The Wideband Arecibo Pulsar Processor - File Format Page
WAPP File Format
WAPP files have three sections that appear sequentially in this order: an ASCII header description, the binary header, and finally the actual binary data. The first section is a raw copy of the C-language include file (wapp_header.h), terminated with a single null character. This is included for reference only and does not include any information specific to the observation. After the ASCII header comes the actual header values that describe the observation.
. The WAPP realtime observation engine does not produce files larger that 2 Gbytes. At one time this represented a fundamental limit to Linux files. However, the reason behind this limit is to avoid difficult to manage megafiles. As a consequence, most observations produce a sequence of data files. Each file will have it's own header, even when the files are simply continuations of a single observation. The only difference in header values in this case will be the timeoff parameter, which indicates the time offset from the start of the observation.
The most recent version of this structure is maintained in the ~wapp/include/wapp_header.h subdirectory. Although the include file is available, it is recommended that the user extract the ASCII header from the WAPP data files. This approach will avoid problems with header versions. To view the header file embedded in each data file, type the following:
more wapp.nnn[ret]
WAPP files are written by a i386 Linux machine and use the native byte ordering of that machine: little endian. Thus in memory a 16 bit integer will have it's most-signficant byte appear first. For example, decimal integer 4600 (1234 hex) would appear in memory as follows:
The final portion of the WAPP data file is the actual correlation values. It is written as raw unsigned integers value of 16 or 32 bits (see lagformat). It is written such that lag 0 appears first, then lag 1, etc. If there are multiple IFs, they appear as A, then B, then ?x?(AxB or BxA) and finally ?x?.
WAPP Header Parameter Description
The following table describes the various parameters in the WAPP Header file. The details of this header may evolve, so it is always useful
| Variable | Type | Bytes | Description |
| header_version | long int | 4 | Header revision, presently 1 |
| header_size | long int | 4 | Number of bytes in header (binary portion only) (nom.=2048) |
| src_ra | double | 8 | Requested RA J2000 (10000*hr+100*min+sec) |
| src_dec | double | 8 | requested dec J2000 (10000*deg+100*min+sec) |
| start_az | double | 8 | telescope azimuth at start of scan (deg) |
| start_za | double | 8 | telescope zenith angle at start of scan (deg) |
| double | 8 | AST at start of scan (sec) | |
| double | 8 | local siderial time at start of scan (sec) | |
| cent_freq | double | 8 | Center Frequency of Observation (on Sky) in MHz, example 1500.0 |
| obs_time | double | 8 | User requested period of observation on this source in seconds. |
| samp_time | double | 8 | user-requested sample time (us) |
| wapp_time | double | 8 | actual sample time (us) i.e. requested+dead time |
| bandwidth | double | 8 | total bandwidth (MHz) for this observation (50 or 100) |
| num_lags | long int | 4 | User-requested number of lags per dump per spectrum |
| scan_number | long int | 4 | An integer built by WAPP from year+daynumber+3-digit-number |
| src_name | string | 12 | Source Name (usually Pulsar name) example: B2110+27 |
| obs_date | char | 12 | built by WAPP from yyyymmdd |
| start_time | char | 8 | UT seconds after midnight (start on 1-sec tick) |
| project_id | string | 8 | user-supplied AO proposal Number (XYYYY) |
| observers | string | 24 | user-supplied observer(s) name |
| nifs | int | 4 | Number of IFs to be recorded (1,2 or 4 for full stokes) |
| level | int | 4 | Sample Quantization 3 level (level=1) or 9 level (level=2) |
| sum | int | 4 | Is the correlation a summation of two IFs, 1 = Yes, 0 = No. |
| freqinversion | int | 4 | Is the band inverted (due to a high-side conversion) 1=Yes |
| timeoff | long long int | 8 | Time offset from the start of the observation, important for muliple file processing and SNAP processing. Integer value which indicates the number of records (multiply by wapp_time to give elapse time) |
| lagformat | int | 4 | 0=16 bit unsigned integers, 1=32 bit unsigned integers |
| lagtrunc | int | 4 | (future) if we truncate data -> bits truncated (0 no trunc) |
| power_analog[2] | double | 16 | (future) This pair of values that will hold the total power meauserd by a diode detector. The two values are for each IF. |
| psr_dm | double | 8 | pulsar's dispersion measure (cm-3 pc) |
| rphase[9] | double | 72 | reference phase of pulse (0-1) |
| psr_f0[9] | double | 72 | pulse frequency at reference epoch (Hz) |
| poly_tmid[9] | double | 72 | mid point of polyco in (MJD) modified Julian date |
| coeff[144] | double | 1152 | polynomial coefficients calculated by TEMPO effectively [9][16] |
| num_coeffs[9] | int | 36 | number of coefficients |
| filler | char | 420 | Extra space to make header = 2048 bytes |
| ===== | |||
| 2048 | Total size of header (bytes) |
This document maintained by
wapp@naic.edu.
Material Copyright ©
2000 Arecibo Observatory - Cornell University