lband RFI measurements.
last updated 13jan16
high time resolution power levels in lbw (checking the radar
rfi power levels in lbw receiver.
04apr18: GPS L3
signal remains on for 3+ hours.
p3058 puppi saturation from lband radars.
p3094 on/offs lbw. gns rfi.
gps L3 rfi in a2048 data for month of jul 2016.
puppi data sees gps satellite
13jan16: a2981 on 03jan16 saw
66Khz comb at 1667.5621 Mhz
1670-1675 rfi on/off transmitter tests with lightsquared.
08jul15 wapp data taken with 1670-1675 rfi present
1670-1675 Mhz rfi
added p2030 zaplists through 23jun15
monitor image of lband radars
p2030 pulsar search zaplists and periodicities created by puerto
punta salinas not using modeA starting jul14
05aug14: added monthly
plots of 1325 mhzband from the hilltop monitor system.
punta salinas blanking sector changes from 78 degrees to 22
degrees in around jun12
29nov13: fraction of time gps l3
was seen in A2811,A2754 data during nov13
check the duration of the punta salinas blanking (in the AO
punta borinquen radar blanking re-enabled.
1419.963,1667.957 birdies from dustbin controller on az arm
narrow band tone at 1200 Mhz.
measuring the punta Borinquen radar blanking in the AO
punta Borinquen carsr 1274,1332 Mhz radar
see broadband rfi in alfa
x111 dynamic spectra for lband nov/dec12
glitches in rf from alfa monitoring are only present with bias
17may12: galfacts see galileo
rfi at 1278.7 Mhz
creates intermods in alfa
shutdown of visitors center.
Mhz rfi coming from the AC unit atop building I.
Mhz rfi in alfa data.
16aug11: 1360 rfi azswing
narrow band rfi
polB compresses with longer recovery time.
intermods in galfa spectrometer.
14mar11- 1357.2 Mhz rfi.
01oct10: 1340 Mhz tone after
replacing radar blanker gasket.
28sep10: 1340 Mhz tone coming from
radar blanker lo.
27sep10: freq scan of
lbw. dynamic spectra, avg spc, rms/mean, tp vs time.
27sep10: gps l3 in a2010 data.
on for 26 minutes
08nov09: 1375 mhz rfi.
rfi in alfa coming from aerostat
seen in alfa/pdev
of mixer intermods for alfa/pdev
from lbw crosshead.
looking rfi in alfa data at 1403,1424, 1475 Mhz.
11apr09: gps l3 in
a2010 data jan09-mar09
20mar09: 1422.5 MHz birdie from tv channel 54.
L2, GLONASS L2, and COMPASS E6 rfi 1200-1300 MHz
1424 birdies seen in alfa.
Mhz birdie in alfa from aerostat.
baseband data take during gps L3 test.
comb seen 28mar07 in serendip 5 using alfa.
rfi plot of lbw around 1100 Mhz.
L3 test on 06dec06. A2010 data.
1548 Mhz birdie generated by the new cell phone detector
05sep06: A 1378 Mhz cfr, 3.2 Mhz wide
radar appears for 430 seconds.
salinas radar blanking test.
18jul06: az,za dependence of
rfi using transmitted birdies
17jul06: 1200 Mhz narrow birdie.
12jul06: 1188 birdie, 66 Mhz comb from
kronos timecard reader
06jul06: birdie near 1320 Mhz.
07apr06: distomat birdies after
the harmonics in the alfa/wapp receiver with cfr=1385.
30nov05: compression in
the alfa receiver with the 100 Mhz filter installed.
of 1350 faa radar aliases in alfa.
28sep05: 1300/1400 Mhz birdies coming
from the wapps.
phone harmonics in lbw when transmitting inside the dome.
baseline variation with long recovery in alfa (from faa radar).
12jun05: 1428 Mhz birdie in alfa 18may05 thru 23may05
compresses alfa receiver when oh filter bank amps were in.
1350 Mhz radar appears at 1380 Mhz in alfa survey data.
26feb05: punta salinas not staying
in mode A
Mhz drifting harmonics at lband.
18nov04: measure the distomat
birdies after some shielding work.
17nov04: punta salinas runs
with chn 18 (1367/1382) on during the night.
drifting 60 Mhz harmonics are not coming from the dome AC units.
14oct03: 1289.7 Mhz birdie for
a1803 (punta salinas??).
06oct04: faa,punta salinas,
and aerostat Radars. total power versus time.
28sep04: az dependence of faa
radar compression in alfa receiver.
signal compression by the faa radar.
02jul04: 1400 Mhz birdies
coming from 100 Mhz distribution/wapps
29jun04: birdies around
1400 Mhz while doing an azimuth swing.
harmonics in lband with 60 Mhz spacing.
30may04: distomat birdies after
communications changed from copper to fiber.
28may04/07jun04: 1374.495 Mhz
28may04: 1400 Mhz birdie.
13oct03: 1422.5 birdie from
tv channel 54
19sep03: lbw iridium signal
1290,1350Mhz radars at 1ms with lbw. Show rotation drift and
(using lbw,lbn) compression by the 1330/1350 FAA radar .
za dependence of 1350,1290, and aerostat radars.
04apr03. Radar power levels in the downstairs IF
Measuring the power levels at the output of the lbw dewar.
01mar02. 1400 Mhz biridie
with azimuth swing
06nov01. 1381 gps L3 birdie on
all the time.
05oct01. tone at 1350 Mhz.
Radar harmonics created at the 2nd IF.
caused by new air conditioner.
20sep01 lbn. Rfi
from color camera on the service platform.
from tertiary electronics and motors.
Measuring the power levels at the output of the lband wide dewar.
1407gr. 300Khz wide birdie at 1407 (tv station).
26dec00 lbn. Birdies near 1420
MHz. Azimuth swings with dome at 19 degrees za.
03jan16: a2981 see 66Khz comb at 1667.562
A2981 took interim cor data at the oh bands
(looking at a comet).
The data was 1 minute integrations using a hexmap pattern.
Ellen reported a 66khz comb in the 1667 Mhz band.
Data was taken on :
Processing the data:
- 20dec15,24dec15,26dec15,31dec15, and 03jan16.
- for each record of each day i fit a linear baseline and
- I then averaged the days data.
- The y axis for the spectra is in units of Tsys (since the
spectral baseline was close to 1.)
- the 7 minute hex maps were doppler corrected, so averaging a
days worth of data, spread any rfi by the doppler correction.
The plots show the daily
spectral averages for 1667 Mhz band (.ps) (.pdf)
- Each daily average has been offset for display purposes.
- The dashed lines show the rfi and comb
- f0=1667.5621 +/- .0659 Mhz
- The top frame show the entire bandpass (390Khz, 1024 channels)
- The bottom frame is a blowup around 1667.5621
- The data has been hanning smoothed
- In the daily integration, the line was not resolved in the
380 Hz channels.
20mar09: 1422.5 Mhz birdie from tv
Channel 54 is in the process of installing their
digital transmitter. Some of the shielding around the transmitter
has been removed to allow them to do this. The video carrier at
711.25 generates a 2nd harmonic at 1422.5 that we have seen in lband
On 20mar09 an azimuth swing was done from az=180
to az= 413 at .1 deg/sec. Lbw in linear polarization and the 1370
hipass filter was used to check the rfi. A 3.125 MHz band
centered at 1423.4 with 1.5 Khz freq resolution was recorded
using the interim correlator once a second (using 9 level sampling).
The wider band was used to see if the 1424 Mhz birdie that has been
reported recently was also coming from chn 54.
Since the birdies are narrow band, a median
filter of 34 channesl was used on each 1 second spectra to remove
The plots shows the
of the 1422.5 MHz birdie (.ps) (.pdf):
- Top: Averages spectra for the entire az swing (2345 seconds).
- 1422.5 is about 1.06% of Tsys on average in a 1.5 KHz
- 1424.051 and 1422.082 are flagged in red and green.
- Bottom: power in 1.5 Khz channel vs azimuth for each rfi
- No hanning smoothing was done on the frequency spectra
before using the channels.
- The azimuth motion was smoothed to .9 degrees (9 secs at .1
- Black is 1422.5, red is 1424.051, and green is 1424.082.
- The dashed blue line is at the azimuth of chn 54: 196
degrees (from web antenna web site.. I'm not sure of the
accuracy of this number. It probably originates from the fcc
- The dashed purple line is 90 degrees from the tower
direction: 286=196+90. There is a peak in the rfi
here. The azimuth arm is 90 degrees from the direction of the
- The signal to noise in the 1424.051 Mhz birdie is not large
enough to verify that it is coming from chn 54 tower. Probably
need to do the swing with a lot narrower bandwidth.
- The strongest signal was 11% of Tsys in 1.5KHz chan at an
azimuth of 178.6 degrees.
- The 1422.5 Mhz harmonic from chn 54 is 11% of Tsys in 1.5 KHz
channel at az=178.6
- There is another az peak of the 1422.5 MHz birdie at az=286.
This is 90 degrees from the chn 54 tower direction (196
az). It occurs when the azimuth arm is perpendicular to the
- A 1424.051 and 1424.082 birdies are also seen but are too weak
in the 1.5 Khz chan to determine their az dependence.
- Data taken on 13oct03 also had an azimuth peak for the 1422.5
MHz birdie at az =169.8 deg.
- The telescope triangle is oriented 2.87 degrees east of true
north. This puts the southern face of the triangle pointing at
- chn 54 tower is 196-182.87 = 13.13 degrees east of the
southern triangle beam.
- 182.87 - 13.13= 169.74 degrees. So the large birdie at 169.9
is a specular reflection off of the southern face of the
0n 05sep06 around 3:43 am (ast) a radar appeared
for about 400 seconds. The project a2010 was doing 600 second drift
scans (1 second sampling) using alfa. The telescope was parked at
az=360,za=4.2. The radar specs were measured to be:
05sep06: A 1378
Mhz cfr, 3.2 Mhz wide radar appears for 430
The first image is the dynamic
for beam 3 polA (.gif) for the 600 seconds of data.
- It was seen in all 7 beams of alfa and both pols.
- period: 12 seconds
- amplituded about 10% of Tsys.
- centered at 1378.8 Mhz.
- bandwidth: 6.2 Mhz. FWHM:3.2 Mhz
- sidebands +/- 13.2 Mhz from the center
- It lasted for about 430 seconds. It only appeared once during
this nite in the data.
- The birdie appears to be aligned in time with the faa radar
(1350 Mhz). The two pulses drifted slowly (3 seconds over the
430 seconds of data) so the alignment is probably a
- The radar increases in strength for about 2 minutes, stays
relatively constant for about 4 minutes and then takes about 1
minute to decrease to zero.
The 2nd plot has the average
and the time variation of the rdr (.ps) (.pdf):
- The new radar stretches from 1375 to 1380 Mhz. You can
barely see the sidelobes at 1366 and 1392. The radar at 1350
is the faa. It has harmonics at 1380, 1405, 1410.
What is the radar isn't:
- Top: The average spectra when the radar was
pointing at the AO. Black in polA, red is polB. Vertical flags
have been placed at: faa radar ,
side bands, and the center of the new radar.
- Middle: The time series for the faa radar (brown flagged channel) and the that
of the new radar (blue flagged
channel) are plotted versus time (with 1 second resolution).
At first glance the peaks overlap each other. This looks like
the birdie is coming from the FAA radar.
- bottom: A blowup in time of the middle plot. Each
sample is now marked with an *. You can see that the new
radar peak starts 1 second after the faa radar. It then drifts
so after 300 seconds it is 1 second before the faa peak. The
rotational periods of the two radars differ by 3/300 or 1%. So
the new radar is not coming from the FAA radar.
- It's not the faa radar
- If it's the aerostat then it must be a second harmonic since
the 3.2 Mhz fwhm is twice the bandwidth of the aerostat radar
(1.6 Mhz). It's appearance and then disappearance during the
scan is hard to explain since the aerostat and the gregorian
dome were stationary (at least i assume the aerostat was
stationary). The frequency does not match the known aerostat
harmonics in the iflo.
To check if birdies are coming from the control
room area you could compare the az,za dependence of the unknown
birdies with the az,za dependence of known birdies from the control
az,za dependence of rfi using transmitted birdies. (top)
On 14jul06 birdies were transmitted from the
control room and from the control parking lot. Data was then
recorded with the lband wide receiver in linear polarization
mode while the dome was swung in azimuth and then stepped in
za. The setup was:
Various plots were made to show how the power
varied. The azimuth swings were broken up into 4 90 degree plots to
give better plot resolution. Each of the plots may have different
- Use short whip antennas from the cell phone detector for the
xmit antenna (antennas with the white dots).
- Transmit 1391.953674 Mhz from the table next to the observer
in the control room.
- Transmit 1390.902344 Mhz from the control room parking lot.
The antenna was hung from the rail in front of the central
- Use two Hp synths for the power source. Set the amplitude to 0
- Set the interim correlator to 390 Khz bw and 1024 channels
giving 762 hz resolution after hanning smoothing.
- Swing the azimuth from -90 to 270 azimuth and .3 deg/sec. Do
this for dome za's of 19,17, and 15 degrees. Sample the data at
- The control room and the parking lot were
chosen to compare different locations. We assumed that the
transmitted power was constant for each location. This lets us
measure the change in received power caused by the motion of the
azimuth and za at each location. We did not measure the absolute
transmitted power from each location so we can not say how good
the control room shielding is compared to the parking lot. The
data processing was:
- Input and hanning smooth the data.
- Compute the median baseline between the two birdies for each
- Subtract the median baseline from each sample. This should
remove continuum sources and the sidelobes from the sun.
- Compute the electronic gain by taking the median over all
samples of the baseline computed in 2. Divide the results of 3
by this value. This should put the data in units of Tsys
(averaged over the 3 za's).
- Extract the time series for the two birdies (polA and polB are
- Compute a robust mean for each time series over all az, za and
then divide each time series by this mean. The data should be
normalized to the mean power received over all az, za.
The plots show the
dependence of the xmitted birdies (.ps) (.pdf):
- Page 1: az,za. Pol A from the parking lot. Peaks around az =
(-25 to -20) degrees.
- Page 2: az,za Pol B from the parking lot. Peaks around az=(-25
to -20) and (175 to 180) degrees.
- Page 3: az,za Pol A from the control room. Peaks near az=(-30
to -20) deg.
- Page 4: az,za Pol B from the control room. Peaks near az=(-20
to -5) deg.
- Page 5: az,za (polA + polB) Parking lot.
- Page 6: az,za (polA + polB) Control room.
- Page 7: az,za = 19. (polA + polB). Compare parking lot and
control room (Black = parking lot, Red=control room)
- Page 8: az, za=19. Control room. Compare polA and polB
(black=polA, red=polB). Around az=-20 you can see that the polA,
polB peaks are shifted by a degree or two.
- The peaks can be narrow. The spacing between the peaks is
determined by the length of the scattering member.
- PolA and polB peak at different az.
- The peaks shift in az when the za changes.
- The strength in the peaks can change by factors of 60
depending on the azimuth and za.
- Repeat this another day to see if the peaks are repeatable
(maybe small changes in the platform/tiedowns will shift them).
- Fill in the other za's.
Project A2049 was doing double position switching
using lbw on 15jul06. The source A1852 (scans 619601431 -
1434) had a narrow birdie at 1200 Mhz. The birdie properties were:
Mhz narrow birdie (top)
The plots show the az
dependence of the 1200 Mhz birdie (.ps) (.pdf):
- The birdie was not resolved in a 25 Khz channel width (after
- The previous scans were centered .5 Mhz away and they still
had the birdie centered at 1200 Mhz. This implies that the
birdie is not a harmonic created within the IF.
- The birdie has an azimuth dependence. When the on/offs
retracked the same az,za the birdie strength repeated.
- The x111 rfi monitoring birdie also shows this 1200 Mhz
birdie. I looked at data from the start of the year (2006) and
it was present (this was the first date i looked at, not the
first time it appeared).
- This could be the 100 Mhz harmonic birdie caused by the
wapp lags being driven at 100 Mhz (see 1400 Mhz birdies from the wapps).
Narrow birdies at 1300 Mhz and 1400 Mhz were not seen (or they
were very stable). A small birdie at 1600 Mhz was seen.
You can see that the 1200 Mhz biridie repeats for
the same az, za. This shows that the birdie is not coming from
inside the dome.
- Fig 1: top plot is polA, bottom plot is polB. The data at 1200
Mhz is plotted after being normalized by adjacent
channels. The on,off source and on,off calibrator are
plotted in different colors.
note on DPS pattern:
- The double position switching did not repeat the same az,za
for the on,off source.
- The off was done first followed by the on. Looks like the on
src started about 1 degree in azimuth late.
We are installing a new employee time card system
made by kronos. The readers will be distributed at various locations
at the observatory for employees to punch in. The current system was
checked for rfi and found to have a 66 Mhz comb that extends through
lband. The computer and reader are in a single plastic box.
12jul06: 1188 Mhz
birdie, 66 Mhz comb from kronos timecard reader. (top)
Data was taken on 12jul06 with the lband wide
receiver to check if the rfi got into the receiver. The interim
correlator was used with a 25 Khz channel resolution. Data was taken
while the kronos device was in the control room and then when it was
in the parking lot next to the control room. A few minutes of data
was taken with the device on and then with the device powered off.
There was no ethernet cable plugged into the device when the data
The plots show the rfi
created by the device at 1187.87 Mhz (.ps). (.pdf)
- The plots show KronosOn/kronosOff -1. The black plot is the
device in the control room (3 minutes on). The red line is the
device in the parking lot (4 minutes on). The vertical scale is
fraction of Tsys for lbw (about 32 K at 1188 Mhz).
- In the parking lot, the birdie at 1187.87 Mhz is .2% of
Tsys . It is about .5 Mhz wide.
- In the control room we can not see the birdie. This is
probably because the shielding on the control room windows gives
about 20 db of suppression.
- The birdie is a comb that is spaced about every 66 Mhz.
- The control room data was taken with the az=360 and the za=18
degrees. The parking lot data was taken with the az=306 and the
za=18. These are not the az, za that give the maximum
leakage into the dome (see az
of 1400 Mhz birdie (.ps) page 2 center plot). At the
maximum position the single could be 13 db higher
project a2049 was doing double position switching
with various sources on 06jul06 when a birdie close to 1320 Mhz
appeared. The plots show the birdie
1320 Mhz (.ps) (.pdf):
near 1320 Mhz: (top)
of the 15th scan (.gif) was made to show the frequency jump.
- Page 1: on/off-1 with display offsets. The top plot is polA,
the bottom plot is pol B. Each colored line is on/off-1 for a
particular source. Different on/offs were centered at slightly
different frequencies. Dashed vertical lines are drawn close to
where the problem occurs.
- Most of the on/offs have the birdies going negative. It
turns out the birdie is fairly stable. The on/offs go negative
because of the continuum in the ons.
- When the center frequency changes, the birdie remains at the
same frequency. This tells us that the birdie is not an
intermod or harmonic of some other birdie (if a harmonic then
it would move differently than the 1st lo).
- Page 2: rms/mean by channel for each scan. Each colored strip
is the rms/mean by channel for a particular scan. The
colors show on/off pairs. These plots are made in time order
(bottom to top). An offset has been added for display. The two
vertical lines flag the frequencies of the problem.
- Within most scans the birdie is very stable. The exceptions
(where we see a larger rms near the birdie) are caused by the
birdie moving in frequency within the scan.
- Page 3: average bandpass when the birdie frequency jumped.
This is the 15th scan pol A (2nd from top strip on page 2). The
rms/mean was large in page 2. The black line is the average of
the first 30 seconds of the scan, the red line is the average of
the last 30 seconds of the scan. The bandpass was flatten using
the source deflection from the first pair of the day.
- You can see two peaks before and after the frequency jump.
The jump was about .9 Mhz. The fwhm of the birdie is about 2
What we know:
- At 190 seconds into the scan the frequency jumped from 1318.94
to 1319.79 (.9 Mhz).
- The birdie was seen at 1318.9 and 1319.79.
- It is about 2 Mhz wide and .1 Tsys high.
- When sitting at 1 frequency it is relatively stable in
frequency. The rms is small for an entire scan. This rules out
any strong az dependence. It hints that it may be inside the
- Stays at the same rf frequency when the first lo is
changed so it is probably not a harmonic.
- If it is an external birdie then it remains very stable even
as we move in azimuth.
The windows on the distomat cans were replaced in
feb/mar06. On 07apr06 lbw was used in linear polarization mode
centered at 1400 Mhz with a 12 Khz resolution. 180 one second
records were taken while pointing at each of the 6 distomats (the
dome was at 18 degrees za). The 6 distomats were measured while
moving clockwise and the 5 distomats were measured moving
counterclockwise (we ran out of time to complete the ccw swing).
distomat birdies after window replacement. (top)
The distomats were set to run on a 60 second
measurement cycle so there were 3 complete cycles of turnon, turnoff
while looking at each distomat. About 7 seconds after the minute the
distomats begin their distance measurements. The data was
synchronized to the start of the 60 second cycle. Birdies were seen
at 1399.84 and 1400.01 Mhz. I averaged the clockwise and
counter clockwise swings. The plots show the current state of the
The distomats with the birdies have not changed since the new
windows were installed ( see
nov04 measurements). In nov04 the birdies lasted for a longer
period of time. That is probably because the old screens were not as
transparent as the new ones so it took longer to integrate up to the
signal to noise that the distomats needed. The strengths look a
little smaller in nov04. This is probably because the nov04
bandwidths were 25 Khz while the apr06 measurements used 12 Khz (the
birdies are smaller than 12 khz).
(.gif)) dynamic spectra (time vs frequency of
spectral density). The 6 images correspond to the 6 distomats.
Horizontal dashed lines are drawn at the start of each 60 second
cycle. You can see the 1399.84 and 1400.01 Mhz birdie in
distomat 3 (pol B is stronger). The spectral resolution used did
not resolve the birdies.
for 12 kHz channels at 1399.84 and 1400.01 Mhz (.ps)
plots shows the time series (180 seconds) for a single frequency
channel. Page 1 is at 1399.84 while page 2 is at 1400.01 Mhz. On
each page the 6 plots correspond to the 6 distomats. Black is
polA, red is polB, and green shows where the distomats turned
on. The birdies strengths are:
- Distomat 3 has the strongest birdie. It is stronger in polB
than polA. It is a little stronger at 1399.84 than 1400.1 (.1
vs .08 Tsys).
- Distomat 4 has a little of the 1400.01 birdie. the strength
is about 3% of Tsys.
We looked at distomat 3 with the portable
spectrum analyzer and a we saw the 1400 mhz birdies. They were
coming from the front of the distomat (through the window).
Compression in the alfa receiver with the 100 Mhz RF filter
A filter bank was installed in the RF after
the alfa dewar. It has a set of switch able 100 Mhz filters centered
at 1440 Mhz as well as the straight through option (no
filter). These filters should help reduce the compression of
the system caused by the radars. The main culprit is the FAA radar
at 1330/1350 Mhz.
The faa radar specs are:
- 5 usecond pulse
- 2.5 millisecond average prf (pulse repetition frequency). 5
ipps close to 2.5 milliseconds used.
- 12 second rotation period.
It transmits at 1330 and 1350 Mhz every prf (one pulse at 1330
followed by 1350).
On 30nov05 data was taken with the wapps in pulsar mode with the
100 Mhz filters in. The setup was:
The data processing of the data was:
- the telescope was stationary at az=340, za=16.6 degrees. 340
azimuth had large compression when the
swings were done back in sep04 (.pdf) . (although
the dome is now at 16.6 rather than 18 degrees za).
- 3 level 100 Mhz was taken centered at 1440 Mhz.
- Data was sampled at 128 Usecs. for about 300 seconds.
- The alfa rotator was set to 0 degrees.
The plots show the results of the measurements:
- Compute the total power for each time sample.
- Remove Tsys by normalizing to a 1 second median and then
- Plot the data and find all the points that are 7% below Tsys
(100 MHz and 128 usecs gives a delta Tsys of .008).
- Knowing the 5 periods of the FAA radar find the phase of the
faa compressed signal in the data.
- compute the "best" rotation rate for the radar so the 12
second pulse remain remain stationary for the az spin time.
time when the faa radar pointed at the observatory (.ps)
- The total power (100 Mhz) vs time is plotted.
- There is 1 page for each beam of alfa. On each page polA in
on the top and pol B is on the bottom
- Each plot shows 250 milliseconds when the FAA radar pointed
at the observatory. There are 10 consecutive ipps plotted (12
seconds per ipp). The vertical scale is in units of Tsys
(with offsets for display).
- Negative going spikes separated by about 2.5 milliseconds
and lasting for 80 milliseconds are the alfa system
compressing when the faa radar beam sweeps through the
- You can see compression in beams 0b, 1b, and 6b.
between adjacent negative going spikes (.ps) (.pdf):
time between adjacent negative going spikes is plotted. The faa
radar uses 5 separate ipps. These are plotted in red. The
measured time differences match up with the expected ipps of the
faa radar (to within the 128 usec sampling). This proves that
the compression is being caused by the faa radar (and not say
- The alfa filter bank reduced the compression in the alfa
- Compression was only seen in 3 pixels (0b,1b, and 6b).
The maximum value was about 10%.
Narrow birdies have previously been seen at 1300
and 1400 Mhz (more info: 2004). Some of these
birdies were found to come from the correlator/wapp room. During the
summer of 2005 the shielding of the doors and the windows of the
wapp/correlator room was worked on. Measurements were taken on
28sep05 to see any remaining signals.
1300,1400 Mhz birdies coming from the wapps. (top)
- An lband helix antenna and the Tektronix portable
spectrum analyzer were used to measure the strength of the
signal coming from the clock room (rbw 100 hz, Tsys about 700
- The measurement was made at the entrance to the walkway
between the control building and the cliff (next to the
- The strength of the birdie at 1400 Mhz varied between 20 and
25 db (above the noise floor). In sep04 the value was closer to
30 db above the noise floor (same setup).
- Walking down the walkway (between the cliff and the building)
the signal got as high as 40 db above the noise floor.
Data was also taken with the lbw receiver (in
linear pol mode) using the interim correlator. The
correlator was set to 95 hz resolution (190 hz after hanning
smoothing) . 2 bands were centered at 1300.001 and 1400.001 (the
offset was used to get away from the spike in the center of the
interim correlator). To verify that the signal was coming from the
wapp/correlator room, the 100 mhz input reference for the wapps
was taken from an hp synthesizer. Data was taken with the
synthesizer locked to the station clock and also with the
synthesizer free running. When in free running mode, the 100 Mhz
frequency shifted down by 85.4 hz. This offset got multiplied by
13 and 14 to get to 1300 and 1400 Mhz. The plots show the results
of the measurements:
Mhz birdies when the 100 Mhz reference was unlocked (.ps)
was taken with the wapps unlocked (red line) , locked (black
line) , and then unlocked (red line). Each section lasted for
about 60 seconds. The dome was at 19 deg za and the az was at
327.439 deg (this position was a maximum for the 1300 Mhz polA
birdie). When the reference to the wapps was unlocked the
birdies moved away from 1300/1400 Mhz. When the reference was
relocked, they moved back. When shifting, a residual birdie
remained at 1300 Mhz. This is probably because there are other
sources of 100 Mhz that we were not drifting (the interim
correlator, the 100 Mhz in the dome). Since 1300 is an odd
harmonic of 100 Mhz you would expect it to be stronger than 1400
Mhz. The top plots are polA while the bottom plots are polB. The
birdie strengths (in units of Tsys) are:
the birdie versus azimuth (.ps) (.pdf):
sets of azimuth spins were done over 1.5 hours (colors
differentiate the different spins). The first (black) swing was
done at .4 deg/sec. The last 3 used .1 deg/sec. The 3rd (green)
and 4th (blue) swings went over the same azimuth twice. The za
was set to 19 degrees. The vertical scale is linear in power
(the baseline would be Tsys).
- Fig 1 top: The 1400 Mhz polA birdie versus azimuth. The max.
value is about 1*Tsys.
- Fig 1 2nd plot: 1400 Mhz polB. The max. value is about
- Fig 1 3rd plot: 1300 Mhz polA. The max. value is 4*Tsys.
- Fig 1 bottom: 1300 Mhz polB. The max. value is 4*Tsys
- Fig 2 top: A blowup in az of the 1300 polA swings 2
and 4 shows that the birdies FWHM are about .8 degrees in
- Fig 2 bottom: This shows the platform vertical motion during
the measurements. The dashed lines show when each az swing was
taken. During the 3rd (green) swing there were no distomat
measurements (it was pouring rain).
- A large fraction (over 50%) of the 1300/1400 Mhz birdies
are coming from the wapps (the correlator chips are being
clocked at 100 Mhz).
- The 1300/1400 Mhz birdies maximums do not come at the same
- Within say 1/2 hour the azimuth positions of the peaks are
- The maximum 1400 Mhz birdie in sep04 was about 25*Tsys. It is
now about 3*Tsys so the shielding has improved things.
- The signal strength at the entrance to the corridor between
the cliff and the control room (measured using a lband helix and
spectrum analyzer) decreased from 30db (sep04) to about 25db.
punta salinas transmitter left at wrong frequencies?? (top)
On 26feb05 project A2010 was doing 100 Mhz
drift scans centered at 1385 using alfa starting at 21:15 ast.
Interference that looked like punta salinas radar was seen in the
band. Punta salinas was called at 21:25 ast and they claimed they
were in mode A (channels 2,3). The plot shows the
monitoring 1325 Mhz band for 26feb05 (gif). Lighter color is
Each punta salinas channel has two frequencies
separated by 15 Mhz. Each channel pair is flagged with a different
color. For most of the day there was power in :
It seems unlikely that punta salinas was in mode a and someone else
was using most of their channels...
- Chan 3,4 (mode A)
- Chan 6,7 (mode B)
- chan 10,11 (mode C)
- chan 18 (diagnostic channel)
- chan 5 (maybe).
The birdies at 1330/1350 Mhz are the faa radar. The 1290 signal is
the remy radar.
distomat birdies still there after added shielding. (top)
The shielding for the distomats was worked on an
then more measurements were taken. On 18nov04 lbw was used in linear
polarization mode centered at 1395 Mhz with 24 Khz resolution. 180
one second records were taken while pointing at each of the 6
distomats (the dome was at 18 degrees za). The distomats were set to
run on a 60 second measurement cycle so there were 3 complete cycles
of turnon, turnoff while looking at each distomat. About 5 seconds
after the minute the distomats begin their distance measurements.
The data was synchronized to the start of the 60 second cycle.
Birdies were seen at 1400 Mhz. The plots show the current state of
The distomats with the birdies have changed since the may04 measurements.
(.gif)) dynamic spectra (time vs frequency of
spectral density). The 6 images correspond to the 6 distomats.
Horizontal dashed lines are drawn at the start of each 60 second
cycle. You can see the 1400 Mhz birdie in distomats 3 and 4
(although 4 is weaker). The spectral resolution used did not
resolve the birdie (so they may be in other distomats at higher
for 24 kHz channels at 1400 Mhz (.ps) (.pdf).
plots shows the time series (180 seconds) for a single frequency
channel at 1400 MHz. The 6 plots correspond to the 6 distomats.
Black is polA, red is polB, and green shows where the distomats
turned on. They remained on for about 13 seconds. The strongest
signal in polA of distomat 3 where the strength is about 4% of
On 29mar05 we went down and looked at distomats
3,4,5,6 using the portable spectrum analyzer and a helical lban
antenna. With a 50 Khz span we saw the 1400 Mhz birdie in all of
these distomats. It was strongest in D3,D5, and D6.
On 17nov04 project A1961 had two radars at 1367
and 1382 Mhz. They observed during the early morning hours. A peak
hold of a 162 second scan showed that this was punta salinas channel
18 which had been left on. The plots
show the radar signature (.ps) (.pdf).
radar.. punta salinas chn 18 left running.
The hilltop monitoring showed that punta salinas was in mode A,B,C
and channel 18 on from midnight 17nov04 till about 15:00 17nov04.
This only occurred on 17nov04. 16nov04,18nov04 did not have this
- Top plot: Peak hold by channel for 162 seconds of the wapp
data. The black and red lines are polA and polB. The dashed
color lines are the frequencies for the long range (blue) and
short range (green) pulses transmitted by the radar.
- Middle: A blowup of the 1367 Mhz pulse. You can see the wide
frequency, lower power density of the short range pulse, and the
narrower frequency higher density of the long range pulse.
- Bottom: A blowup of the 1382 Mhz pulses.
On 06oct04 data was taken with pixel 2a,2b of the
alfa receiver. The band was centered at 1410 Mhz. The front panel
outputs were passed thru attenuators (pix2a 15db, pix2b 0b) and then
detected with a 20 usecond time constant. The signal was then
sampled at 10 usecond sampling for 48 seconds. The telescope was
parked at az=270, za=1.9 degrees.
salinas, and aerostat Radars. total power versus time.
The attenuated version was to let us see an
unclipped version (in the A/D's) of the radar signal when it pointed
at the AO. The unattenuated version was to see detail when the
radar was not pointed directly at us. These plots can be used as a
reference when someone is looking at a square law detected
signal on the oscilloscope in the control room.
- Top: The un attenuated version of the data. Dashed
colored lines have been drawn where the faa, punta salinas,
and aerostat radars appear. The red dotted lines are the
sidelobes of the aerostat as it spins around (when the
aerostat points at AO, it blanks its transmitter).
- Bottom: This is the data with the 15 db of
radar power versus time. (.pdf)
The FAA radar at 1330 and 1350 Mhz was flagged with the
green lines in the first plot. It has a 5 useconds pulse
at 1330 Mhz followed by a 5 usecond pulse at 1350. This gets
repeated every ipp useconds. There is a sequence of 5 ipps that
are used by the radar (2500 to 2500 useconds). The radar does
not blank when it points at the observatory.
- Fig 1: This has 120 milliseconds centered on when
the faa radar pointed at the observatory. You see the
beam sweep by the observatory. This repeats every 12 seconds
(11.985 seconds seemed to give the best alignment of the 4
beams). The FWHM of the beam is about 50 milliseconds. This
translates to a beam width of 1.5 degrees (.05/12 *360). This
data was taken from pix2a with the 15 dbs of attenuation. The
red and green dashed lines show the data that is used for
- Fig 2: The top figure shows 6 ipps. The measured
spacing (with 20 Usec time constant) is close to that measured
with higher time resolution (i'm calling it the exact
version here). The bottom figure is single radar pulse. It has
been smeared out by the 20 usecond time constant.
Salinas frequency agile radar. (.pdf)
punta salinas radar was flagged with the blue lines in the first
plot. It is a frequency agile radar (more
info..). It transmits two frequencies simultaneously and
can hop between up to 20 different pairs. On each transmission
there is a narrow strong pulse and a wider weaker pulse for the
short range and long range detection. The rotation rate of the
radar is 12 seconds. They supposedly blank in the
direction of the observatory.
- Fig 1: Shows the 4 times when the radar pointed at
the observatory. There looks like there are two separate
beams in each plot. The central blanking last for 40
milliseconds. The beam width is about 100 milliseconds min to
min (looking at the 3 sweep). The third picture also shows
that there are two blanking periods. During each blanking
period all of the pictures show two strong pulsed during the
- Fig 2: The top plot is the first sweep of fig 1.
The colored lines show that data used for the following plots.
The second plot has the two strong pulses during the blanking
period. The first pulse is 800 useconds wide while the second
is 100 useconds. They are separated by 4.2 milliseconds. The
3rd plot shows 11 consecutive pulses. Each of them are spaced
by 1420 useconds. Using the entire data set, ipps of
[1490,1340,1670, 1420] useconds were found. The bottom figure
is a single pulse which is about 100 useconds wide. There is
supposed to be a 52 useconds narrow pulse and a 400 usecond
long pulse. The measured widths may be from the time constant
or two pulses following one another. The 800 usecond long
pulse in the middle of the nulling period is probably two of
the 400 usecond pulses.
balloon radar. (.pdf)
aerostat balloon radar flies above lajas (more
info..) . It has 4 frequencies it can transmit on
(1241, 1244,1256, and 1261). It usually transmits on a pair of
these. The pulse length is 160 useconds long with a band width
of 1.6 Mhz (it is chirped). The two frequency pulses are
transmitted back to back giving a 320 usec long transmission.
There are 7 ipps that are cycled through (2800 to 3800
useconds). The rotation rate for the radar is 12 seconds. It
blanks for about 1.4 seconds (42 degrees of azimuth) while it
swings by the observatory. There are strong back lobes when the
radar is at +/- 110 degrees from the observatory direction.
- Fig 1: The first figure shows the entire 48
seconds. The red lines are where the aerostat points at the
observatory (and is blanking). The green lines are 108 degrees
before the observatory direction while the blue lines are 115
degrees after the AO direction.
- Fig 2: This shows the 4 times the aerostat pointed
at the observatory. The blanking lasts for about 1.4
seconds. The relative size of the signal on both sides
of the blanking region shows how well the blanker is aligned
with our direction (it looks pretty good here).
- Fig 3,Fig4: These are the signals from the
aerostat when it pointing -108/+115 from the observatory
direction. Since it is symmetric about the AO position, it is
probably from a sidelobe/backlobe of the transmitter rather
than a reflection from a tower.
- Fig 5: The top figure shows 8 contiguous ipps. It
shows the 7 ipp sequence of the radar. The measured values are
within the time constant of the values measured with higher
time resolution. The bottom plot shows a single pulse. It is
367 useconds wide. This is close to the expected width of 320
useconds (given the time constant). The two peaks come
from the two different frequencies (160 useconds one frequency
followed by 160 of the second).
- The time variability of the 3 radars is shown.
- You can use the time variability to distinguish between the
- The rotation is the same for all 3
- The pulse widths are: 5 usecs faa, 100/400 punta salinas,
320 usecs aerostat.
- The aerostat blanks for 1.4 seconds when it points at the
- The aerostat has back lobes +/- 100 degrees from the ao
- punta salinas blanks for 40 milliseconds when it points at
the ao. It looks like it has a second blanking period.
- It would be nice to know what exactly the 2 punta salinas
pulses during there blanking period are doing.
- The punta salinas pulses were the strongest of any of the
other radars at this az,za position.
dependence of faa radar signal using alfa receiver. (top)
The faa radar specs are:
When the radar beam points in the direction
of the observatory there is a large signal at 1330 and 1350
Mhz. Frequencies outside the 1330/1350 range go into compression
(they decrease in power). The depth of the compression can be used
to measure the strength of the faa radar in the receiver system.
- 5 usecond pulse
- 2.5 millisecond prf (pulse repetition frequency)
- 12 second rotation period.
- It transmits at 1330 and 1350 Mhz every prf (one pulse at 1330
followed by 1350).
On 28sep04 an azimuth swing was done from az=270
to az=630 degrees. The azimuth moved at .3 degrees per second. The
dome was at 18 degrees za. A 100 Mhz band centered at 1420 Mhz
was sampled every 64 useconds using the alfa receiver and the wapps.
The alfa rotator was set to 0 degrees. The processing steps were:
The first plots shows power
time when the faa radar pointed at the observatory (.ps)
(using pixel 4 of the alfa receiver).
- Compute the total power for each time sample.
- Remove Tsys by normalizing to a 1 second median and then
- Plot the data and find all the points that are 10% below Tsys
(100Mhz and 64 usecs gives a deltaTsys of .0125).
- Knowing the 5 periods of the FAA radar find the phase of the
faa compressed signal in the data.
- compute the "best" rotation rate for the radar so the 12
second pulse remain remain stationary for the az spin time.
Each line has 120 milliseconds centered on when the radar pointed
at AO. Successive plots have been offset for plotting
purposes. While this data was being taken, the azimuth was
moving at .3 degrees/second. You can see that sometimes the
compression was large and sometimes it is hard to see. The
successive spikes are the radar pulses very 2.5-3 milliseconds. It
takes about 80 milliseconds for the faa beam to sweep by the
The second plot shows the
versus azimuth angle (.ps) (.pdf)
The 7 plots are for the 7 beams of alfa. The black color is polA
and the red color is polB. The farther negative the plot, the
stronger the compression. This is a linear scale. When the
value reaches -.4, the system temperature has compressed by 40%
(Tsys is now 60% of the original value). You can see large
values of compression near az=10,40,80, 340 degrees azimuth in
most of the beams.
02jul04: 1400 Mhz
birdie coming from 100 Mhz distribution/wapps. (top)
measurements showed that the 1400 Mhz birdie was not coming
from inside the dome. On 02jul04 eddie castro and myself looked in
the control room with the new tektronix spectrum analyze (ybt250)
with 100 hz resolution. The birdie was 20 to 30 db above the noise
floor (100 hz resolution) in the receiver room. The signal got to 50
db above the noise floor in the wapp,correlator room. We did the
It's not surprising that the wapps had the largest contribution to
the 1400 Mhz birdie since all of the correlator chips are running at
- We unlocked the phase lock loop for the 100 mhz distribution.
This caused the birdie to drift by many khz.
- We pulled the power on the 100 Mhz distribution and the birdie
in the receiver room went away.
- There were 3 cables leaving the 100 Mhz distribution:
correlator, receiver room, and wapps. We replaced the 100 Mhz
phase locked reference with a 100 Mhz from an hp synthesizer for
these 100 Mhz (one at a time). We monitored the birdie in the
receiver room while we did this. The portion of the 1400 Mhz
birdie that came from the "different" 100 Mhz reference would
jump be a few 100 hz (since the hp synthesizer was not synched
to the maser). Of the 3 signals, the wapps had the strongest
We spent some time looking at the strength of the signal outside
in front of the control room. The results were:
- The signal was barely visible (2-3 db above the noise floor)
in front of the main windows. The shielding placed on the
windows is really working.
- The signal is 10 db above the noise floor in the office to the
left of the control room (hectors office).
- At the entrance to the pathway between building 1 and the
cliff, the signal is 30 db above the noise floor. Walking down
the pathway, the strongest signals were coming from the plywood
around the air conditioners that were installed through the
building walls. This hole is breaking the faraday cage that
encloses the floor, ceiling, walls of the correlator/computer
rooms. Some signals are probably also coming from the
doors/windows that are also not shielded.
- The other 100 Mhz harmonics are also visible. The odd
harmonics are stronger (1300,1500..)
The azimuth spin looking at this birdie shows how a birdie from
the control room should vary as a function of azimuth. Any other
birdies with this azimuth dependence most likely come from the
birdies around 1400 Mhz during azswing. (top)
Birdies close to 1400 Mhz were looked at with
high spectral resolution during an azimuth swing. The correlator was
set to 190 hz resolution (after hanning smoothing). The telescope
was swung from az=540 to az=180 and then back at .4 degrees per
second. Data was dumped at 1 second intervals. Lbw was used in
linear polarization mode with the 1370 hipass filter in. The two az
swings took about 2000 seconds.
- The image
dynamic spectra shows the time variation during the
swings. The x axis is KHz from 1400 MHz. There are strong
birdies at 1399.987 and 1400.00 Mhz. A more intermittent birdie
is at 1400.009 Mhz.
- The power
the birdie channels versus azimuth shows how the signal
varies with azimuth. The three plots are for the frequencies
1399.983,1400.000 and 1400.009. The data covers 190 hz. The
black plot is the counter clockwise spin while the red plot is
the clockwise spin.
- Figure 1. This has the complete azimuth swings. The black
and red lines overlay each other so the signals are not coming
from inside the dome. The first birdie is strong at 0 and 180
degrees azimuth. The second birdie (1400) peaks at -20 degrees
azimuth. The 3rd birdie peaks around an az of 145 degrees. The
1400 birdie is 30 times the system temperature (in a 190 Hz
- Figure 2: This is a blowup of the horizontal scale about the
strongest peak for each birdie. The first two birdies overlap
exactly in azimuth (comparing the clockwise and counter
clockwise spins). The 3rd birdie has an offset of a few
degrees between the two spins.
- Figure 3: This plots the power in the birdie versus time
(minutes from midnight). The green lines at the bottom of the
plot mark the even minutes. 7 seconds after the even minutes,
the distomats begin their measurements. You can see that the
bottom birdie aligns with these even minutes so it is coming
from the distomats. The az of 140 is distomat 3. There is a
beating between the position of the azimuth and the distomat
turning on/off that make us loose some of the distomat birdies
from the other distomats (seedistomat az swing).
- there are 3 distinct birdies around 1400 Mhz. Previous
measurements with wider resolutions have missed this.
- All three birdies are coming from outside the dome.
- The strongest birdie is at 1400 Mhz. It is 30 times Tsys in a
190 hz channel and has maximum strength at an az of -20 degrees.
It was not resolved in a 95 hz channel spacing. This birdie is
coming from the 100 Mhz buffers in the controlcorrelator
room. Mainly the wapp's who are running at 100 Mhz.
- The second birdie at 1399.983 is about 2*tsys in 190 hz
channel and peaks up at 0 and 180 degrees.
- The 3rd birdie at 1400.009 is from the distomats (in this case
birdies still there after converting to fiber communications. (top)
The communications between the distomats and the control room were
converted from copper to fiber in xxxx 04. On 30may04 I
checked to see if the distomat birdies were still present after this
Lbw in linear polarization mode centered at 1400
Mhz with 48 Khz resolution (after hanning smoothing) was used. 180
one second records were taken while pointing at each of the 6
distomats (the dome was at 18 degrees za). The set of birdies used
was at 1399.83 and 1400.00 Mhz. The distomats were running on a 60
second measurement cycle. About 10 seconds after the minute the
distomats begin their distance measurements. The data was
synchronized to the start of the 60 second cycle. The plots show the
current state of the birdies:
The conversion of the distomat communications had little affect on
the distomat birdies that occur during the measurement (see 28apr02 measurements).
dynamic spectra (time vs frequency of spectral density).
The 6 images correspond to the 6 distomats. Horizontal dashed
lines are drawn at the start of each 60 second cycle. For polA
you can see the birdies at distomat 4 (1399.83) and distomat 6
(1399.83 and 1400). For polarization B you can see the birdies
at distomat 4 (1399.83,1400) and distomat 6 (1399.83, 1400).
This spectral resolution is a lot wider than the distomat birdie
so it is probable that you could see the birdies in the other
distomats if narrower channels were used.
for 48Khz channels at 1399.83 and 1400 Mhz. These plots
shows the time series for a single frequency channel. The top
plot is 1400 Mhz, the bottom plot is 1399.83 Mhz. The black
color is polA while the red color is polB. For each plot the
distomat values are number 1 (bottom) to 6 (top). The offsets
have been added for display purposes. The vertical dashed blue
lines are the start of each 60 second cycle. You can see the
birdie increase a few seconds after the start of each cycle. The
vertical scale in Tsys units. The strongest birdie is distomat 3
polB where it gets up to .15 Tsys in the 48 Khz channel
Project a1861 had a birdie at 1374.495
Mhz. Lband wide was used in linear polarization's with the radar
blanker enabled.. They did 5 minute on/off position switching. The
correlator was setup for 2048 channels over 12.5 MHz and 1 second
dumps. The birdie was less that 1 channel wide (6.1 Khz). The
plots (ps) (pdf)
show the birdie during 4 on/off patterns (300*2*4=2400 samples).
Mhz birdie. (top)
This birdie is coming from outside the dome. The birdie strength
correlates with azimuth position so it is probably ground
based. It does not correlate as well as the 1400 Mhz birdie above
and does not line up with the distomat locations.
- Fig 1 plots the 2400 samples. The top plot is polA
and the bottom plot is polB. The black lines are the birdie at
1374.495 Mhz (1 channel of 6.1 khz). The red lines are an
average of 6 adjacent channels. The step at 600 sample
boundaries are the start of new on/off pairs at different za's
(and possibly different correlator attenuations).
- Fig 2(polA) and Fig 3(polB) plots
(1374.495 MhzChn - avg6adjacent_channels) versus azimuth. Since
position switching was done, the telescope tracks the same part
of the dish for the on position and the off position. The black
lines are on source while the red lines are off source. Each
frame (4 per page) is a separate on/off pattern. The strength of
the birdie repeats with the telescope position. This means that
the birdie is not coming from within the dome. It also probably
means that is ground based (unless it is a geostationary
satellite). The average za's for the for on/off patterns are:
14.5, 12.4, 11., and 11.2.
- Fig 4 plots the birdie strength versus time. The top
plot has 1000 seconds with polA in black and polB in
red. The bottom plot is a blowup m(150 seconds) of
polA. The peaks look like sidelobes drifting through the
interference. They last from 20 to 25 seconds. To move through
the 5 peaks takes about 26 seconds. ian shape and lasts for
about 25 seconds.
On 07jun04 base band sampled data with 10 Khz bandwidth was taken
to see the high resolution structure of this birdie. The image
shows dynamic spectra for 500 seconds. The bandwidth is +/-
245 Hz about 1374.494794 Mhz. The frequency resolution of the
image is .6 hz. The top plot is polA while the bottom plot
is polB (lbw was set to linear polarization mode). The signal is
drifting by about 5 hz over 500 seconds. The sidebands of the
signal are spaced by 60 Hz. The instantaneous width of the line is
less than 1 Hz wide.
28may04 1400 Mhz birdie.
Project a1861 had a birdie at 1400. Mhz.
Lband wide was used in linear polarization's with the 1370 wave
guide filter in. They did 5 minute on/off position switching. The
correlator was setup for 2048 channels over 25 MHz and 1 second
dumps. The birdie was less that 1 channel wide (12.2 Khz). The
plots (ps) (pdf)
show the birdie during 2 on/off patterns (300*2*2=1200 samples).
There is a distomat located at an azimuth of 207 degrees (distomat
4). Normally the distomat birdies turn on for a few seconds every
120 seconds and then turn off. This birdie looks like it is on all
the time. The blowup of the polA birdie shows that it is the
beam sweeping thru the birdie that makes it go up and down, and not
the distomat turning on and off (that would not give a gaussian
profile). We need to look at this with higher resolution.
- Fig 1 plots the 1200 samples. The top plot is polA and the
bottom plot is polB. The black lines are the birdie at 1400 Mhz
(1 channel of 12.2 khz). The red lines are an average of 6
adjacent channels. The step every 300 samples is because the
source had some continuum flux. The birdie is stronger in pola.
- Fig 2 plots (1400 MhzChn - avg6adjacent_channels) versus
azimuth. Since position switching was done, the telescope tracks
the same part of the dish for the on position and the off
position. The black lines are on source while the red lines are
off source. The top plot is polA while the bottom plot is polB.
The strength of the birdie repeats with the telescope position.
This means that the birdie is not coming from within the dome.
The strongest position was close to an azimuth ot 206.2 degrees
azimuth (the za was 11.945 deg).
- Fig 3 plots the birdie strength versus time. The top plot has
polA in black and polB in red. There are 6 minutes between the
two peaks (5 minute on with a 1 minute move). The peaks in polB
(red) are occurring at 50 or 100 second intervals. The bottom
plot is a blowup of one of the peaks in polA. It has a gaussian
shape and lasts for about 25 seconds.
birdie during a1803 experiment. (top)
Experiment a1803 has been having problems with
radars at 1289.7 and 1298.7 Mhz. I took some data during the day on
14oct03 using lbn, the 1280Mhz hipass filter in, 25Mhz bandwidth
with 1024 channels, and 209 1 second dumps. I covered 1278.5 to
The 15 Mhz spacing looks like the
salinas radar. It also has a 12 second period like the
punta salinas radar. It could be punta salinas channels 9 and
10 if their reference frequency was off by 3 Mhz. The bandwidth is a
bit narrow but we may only be seeing the long range pulses (which
are stronger and narrower in freq). I did not get a chance to
measure the pulse duration of the signal. It went off of the air
around 15:30. The temporal duration of the pulse should tell us if
it really is a fps117 radar. By the way, I've notices that the faa
airport radar has not been transmitting simulataneously at 1330 and
1350 Mhz (just one or the other). I wonder if they have changed
- Fig 1 below is a dynamic
image of the 209 secs for the two 25 Mhz bands.
You can see the 12 second rotation period of
the radar. There are two sets spaced by 15 Mhz.
- Figure 2 Is a peak
of the 209 second records with a bandpass normalization of
the median bandpass for the 209 seconds. Black is polA and blue
is polB. The dotted lines show the paired birdies that are 15
mhz apart (Red and green).
13oct03 Color camera #5
birdie at 1417 (top)
On 16sep03 the dome camera #5 (looking at the
klystron tube) showed a birdie at 1417.5 Mhz. The camera was removed
and worked on in the lab. On 13oct03 it was reinstalled and tested
using the correlator. The setup was lband wide, 2048 channels over
195 Khz (95 hz resolution) , with 1 second dumps. The image of
the dynamic spectra show the results.
The camera was removed and returned to the lab for more testing. The
question is how is the rfi getting out of our rfi shielded enclosure
for the camera?
- The horizontal dashed lines show (approximately) when the
power to camera 5 was switched on and off. The vertical axis is
1 second integrations. The image has been normalized by the mean
bandpass. The strongest birdie is at 1417.495 Mhz. When it turns
on there is a frequency drift as it warms up. The comb is spaced
by 15.85Khz which is very close to the 15.75Khz line rate of
standard ntsc video. From record 240 to 330 camera 5 was off and
all the other cameras were on so it looks like this is the only
13oct03 1422.5 Mhz
birdie from tv channel 54.
A test was done with tv channel 54 to see if it
was still generating birdies. The video carrier is at 711.25 Mhz.
The 2nd harmonic of this falls at 1422.5 Mhz. A guard at the
transmitter site opened and closed the transmitter doors while we
were taking data. He also turned the transmitter on and off.
The lband wide receiver (Tsys about 28 Kelvins) was used in circular
polarization mode. Data was taken with the interim correlator
running in two modes: 381 Hz and 95 Hz resolution. Data
was dumped once a second. The
plots show the results of the measurements.
A 1422.5 Mhz birdie is seen from the channel 54
transmitter. It is less than 95 hz wide. It has a maximum strength
close to az=258,za=18 deg. Closing the transmitter doors makes a
difference when the telescope is not in the maximum strength
position. When the telescope is sitting close to the maximum
strength position , closing the transmitter doors make little
- Fig 1(polA) and Fig 2(polB). The telescope was sitting
at an azimuth of 192 degrees and a zenith angle of 18 degrees.
Each plot is a spectral average over time. The y axis is a
fraction of Tsys (about 28K). You can see the 1422.5 birdie. It
was not resolved in frequency by the 380 Hz channels. The top
plot has the transmitter doors open. The second plot has the
transmitter doors closed. The 3rd plot has the transmitter off.
The bottom plot has the transmitter turned back on. You can see
the birdie at 1422.5 with the doors opened and closed. It goes
away with the transmitter off. This az,za was not the position
of maximum signal strength (see fig 3).
- Fig 3 shows the azimuth dependence of the 1422.5 birdie (with
the za=18degrees). The transmitter doors were open for these
measurements. The 95 Hz resolution was used for these
measurements. The single channel at 1422.5 Mhz was plotted. The
top 2 plots show the birdie strength versus azimuth (black and
blue traces). The red trace is an average of the system
temperature close to this frequency channel. The 3rd plot shows
the fractional strength of the birdie (birdie/Tsys) - 1 versus
azimuth. The bottom plot blows up the azimuth around 245 to 265
degrees. There is a maximum around az=258 degrees. It gets
up to about .6 Tsys.
- Fig 4 has the telescope parked at az=257.2 and za=18 using the
95 hz resolution channels. The data is plotted as a fraction of
Tsys versus time. The data samples have been smoothed to 5
secs. At the beginning, the transmitter doors were open.
At second 340 the inner doors were closed. The outer door of the
transmitter building were shut around 350 seconds. There is no
noticeable drop in the birdie strength with the doors closed.
19sep03 iridium signal
Experiment a1840 dumped 100 Mhz bands
every 2 milliseconds with the bands centered at 1370,1470,1570, and
1670 Mhz while looking at interplanetary scintillation's. There were
256 channels per board giving 390 Khz resolution. 14 300 second
integrations were done over two days. I used this data to see how
stable the iridium signal was. The signal has a 90 millisecond
period with 45 milliseconds for downlink ( more
The band of interest were 1621 to 1626 for
iridium and 1610 to 1613 for the radio astronomy band. Unfortunately
the observer setup the filters so they abutted at 1620 Mhz. The plots show
the iridium signal properties..
The satellite signals look pretty stable with time. We can probably
compute the phase of the start of the 90 millisecond cycle and then
use the sps to generate a blanking signal. The blanking duration
will probably have to be close to 50 milliseconds. As a double
check, i need to find someone else's data and see if the phase of
the signal from this data set continues to predict the phase of the
iridium on cycle.
- Fig1 For each 300 second integration the
rms/median() was computed for each frequency channel in the 1600
to 1630 Mhz range. The top plot is the rms/median() by channel
for the 14 scans (300 sec integrations). Each strip has been
offset for display. The middle plot is a blowup with no offsets.
The iridium signal is clearly seen in the 1620 to 1626 Mhz
range. The bottom plot is the median bandpass for each of the 14
scans. You can see the filter falloff in the middle.
- Fig2 The spectral data was bandpass corrected by
scan (using the median bandpass) and then the total power over
the range 1621.35 to 1626.35 was computed for every 2
millisecond sample. The total power data from each scan
was folded with a 90 millisecond period. The starting time
for each scan relative to the first scan was then used to phase
correct each scan to the first scan. The data was then shifted
so that the off period started at T=0. The top plot has
the folded total power in the iridium band plotted versus the 90
millisecond period of iridium. Each scan has been offset for
plotting purposes. You can see a drift of about 4 milliseconds
from the start to the end of the data set. For each scan there
is about 42 milliseconds with no iridium signal. The bottom
plot is the same data but the y axis is now the hours
since the start of the first scan. You can see how the signals
drifted over time.
- Fig 3. The total power was also computed in the
radio astronomy band (1610.6 to 1618.8) and processed the same
as the iridium data. The top plot is the 14 scans of data. The
dashed green lines show were the strongest iridium signal sat.
The bottom plot has the 14 scans averaged together. There
is no obvious difference between the iridium on and iridium off
The system was being driven far into
saturation so that the gain was decreasing during the radar pulses.
I wanted to measure the downstairs IF power levels when the
radars were present to see if it was happening downstairs or
upstairs. I set the power levels into the square law detector so
that the largest pulsed from the radar would not clip. This meant
that you could not see the off pulse noise floor.
04apr03: Radar power
levels in the downstairs IF (if2) (top)
Data was taken with the correlator and the radar interface during
the day on 04apr03. The telescope was parked at za=8.48, az=270.
The new lband wide receiver was used in linear polarization mode
with the 1100 - 1800 Mhz filter in and the 750 Mhz IF.
|Upstairs power level (IF1)
||-40 dbm on meter
-20 dbm into FO xmter
||rf: 6 5 db
if: 0 0
|Downstairs power level (IF2)
||-52 dbm meter
-32 dbm before coupler
||-7 ,-9 or
3db ,1db + 10db amp
|output power IF2 FO RCVR
|-32 at meter coupler
-26 before 4 way splitter
-34 before gain/attn
-52 before 18db amp (output FO RCVR)
|output power bottom IF2
||IF2 FO rcvr out + 22 db
|square law detector
||2 usec time constant
|op Amp after sqrLaw
|8 bit digitizer
||0 to 2.5 volts sampled at 1Mhz
The 1st sampled pair of 500 MHZ bandpass was taken from
the 750 Mhz front panel output. This had a 20 db pad so the power
level was the same as that measured by the power meter. It was
sent to a square law detector and detected with a 2 usec time
constant. This voltage was multplied by 5 in an opamp and sent to
the 8 bit A/D converter (0 to 2.5 volts full scale). The 2
signals (polA,polB) were sampled at 1 Mhz.
The noise power levels into the square law detector were -52
dbm so we could only see when the radar was on and pointing at
us. This insured that the a/d square law were not saturated.
A second set of signals were taken from the bottom of the IFLO
buffer amps. The output power of the buffer amps is 14db +
gainUsed greater than the input power. The 8db gain used set the
output power 22 db above the input power. The data was taken out
of the front panel input, passed through 23 db of attenuation
(to put the levels close to the first pair of signals) ,
detected, and then sent to the AtoD converters.
The correlator ran at 1 second dumps, 3 level interleaved
sampling 1225 to 1425 Mhz. This let us identify which radars
To calibrate the system, a -20 dbm and then -17dbm tone
at 750 Mhz was injected into the IF/LO at the transfer switch
input (right after the FO rcvr output). This data was recorded and
used to map the a/d levels into dbm in the downstairs if/lo. The
power at the output buffer amps of the IFLO were 22 db above the
input. The -20 dbm level would be + 2db output at the buffer amps.
The radars seen:
Three radars were seen: 1290 Mhz remy radar, 1330/1350
FAA airport radar, and the punta salinas fps117 frequency agile
radar (the aerostat was not broadcasting). Each of these radars
has a 12 second rotation period. Three distinct peaks could
be seen in any 12 second section of data. They occured when the
radar pointed in our direction. The radar peaks were identified in
the total power data stream by their radar ipps (since they
were known ahead of time).
The plots show polB,polA from the upper section of the iflo
rack and then polA polB from the buffer amplifiers. A gaussian
is fit to each sweep of the radar to measure the beam width as
it passes in front of the observatory. The second page shows a
single sweep (rotation) of the radar, ipps, and a single pulse.
This is a single pulse (5 usec), single ipp (2781 usecs) radar
with a rotation period of 12 seconds. The power level gets to
-17 dbm at the input. This is 5 dbm output at the buffer amps.
Ther is compression in some of the sweeps since the *'s
sit below the gauss fit.
The second page shows the largest sweep (polB top of iflo). The
single pulse does not show any negative going gain.
This is a single pulse (1 usec), 5 ipps cycle, dual frequency
(1350,1330) radar with a period of 12 seconds. The 1350 pulse is
sent and then 8 usecs later the 1330 usec pulse is sent.
This radar got close to the -17dbm power level (+5 dbm at output
of buffer amps). The top of the stronger sweeps (black,yellow)
seem to be flat topped a little so it has probably gone into
compresssion. The 6 pulse plot shows the 5 different ipps.
The single pulse has the two frequencies smeared together with
the multi path scattering. There is no negative gain.
frequency agile radar.
This is a frequency hopping radar with 18 channels. Each channel
has a long pulse (400 usecs two frequencies) and a short pulse
(51.2 usecs two frequencies). It is hard to charaterize the ipps
because of the frequency hopping. They supposedly blank the
radar when it points in our direction.
- Fig 1 is a single sweep of the radar past the observatory.
The power levels reach the -20 dbm iflo input level (buffer
amps +2 dbm). The four strong pulses in the center are the
400 usec long pulse. The smaller closer spaced ipps I think
are the 50 usec pulses. The 50 usec pulses look like they
are sweeping out a beam, but then they disappear. I don't
know if this is blanking or what. The two levels we see with
the 50 usec pulses are probably different frequencies.
- Fig 2 shows the sweep for polA top of iflo. It then plots
the 4 400 usecs pulses and then a single 400 usec pulse. The
power definitely goes down during the pulse. I don't know
whether this is a part of the transmission or we are seeing
some negative gain here. The bottom two plots show the 50
usec pulse. It should be stronger than the 400 usec pulse
(higher power for shorter period of time) but it isn't. The
single pulse last longer than 50 usecs. I don't know if they
are transmitting the two frequencies staggered in time or
- Fig 3. This is a single 51.2 usec pulse. It is 102 usecs
long so there are probably two pulses here (the dual
frequency). All four of the outputs show a dip in the center
of each pulse. The power levels in the downstairs iflo are
all less than -17 dbm input or +5dbm at the buffer amps. If
this is saturation and negative gain, then it must be
The power levels from these radars are not close to the
1db compression level of the system (+16dbm for the amplifiers at
the end). There is some compression occuring so it must be
upstairs. The punta salinas 51 usec pulse may be going
negative. If this is true, it must be happening upstairs since the
downstairs levels have not arrived at saturation.
On 23apr03 we placed a 20 Mhz IF filter
centered at 1435Mhz before the fiber optic transmitter.
We saw saturation with the 1350 FAA radar. We placed a
1435 rf filter before the mixer and saw no saturation.
This shows that the compression is occurring in the 750 Mhz
The power was measured at the output of the lbw
dewar on 18sep02. The azimuth was stationary. Peak holds were done
at 1300 Mhz 1Ghz bw and 1400 Mhz 400 Mhz bw.
the power levels at the output of the lbw dewar. (top)
The peaks 1,2,3,4 are related to the cellar phone bands. This rfi
is the reason you must run with at least the 1100 -1800 Mhz filter
between the dewar and the postamps.
Peaks 5,6 are the aerostat radar balloon.
Peaks 1,2 are the aerostat radar.
Compare these plots with the data taken may01
from after the lbw dewar. This data was taken with a 30Khz
Resolution bandwidth filter. The radar bandwidths are .2 to 2
Mhz so this setting undersampled the radar power by up to
1.6Mhz/30Khz= 55 or 17 db low.
Peaks 3,4 are the san juan airport radar. They are about -45 dbm.
In the 1 Ghz scan they were -68 dBm. This illustrates the problem
in using a swept oscillator system to measure a pulsed radar. The
radar has a 1 usec pulse, ipp of about 2 millisecs, and a rotation
period of 10 seconds. In the 1 Ghz plot the spectrum analyzer was
not over the 1330 freq bin when the radar was pointed at us and
Peaks 6,7,8,9,10.. are the punta salinas radar.
Experiment A1475 (using lbn) had interference at
1399.83 with a 1 minute period. It repeated in the on position
and the off position. This occurred in multiple scans that were
close to azimuth 175 to 191 deg. At the time the distomats were
being run on a 60 second cycle. The rfi window had been installed on
the distomats months ago. The plot shows the power
a 12 Khz channel at 1399.83 Mhz (black) and two adjacent
channels 24 Khz away (red,green). The rfi is strongest in polB
(upper plots). The data was sampled with 6 second intregations.
camera birdies still present. (top)
Distomat biride at 1399.83 (top)
The dome color cameras were checked on 24may02. Camera
#1 that caused the problem on 09apr02 was not installed (so there
were only 5 cameras). The setup used a 390Khz setup with 190 Hz
resolution. There was a band at 1388.86 and a second band at
1417.5. lband narrow was used while tracking blank sky with
the radar blanker on. In the test all of the cameras were
turned on/off using the downstairs switch.
The first plot shows 200
seconds cameras on followed by 200 seconds cameras off. 10 20
second integrations were done with the camera on followed by 10 20
second integrations with the cameras off. The black trace has the
cameras on while the red trace is the cameras off (both figures are
polA). The birdie at 1388.858 went away when the cameras were turned
off. The smaller birdie at 1388.86 was there all of the time. The
lower figure shows the birdie at 1417.495. The smaller birdie at
1417.482 was only in one of the 20 records. The birdie was not
resolved with the 190 hz resolution. The signal strength in the 190
Hz wide channels is about 23% of Tsys or (.23*28K)=6 kelvins.
The second image
shows 1 second dumps while the cameras were turned on and off.
The horizontal axis is frequency while the vertical axis is time (in
seconds). The dashed horizontal lines are where the cameras were
turned on or off. The cameras had been on for awhile before the test
started. You can see that when the cameras were turned on, the
birdie drifted by about 2 Khz. After 100 seconds it had still not
come back to the frequency we started with. This is probably a
temperature related drift when the power gets applied.
Two birdies were found:
1388.858 and 1417.495 with a spectral width < 190 Hz. This
would be a comb frequency of 14.3185 Mhz (corrsponding to the comb
measured before shielding..see 20sep09). The other members are
probably also present. I had initially used 25Mhz/1024lags =
25 Khz resolution to look for the birdies. With this setup you
couldn't see these birdies in 1 second dumps. The frequencies
drift up to 2 Khz when there power if first applied. When
debugging this problem you will need to use the high frequency
Rfi at 1417 Mhz appeared in early apr02.
Turning off the color cameras in the dome made the birdie go away.
On 09apr02 the 6 cameras in the dome were turned off 1 at a time (by
disconnecting the power to the camera). The correlator was run with
a 6 Mhz by 2048 channels setup with 1 second dumps on lbn receiver.
The plots show how the rfi at 1417 changed as each camera went on
rfi from color cameras in the dome. (top)
The problem with camera 1 was not there on 20sep01 so something must
have changed since then. Maybe the shield on the cable that goes out
to the camera has a break in it somewhere. Camera 1 should be left
off until it is fixed. We also need to do a better job of shielding
the other cameras.
density as cameras turned on off. The cameras went onoff
camera 1 is causing the problem. The off glitches at 100 and 460
were caused by camera 1 being turned off to get at the cord for
- 30-65 camera 1 on, off (on the stairwell).
- 130-160 camera 2 on, off.
- 175-190 camera 3 on, off.
- 220-255 camera 4 on, off.
- 312-345 camera 5 on, off.
- 382-412 camera 6 on, off.
- 525-560 camera 1 on,off.
- 600-630 camera 2 on,off.
single correlator channel at 1417.49 Mhz. This show the
vertical line from the image for the single channel
(width=6Mhz/2048). Black in polA and red is polB. The turning
on/off of the cameras is in green/blue. The vertical scale units
is tsys (approximately..). The record numbers here are 281
counts larger than those in the image. The birdie is up to 20%
turned off and then 4 min on,off position swithing was done on
blank sky. The cameras (less camera 1) were turned on for
the on position and off for the off position. 3 of these on/off
pairs were done. The onoff position switch plots show the
individual on/off -1 in fig 1-3 while figure 4 is
the average of the last 2. The first on/off position switch
accidentally had the cameras turned on then off during the on
position for a few seconds. This caused a noticable birdie. The
vertical green lines are the 14.3341 Mhz comb measured 20sep01
before shielding. The baselines are lousy since this was done at
5pm when the standingwaves from the sun are bad. Even with the
lousy baselines it looks like the cameras are still making
birdies at about .2% of Tsys. This is .002*30K=30 milliKelvin.
This was not seen on 20sep01 because the intergration wasn't
Note: 30jul02 the gasket material used to make
good contact for the doors of the camera box degrades with time. see
2 mhz comb
at 327 Mhz: resolution.
swing with 1400 Mhz birdie (top)
On 01mar02 I did an azimuth spin with the dome at
18 degrees za ]moving at .4 degrees/second. The
azimuth spin plot shows that the peak strength is in the
azimuth direction 340 degrees (toward the control room). This is the
same direction found for the
peak of the 1350 Mhz birdie.
28feb02 1400 Mhz
3 On/off scans of 5 minutes each were taken with
plots show a 1400 Mhz birdie. The resolution was 6 khz and it
is not resolved (on 01mar02 I used 190 hz channel width and it did
not drift and it was not resolved). The birdie divides out between
the on and the off so it is very stable.
The distomats were turned off during the off of the first scan. The
times (seconds from midnite are:
- Fig 1 shows the spectra for a 6 second integration.
- Fig 2 has the channel corresponding to the 1400 Mhz birdie
plotted versus start time of the on or the off for the 3 scans.
The on and the off are overplotted. Black is polA, red is polB.
There is a 100 second cycle that repeats in the on and the off.
Since the start times between the on and the off is not a
multiple of 100 seconds, the periodicity is in space rather than
- Fig 3 plots the 1400 Mhz channel for the 3 scans versus az.
The on and the off track each other. It is strongest in the
first scan with az=240 (pointing toward T8). It gets weaker as
we move away but there is still correlated power in the on an
the off.This was done at low zenith angle.
The laser ranging points come every 2 minutes
so the 2 minutes after the last point (78962+120=79082) the
distomats were off. This is 48 seconds before the end of scan1
off. The on and the off for scan 1 track identically so turning
off the distomats had no affect.
|scan1 on start
|scan1 off start
|scan1 off end
|laser ranging last pnt
|laser ranging off by
What it could be?? It's narrow, stable and
exactly at 1400 Mhz so it's probably us. It varies as you move the
az,za and repeats for the same az,za so it is fixed to the
ground. The strongest az points at T8 so is could be something at
the tiedown but I don't know of any locked signal (other than the
1 second tick) that goes to the tiedowns. The az direction of 240
degrees should not be relied on since the actual scattering path
into the dome varies a lot.
06nov01. 1381 gps L3 birdie
on all the time. (top)
There has been a birdie at 1381 Mhz for the
last few weeks that is 1 Mhz wide. It looks like GPS L3 at 1381.05
Mhz but occurs more often than the scheduled usage of GPS L3. The
people at the VLA do not see this unusual usage of GPS L3.
Experiment # A1517 has been running from 19:00 AST thru 1:00 AST the
next morning. Below is a list of what it has seen at 1381. The
dates are the AST end dates (1am AST). The times are AST (UTC-4).
06nov01 was different from all of the other days. On this day, the
bursts would last for a minute or two but the signal would not turn
off completely after the bursts. A low level interference would
||19.0->0.8am 4 bursts of 1 to 2 minutes duration.
||19.0->0.8am 1 two minute burst
||19.0->20.8 ...1 to 2min on for each 4 minute scan
23.2-> 0.8 on .. 1 to 2 min of for each 4 minute
||19.->.8 on most of the time.
There were the 1 or 2 minute bursts that were
When the strong bursts stopped, a lower level emission
||3 bursts of: 2min, 1 min, 1min for the entire night (19.
||4 bursts of:1min,2min,1min,1min for the entire night
||3 bursts for entire night (19->.8am)
shows two plots from 06nov01 when the birdie was on all the time
and 1 plot from 08nov01 when the bursts were normal. The data was
sampled at 1 second intervals. The telescope was tracking a
celestial source while this data was taken on 06nov01. On 08nov01
the telescope was stationary while the data was taken. (The links
for fig1,2,3 below are the the individual pages from the "figure"
link above. They were added for the braindead browser at
1 top is a linear plot of 1 Mhz total power about
1381. At 150 seconds it is the strongest. An image of
spectral density versus time showed that that the signal
remained there at a lower level for the entire scan.
- Figure 1 middle plot is the same plot but with a db scale. The
FWHM for the peak at 150 seconds is 16 seconds. This
is close to the sidereal rate of 13 seconds for the main
beam. The signal is moving through one of our
sidelobes because the telescope is tracking a celestial source
and the rfi source is probably also moving ( GPS
satellites have 12 hour nearly circular orbits so they are move
in their orbital plane at about twice sidereal rate).
- Figure 1 bottom is the spectral density for 100 contiguous
seconds around the peak. An offset has been added for display.
2 top occurred later in the night. The signal was
off and then turned on at 136 seconds ( 1:58:42 UTC
06nov01). It then looked again like it was drifting
through a sidelobe. At 200 seconds it was turned off and
then came back on at 230 seconds.
- Figure 2 bottom is the spectral density function for 100
seconds around the first turn on. An offset has been added for
3 was taken on 08nov01 during the day with the telescope
The top plot is 1 Mhz power (linear scale)
about 1381 Mhz sampled at 1 second intervals. PolA is the upper
trace and pol B is the lower one (lbn was used with circular
polarization). You can see the satellite move through the
sidelobes taking 54 seconds to move through 4 sidelobes (13.5
secs/ sidelobe). This burst was a clean burst where it turned on
and then off completely. The telescope was stationary so there is
no mixing of the motion of the satellite and the telescope. The
difference in polarizations is probably do to the unknown
polarization properties of the far out sidelobes.
The bottom plot is the spectral density versus
time for this period (offsets were added for display). The
spectral shape does not have the modulation that was seen in the
spectra on the 06nov01.
GPS L3 is used for nuclear blast detection.
There are on board sensors that monitor electromagnetic and x-ray
events that could be related to nuclear blasts. Using multiple
satellites, the positions of the blasts can be determined. This
data is formatted and transmitted to ground stations on the L3
frequency. The GPS Nuclear Detection System operators were
contacted and they admitted that one of their birds had a problem
with its detector being stuck on. This could have caused the
continual transmissions that we were seeing on 5/06nov01. A single
satellite failure would also explain why we saw it and the vla
The FAA radar is at 1330 and 1350 Mhz. When the
radar blanker was used, the radar pulses were blanked but a tone
remained at 1350 Mhz. On 05oct01 an azimuth spin was done with lband
narrow (native circular) with the dome at 19 degrees za to look for
any azimuth dependence of the tone. The correlator was setup for 25
Khz resolution and was dumped at 1 second intervals. The figures
shows the results. Pol A is black, pol B is red, and the azimuth
location of the control room is flagged with a green vertical line.
tone at 1350 Mhz. (top)
The tone is between azimuth -100 and +10 degrees
az. The peak value is close to the control room values but the
center is not. The peaks last for about 8 seconds (or 1.6
degrees in azimuth). At 25 Khz resolution the tone was not resolved.
- The top figure shows +/- 180 degrees azimuth (from north).
- The center figure smoothes the 1 second samples (.2
degrees azimuth /sample) by 11.
- The lower figure blows up +/- 20 degrees azimuth about the
control room position. Asterisks mark the 1 second samples.
If the control room is causing the problem,
then the center offset may come from the asymmetry in the cliffs
adjacent to the control room blocking the beam. The lidar lab and
the rfi shack are also in this general direction. We need to go up
to the azimuth with an antenna and look around (bringing the data
down into the correlator and using the blanker).
harmonics created in the 2nd IF. (top)
Spectral line observing at lband normally has the following IF
The 1350 Radar is (1481-1350)= 261 Mhz below the band
center so dx = 1 Mhz.
The birdie then falls at (cfr-2*dx)=1481-2= 1479
- The lband wide front end has a frequency response (see link).
- A set of filters between the dewar and the post amps limits
- A high side lo mixes the Fr center frequency to 750 Mhz. It is
then band limited to 500-1000 Mhz.
- A set of up to 4 high side second Los mix down to 260 Mhz.
Each of these is then low pass filtered to 500 Mhz.
- Each of the 260 Mhz bands go through a distribution amplifier
and then to the correlator.
- The correlator first places a 50 Mhz filter about 260 Mhz. It
then amplifies the signal and sends it to the digitizer that
samples the entire 50 Mhz band.
- Digital filters then reduce the band by factors of two from 25
Mhz down to 195 Khz.
In step 5, the second harmonic of any signal 130 Mhz below the
center of a subcorrelator will land centered on the
subcorrelator (130*2= 260 MHz is the center of the band).
If the sub band center is 130+dx Mhz above a radar, then
the radar 2nd harmonic will come 260 -
(260-(130+dx))*2 = -2 dx Mhz above the center of the band.
If abs(dx) < Bandwidth/4 then it will appear in the spectrum.
The table below shows the center frequency 130 Mhz above various
radars. It also lists the range of vulnerability assuming you
use a 25 Mhz digital filter (25/4=6.25)
|| 130 Mhz Above(6 -/+Mhz)
|1233 modeA puntaSalinas
|1242 puntaSalinas modeA
|1248 puntaSalinas modeA
|1257 puntaSalinas modeA
This problem will only occur if the radar signal in the 2nd IF
(260Mhz) is strong enough to create harmonics. These harmonics
have been seen for the 1269 Mhz radar with sbc cfr =1398Mhz . In
this case dx=-1 and the birdie comes -2dx= +2 Mhz above the
center (at 1400Mhz).
shows the 1330,1350 Mhz faa radar as well as 130 Mhz above
each of these frequencies : 1460 and 1480 Mhz.
2 shifts the center frequency of the last two sub bands to
1461 and 1481. The 1330 Mhz harmonic is not visible but the 1350
harmonic is now at 1479. Using the formula above:
The other birdies in the 1330,1350 bands are intermods caused in
the 8 bit A/D converter for the correlator by the radar.
A color camera was placed on the service
platform for monitoring purposes. It had no "extra shielding" added
by us. On 13aug01 the lband narrow receiver was used to measure the
rfi from the camera. 1337 to 1437 in 4 subtends was used. Each with
25 Mhz over 1024 channels. 1 second integrations were done. 120
seconds of data with the cameras on were followed by 120 seconds
with the cameras off. The camera was removed and shielded. The rfi
was remeasured on the service platform on 20sep01 using the same
setup but a 240 second integration.
Rfi from color camera on the service platform. (top)
The image shows that the frequency is moving by
up to 100 Khz/second. This will decrease the average intensity of
the tone when averaged over time.
mb) shows the 240 records versus frequency (polA). They
have been divided by the average of the camera off so the units
are Tsys (28 K lbn). Horizontal lines are placed at the
the average (cameraon/cameraoff)-1. The top plot shows the
average for the 120 seconds on 13aug01. PolA is black and
polB is red. The units are fractions of Tsys. Vertical lines
have been drawn every 14.3341 Mhz. The bottom plot shows 4
minutes of data with the camera on taken on 20sep01 after
A fundamental of 14.3341 would give harmonics at the measured
positions. The strength of the signal increases every 4 of these
units so there are probably two frequencies involved.
The shielding removed the rfi for a 4 minute integration. To go
farther in sensitivity we should do the experiment at night (no sun
and hopefully no punt saloons radars).
We measured the output power levels of the lband
wide dewar using the 22 Ghz spectrum analyzer and a power meter. The
telescope was parked at az=214.4 and the gregorian was at 19
degrees. A 12 foot long semi rigid cable was used to connect to Pol
A's output. The
the power levels at lband wide dewar output. (top)
- Figure 1 plots the spectrum 1400 Mhz +/- 1000 Mhz. The
resolution bandwidth was set to 3 Mhz. The system temperature at
this za is about 40 K. The black line averages 10 sweeps of the
spectrum analyzer. The red line is a peak hold for greater than
20 seconds. The black line shows continuous signals while the
red line includes the impulsive radars. The dear's response goes
from 1 Ghz out to 1.9 Ghz and is 10 db about the spectrum
analyzer's noise floor. The total power measured with the power
meter was -28 dbm. Integrating the spectra gave -22.5 and -17.9
dbm. The -28 dbm and -22.5 dbm should be the same. The -28 dbm
value of the power meter is probably more reliable.
- Figure 2 is a blowup of the peak hold spectrum The known
- 880 Mhz. cellular phones.
- 932, 956 Mhz. cellular communications.
- 1250 Mhz. aerostat radar.
- 1330,1350 Mhz. airport radar.
- 1950 Mhz. cellular phones, digital communications.
- Figure 3 is the 1407 Mhz birdie at the output of the dewar
plotted on a linear scale.
Doing a consistency check on the power levels:
-198 dbm/hz/K + alog10(3Mhz rbw)*10 + alog10(40K)*10 + 40 db
gain -77 dbm.
The spectrum analyzer value is about 5 db higher than this (it
was also 5 db higher than the power meter reading).
The highest continuous signals are in the 880 to 950 Mhz
range. These signals could be filtered out by decreasing the
diameter of the wave guide at the input to the dewar without
affecting the normal observing band (say > 1100 Mhz). If a
radar causes the dewar to go into saturation then 3*(850-950) -
1350 will mix back into the Fr band.
The final stage lbw amp can output 0 dbm of power so the caw
levels are not high enough to cause the dewar to saturate. There
may be a position of the azimuth, dome where these levels are
higher than those measured. These measurements do not tell
whether the radar pulses are causing the dewar to go into
saturation for the duration of the radar pulse. Some of the
pulse lengths are on the order of ascends. The power meter
averages for a lot longer than this while the chances that the
spectrum analyzer peak hold caught the radar pulse when it was
pointed at us is small (the sweep time/number of sampled points
< radar pip).
The 1407 Mhz birdie is seen at the output of the dewar so
it is not being caused downstream in the post amps. The power
levels in the dewar are low enough that it is probably not an
intermod created in the dewar.
A 1407 Mhz birdie appeared in apr01. It was about 300 Khz wide and
seemed to be on all the time. Some of the characteristics of the
signal were measured using the lbw receiver in the dome:
Mhz birdie from tv station. (top)
11may01 - final results: birdie was from call
sign: w20bx channel 20 arecibo. barrio factor. 507.25 Mhz
video carrier. approximate coordinates are:
18:27:46.0,66:38:18.7 (gps receiver close by). This was about
60 degrees east of north from the platform. Tower FCC erg
number: 1211904. Engineer: Juan G. padding.
Some plots of the signal are:
- It is located at 214.4 Deg az and 31.8 degrees az (180
degrees away) and is at high za (19 deg).
- It is very narrow in azimuth. It drops to 10% of maximum in
2 degrees az.
- The strongest it gets is about 5*Tsys(40K) = 200 Kelvins
coming through the telescope.
- The signal appears to be left circularly polarized coming in
- There is a 15.8 Khz comb and a 60 hz comb in the signal.
- There is a side band +4.5 Mhz from the peak and a weaker one
-4.5 Mhz from the peak (tv signal audio offset from video
carrier). The audio was not strong enough to demodulate.
- The signal wanders around by up to 1 Mhz over hours.
- The signal is almost always on. We did see it turn on
at 9:22 am 11apr01.
- Looking with binoculars along the azimuth direction 214.2
, you can see the 107.3 Fm station tower (one of the
strongest sky signal around). The strongest tv station in this
direction is channel 22 with a video of 519.25 Mhz.
- We inspected the 107.3 Mhz tower. It had the fm antennas, a
microwave link, and one other whip. We did not see any
believable 1407 Mhz emission ( the signals were so strong that
we had trouble with intermods in the antenna/amp we were
- From the platform we could not reliably see the signal at
1407 Mhz. We had a hard time getting a sensitive enough
measurement using a log periodic (6 db gain), amplifiers, and
- We looked at the power levels coming out of the dewar. The
caw output levels don't appear to be strong enough to cause
intermods in the dewar. We see the 1407 birdie at the output
of the dewar on the spectrum analyzer at 60% Tsys (Tsys=40K,
40 db gain in dewar, 10 KHz raw).
- There are no harmonics of tv stations that fall at this
- Thinking that it might be the IF output of our satellite
receivers (900-1450) we turned off the amps/block converters
for the tv dishes but the signal remained.
- The radar blanker (with an lo of 1340 Mhz) was turned off
but the birdie remained.
- The signal drift rate was up to 1 part in 10-4 over 5
minutes. This is a pretty lousy oscillator.
- On 10may01 we setup a receiver on the platform: log
Periodic, circulator, 10db gain amp, 25 MHz filter about 1407,
20 db gain amp, mix to 30 MHz, and then sent it down to the
control via the radar blanker rg7 cable (< 10db loss). We
then input the signal to the correlator and integrated. We saw
the birdie and it came from 60 degrees east of north.
- On 10may01 we went out to route 10 overlook.. no results.
Next was the arecibo airport. We got a good signal coming from
- On 11may01 we went farther east till we got a signal from
the north. Then into barrio factor of arecibo where we
located the tower... W20BX channel 20 in arecibo. It was still
transmitting a test pattern and tone. After contacting the
engineer (juan g padding), we did a test turning the
transmitter off and the signal disappeared. The tv
transmitter had been used for channel 28 (555.25) but was now
being used at 507.25. He had been transmitting with an 8 watt
executor at 507.25 Mhz. The engineer claimed that the bandpass
filter was not working correctly. There must have been an lo
of 900 Mhz lurking somewhere in the system.
spectra through the lbw receiver (circular) and the
correlator using 1 second dumps and 6 Khz spectral resolution.
Pol A (left circular) is red. The az=214.4 and za=19. degrees.
Figure 2 shows the power in the signal channels during the
azimuth spins. It peaks sharply at 214.2 and 31.8 az.
MHz bw about the birdie. The upper plot has 1 Khz
resolution and shows the spike and comb. The middle and
lower plots are blowups to show that the comb is 15.8 Khz and
the spike is a 60 Hz comb. These are the horizontal retrace
and vertical retrace frequencies of the tv signal.
of the signal. The audio side bands can be
seen with the wider bandwidth. There is a spike awfully close
to the suppressed color carrier.
showing the variation of the frequency over time. The
image plots 122 channels about the video carrier for 5200
seconds. The signal is wandering about by a few 100
Khz.It has moved by up to 1 Mhz or more.
at the output of the lwide dewar. The dewar can output 0
dbm so we aren't near the Mx (see Measuring the power levels
at the output of the lband wide dewar below). The last page
shows the size of the 1407 birdie at the output of the dewar
on the spectrum analyzer (linear scale, 10 Khz raw).
log periodic antenna on the platform. The plots show the
strength of the signal using a 100 second integration. The
directions are only approximate since the antenna was turned
- The 15.8 Khz and 60 Hz combs are very narrow. If the signal
was mixing with the FM tv station then it would probably
get smeared out.
- I had thought that a harmonic would cause the audio carrier
to move farther out. Jon hagen straightened me out.
Suppose you have a tone of Amplitude A
and frequency w and a side band of amplitude B and
frequency (w+delta) then:
ABexp(i(2wt delta)) + BB*exp(i(2w+2delta)t)
If the video is a 2nd harmonic, then the audio below the video
harmonic could be 3*video-audio.
If B << A then the AB term will be much
larger than the BB term so the delta signal will be stronger
than the 2*delta.
- The wave guide at the input to the dewar will not let a tv
station fundamental get into the dewar. The power levels in
the dewar are not high enough to cause intermods so the signal
must be entering the dewar at 1407 Mhz.
- The inability to see the birdie from the platform on the
spectrum analyzer is probably a signal to noise problem. The
spectrum analyzer noise floor is -107 db/30Khz raw. Using -198
dbm/hz/K gives a noise temperature of 40,000K. The Mx signal
level through the telescope is 5*40K=200K. The log periodic
noise temperature is at least 150K since it is looking at the
horizon. After adding the noise figure of any
amplifiers/filters, we probably have a system temperature
going into the spectrum analyzer of 250 -300K. Our setup was a
filter (1.7 db loss), 20 db gain amplifier, and then the
spectrum analyzer. We saw no 1407 birdie. Assume the signal is
only losing 10 db getting into the dome and we want a signal
to noise equal to what we saw at the output of the dewar. We
would then need: (40dbDewarGain - 10 db loss getting in - 6 db
gain of log periodic) + 300K/40K (8db tsys difference). This
comes to about 30 db gain (and the signal at the output of the
dewar was not very strong...).
- The block down converters for satellite tv receivers at
cband and ku band use (5150-transponder frequency) and
(transponder frequency - 11.3 Ghz)for the If. For cband we
would want a transponder frequency of 5150-1407=3743. The
closest transponder frequency if 3740 (although i don't know
the video carrier offset). I think that the satellite
transmissions put the audio frequency 6.5 Mhz above the video
so it's probably not a satellite IF from a neighbor's busted
- It seems strange that the azimuth dependence is so sharp.
The sidelobe response should not be so sensitive to angle.
Maybe the signal is doing some type of spectral reflection
(say scattering off of the side of the azimuth arm, down into
the dish, and then back up into the receiver. The fm tower
lining up with the peak of the signal is probably just a
coincidence. Scattering off of the side azimuth would also
explain why we see the signal at 214 and 180 degrees away from
that. This would be a tv signal at 120 or 300 degrees.
distomat birdies at 1300,1400 Mhz. (top)
The distomats used for the laser ranging were
generating birdies around 1300 Mhz. To measure this the correlator
was setup for 25 Mhz bw/1024 channels centered at 1295 Mhz using the
lband wide system. The telescope was pointed at each distomat in
turn (n*120 +/- 35 degrees az, za=18 deg). The distomats were
run on a 30 second cycle starting on the minute. The correlator
integrated for 180 seconds looking at each distomat separately. The
distomat program spends about 7 seconds reading the temperature and
tilt sensors and then it starts reading the 6 distomats. The
measurement is usually through in 22 seconds.
- birdies at: 1287.5, 1299.84,1300.,1399.83,
- Every 2 minutes, azimuth dependent.
- clk:27Mhz. looks like +/- 12.5 Mhz above/below the
images (1.2mb ps file) show 1282.5 to 1307.5 Mhz by
the 180 second measurements while looking at each of the 6
distomats. Distomat 4 (az=205) is the worse with birdies
at 1300 Mhz and 1300-12.5=1287.5. For each 30 second cycle
there is a spike at 7 seconds and then a longer duration that
last for 3 or 4 seconds. This corresponds to the number of
seconds it took to read the distomat.
There were two frequencies close to 1300 Mhz:
1299.84 and 1300.0 Mhz measured with a 0.024 Mhz
resolution. On 5feb01 measurements with a spectrum analyzer
at the distomats showed that distomat 3 = 1299.84 and distomat 4
was generating the 1300. Mhz birdie. The birdie from distomat 4
was about 5 times stronger than that from distomat 3. On 5feb01
distomats 1,3,4 all had large windows (distomat 1 window was
changed from small to large after 22jan01). Distomat 1 showed no
birdies on 5feb01 so the reason is not just the larger window.
shows the time series for the power in the 2 channels around
1300 Mhz (averaged). The bottom trace is distomat 1 and the top
trace is distomat 6 (offsets were added for display). The units
are fractions of Tsys. From this plot it looks like all of the
interference is coming from distomat 4. The junk while looking
at the other distomats may just be leakage from distomat 4. The
birdie could be larger than .3 Tsys if its duration is very much
less than 1 second (the correlator integration time).
The manufacturer says the clock rate is 27 Mhz so 1300 is the 48
Th. harmonic. Using azimuths spins from other days (11apr01)
birdies were seen at:1299.85,1399.83, and 1400.
Azimuth swings with dome at 19 degrees za. (top)
A 3 Mhz band pass was used. Swings were done centered at 1420 Mhz
(257 10 second dumps) and 1419.5 Mhz (284 10 second dumps) The plots
Figure 1: The (average - baseline ) for each of the
swings (pol Apollo B separate). The birdies
are at: 1419.36,1419.5, 1419.93, 1420.00,1420.8.
Figure 2: a blowup of figure 1.
Figure 3: The 1420 Mhz birdie channel versus azimuth angle
with adjacent off channels over plotted. The bottom plot shows
the difference between the 1420 Mhz birdie channel and the
adjacent channels. There does not appear to be any azimuth
dependence so the birdie is probably generated inside the dome
(although the tsys bumps at -60, 60, and 180 which are
perpendicular to the triangle sides are very interesting!!)