-200D 7 Dec 2004 193