lband RFI measurements.

2014

11aug14: p2030 pulsar search zaplists and periodicities created by puerto rican radars
08aug14: punta salinas not using modeA starting jul14
05aug14: added monthly plots of 1325 mhzband from the hilltop monitor system.

2013

12dec13: 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
26nov13: check the duration of the punta salinas blanking (in the AO direction).
04oct13: punta borinquen radar blanking re-enabled.
29may13: 1419.963,1667.957 birdies from dustbin controller on az arm
09may13: narrow band tone at 1200 Mhz.
29apr13   measuring the punta Borinquen radar blanking in the AO direction.
23apr13  punta Borinquen carsr 1274,1332 Mhz radar
14feb13: galfacts see broadband rfi in alfa
01feb13: x111 dynamic spectra for lband nov/dec12

2012

01jul12  glitches in rf from alfa monitoring are only present with bias monitoring.
17may12: galfacts see galileo rfi at 1278.7 Mhz

2011

2010

2009

2008

2007

2006

2005

2004

2003

2002

2001

03jan16: a2981 see 66Khz comb at 1667.562 Mhz

• 20dec15,24dec15,26dec15,31dec15, and 03jan16.

• for each record of each day i fit a linear baseline and subtracted it.
• 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.

• 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 channel.
• 1424.051 and 1422.082 are flagged in red and green.
  
• Bottom: power in 1.5 Khz channel vs azimuth for each rfi signal.
• 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 degs/sec).
• 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 database).
• 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 tower.
• 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 tower direction.
  
• 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 182.87 degrees.
  
• 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 triangle.
  

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05sep06: A 1378 Mhz cfr, 3.2 Mhz wide radar appears for 430 seconds.    (top)

• 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 coincidence.
• 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 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.

• 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.

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18jul06: 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:

• 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 parking spot.
• Use two Hp synths for the power source. Set the amplitude to 0 dbm.
• 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 .5 sec/spectra.
•     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 sample.
• 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 separate).
• 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.

• 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.

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17jul06: 1200 Mhz narrow birdie  (top)

• The birdie was not resolved in a 25 Khz channel width (after hanning smoothing).
• 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.

• 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.

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12jul06: 1188 Mhz birdie, 66 Mhz comb from kronos timecard reader.  (top)

• 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 dependence of 1400 Mhz birdie (.ps)  page 2 center plot). At the maximum position the single could be 13 db higher

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06jul06: birdie near 1320 Mhz:   (top)

• 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 Mhz.

• 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 dome.
• 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.

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07apr06: distomat birdies after window replacement. (top)

•  (polA (.gif)) (PolB (.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.
• Time series for 12 kHz channels at 1399.84 and 1400.01 Mhz (.ps)  (.pdf). These 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.

30nov05: Compression in the alfa receiver with the 100 Mhz RF filter in.  (top)

•  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 telescope was stationary at az=340, za=16.6 degrees. 340 azimuth had large compression when the azimuth 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.

• Compute the total power for each time sample.
• Remove Tsys by normalizing to a 1 second median and then removing it.
• 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.

 

• power versus time when the faa radar pointed at the observatory (.ps)  (.pdf)
• 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 observatory.
• You can see compression in beams 0b, 1b, and 6b.
• Time spacing between adjacent negative going spikes (.ps) (.pdf): The 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 aerostat).

Summary:

• The alfa filter bank reduced the compression in the alfa receiver.
• Compression was only seen in 3 pixels (0b,1b, and 6b).  The maximum value was about 10%.

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28sep05: 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 K).
• The measurement was made at the entrance to the walkway between the control building and the cliff (next to the operators office).
• 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:

• 1300 and 1400 Mhz birdies when the 100 Mhz reference was unlocked (.ps) (.pdf). Data 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:

freq	locked polA/polB	unlocked polA/polB
1300.	3.8/0	.5/.2
1299.999	0/0	2.3/.25
1400.	.2/.2	.05/.08
1399.999	0/0	.05/.0

• The strength of the birdie versus azimuth (.ps) (.pdf): 4 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 3*Tsys.
• 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 azimuth.
• 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 azimuth position.
• Within say 1/2 hour the azimuth positions of the peaks are repeatable.
• 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.

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26feb05: 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 hilltop monitoring 1325 Mhz band for 26feb05 (gif). Lighter color is stronger signal.
    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 :

• Chan 3,4 (mode A)
• Chan 6,7 (mode B)
• chan 10,11 (mode C)
• chan 18 (diagnostic channel)
• chan 5 (maybe).

•  (polA (.gif)) (PolB (.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 resolution).
• Time series for 24 kHz channels at 1400 Mhz (.ps)  (.pdf). These 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 Tsys.

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17nov04: 1367/1382 radar.. punta salinas chn 18 left running.

• 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.

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06oct04: faa,punta salinas, and aerostat Radars. total power versus time.  (top)

• The total power versus time for the 48 seconds (.ps)   (.pdf)  A peak hold on each 10 milliseconds of data was done to decimate the data for these plots.

• 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 attenuation in.

• The Faa 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 figure 2.
• 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.
• The Punta Salinas frequency agile radar.  (.pdf)  The 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 blanking period.
• 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.

• The aerostat balloon radar.  (.pdf)  The 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).

Summary:

• The time variability of the 3 radars is shown.
• You can use the time variability to distinguish between the different radars:
• 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 ao.
• The aerostat has back lobes +/- 100 degrees from the ao direction
• 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.

28sep04: azimuth dependence of faa radar signal using alfa receiver.  (top)

•  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).

• Compute the total power for each time sample.
• Remove Tsys by normalizing to a 1 second median and then removing it.
• 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 observatory.

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.
processing:040928/inputtp.pro,doproc.pro,mkplot.pro

02jul04: 1400 Mhz birdie coming from 100 Mhz distribution/wapps.  (top)

1. We unlocked the phase lock loop for the 100 mhz distribution. This caused the birdie to drift by many khz.
2. We pulled the power on the 100 Mhz distribution and the birdie in the receiver room went away.
3. 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 signal.

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 control room.

29jun04:  birdies around 1400 Mhz during azswing.  (top)

• The image of 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 in 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 channel).
• 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 distomat 3).

11jun04 drifting harmonics in lband with 60 Mhz spacing    (top)

30may04: distomat birdies still there after converting to fiber communications. (top)

•  (polA) (PolB) 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.
• Time series 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 width.

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28may04,07jun04 1374.495 Mhz birdie. (top)

• 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 (.gif) 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.

• 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.

14oct03 puntaSalinas? birdie during a1803 experiment.  (top)

• Fig 1 below is a dynamic spectra 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 hold 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)

• 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 bad camera.

 

13oct03 1422.5 Mhz birdie from tv channel 54.

• 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 stability. (top)

• 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 period.

 

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04apr03: Radar power levels in the downstairs IF (if2)   (top)

Setup:

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
Upstairs attenuators	rf: 6 5 db if: 0 0
Downstairs  power level (IF2)	-52 dbm meter -32 dbm before coupler
IF2 attnenuation	-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	gain 5
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 were active.

Calibration:

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.

• 1290 Mhz Remy radar.

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.
• 1350/130 FAA airport radar.

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.
• Punta salinas 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 not.
• 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 occurring upstairs.

Conclusions:

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 mixer chassis.

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18sep02. Measuring 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.
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 pulsing.
Peaks 6,7,8,9,10.. are the punta salinas radar.

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28apr02 Distomat biride at 1399.83 (top)

       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 resolution.

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09apr02 rfi from color cameras in the dome. (top)

• image of spectral density as cameras turned on off. The cameras went onoff at:
• 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.
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 camera 2.
• time series for 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% of Tsys.
• Camera 1 was 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 long enough

01mar02 azimuth swing with 1400 Mhz birdie (top)

28feb02 1400 Mhz birdie. (top)

• 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 in time.
• 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.

 

scan1 on start	78469
scan1 off start	78830
scan1 off end	79130
laser ranging last pnt	78962
laser ranging off by	79082

    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.
    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.
processing: usr/a1400/28feb02.pro

06nov01. 1381 gps L3 birdie on all the time. (top)

 

date (of end time)	astTimes (hours)
03nov01	19.0->0.8am 4 bursts of 1 to 2 minutes duration.
04nov01	19.0->0.8am 1  two minute burst
05nov01	19.0->20.8 ...1 to 2min on for each 4 minute scan 20.8->23.3 off 23.2-> 0.8  on .. 1 to 2 min of for each 4 minute scan.
06nov01	19.->.8 on most of the time. There were the 1 or  2 minute bursts that were strong. When the strong bursts stopped, a lower level emission remained on.
07nov01	3 bursts of: 2min, 1 min, 1min for the entire night (19. ->0.8 am)
08nov01	4 bursts of:1min,2min,1min,1min for the entire night (19->.8am)
09nov01	3 bursts for entire night (19->.8am)

The figure 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 aerospace!!)

• Figure 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.
• Figure 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 display purposes.
• Figure 3 was taken on 08nov01 during the day with the telescope stationary.

    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 didn't.

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05oct01. tone at 1350 Mhz. (top)

• 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).

processing: x101/011005/1350tone.pro

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04oct01 Radar harmonics created in the 2nd IF. (top)

Spectral line observing at lband normally has the following IF processing steps:
• The lband wide front end has a frequency response (see link).
• A set of filters between the dewar and the post amps limits the bandwidth.
• 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)
 

radar freq	130 Mhz Above(6 -/+Mhz)
1233 modeA puntaSalinas	1363  (1357-1369)
1242 puntaSalinas modeA	1372 (1366-1378)
1248 puntaSalinas modeA	1378 (1372-1384)
1257 puntaSalinas modeA	1387 (1381-1393)
1241.75 aerostat	1371.75 (1365-1378)
1244.6 aerostat	1374.6 (1368-1381)
1256.5 aerostat	1386.5 (1380-1393)
1261 aerostat	1391 (1385-1397)
1269 ramey	1399 (1393-1405)
1290 ramey	1420 (1414-1426)
1330 airport	1460 (1454-1466)
1350 airport	1480 (1474-1486)

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).

image 1 shows the 1330,1350 Mhz faa radar as well as 130 Mhz above each of these frequencies : 1460 and 1480 Mhz.
image 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 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 Mhz.

The other birdies in the 1330,1350 bands are intermods caused in the 8 bit A/D converter for the correlator by the radar.
processing: x101/011005/ifharmonics.pro

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13aug,20sep01 lbn. Rfi from color camera on the service platform. (top)

• image 1 (2 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 transition.
• plot of 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 shielding.

 

processing: x101/010813/dorfi.pro, x101/010920/colcamera.pro

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02may01 Measuring 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 birdies are:
• 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.
processing:x101/010502/doit.pro

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27apr01-11may01 1407 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.
1. It is located at 214.4 Deg az and 31.8 degrees az (180 degrees away) and is at high za (19 deg).
2. It is very narrow in azimuth. It drops to 10% of maximum in 2 degrees az.
3. The strongest it gets is about 5*Tsys(40K) = 200 Kelvins coming through the telescope.
4. The signal appears to be left circularly polarized coming in the telescope.
5. There is a 15.8 Khz comb and a 60 hz comb in the signal.
6. 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.
7. The signal wanders around by up to 1 Mhz over hours.
8. The signal is almost always on.  We did see it turn on at 9:22 am 11apr01.
9. 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.
10. 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 using).
11. 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 spectrum analyzer.
12. 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).
13. There are no harmonics of tv stations that fall at this frequency.
14. 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.
15. The radar blanker (with an lo of 1340 Mhz) was turned off but the birdie remained.
16. The signal drift rate was up to 1 part in 10-4 over 5 minutes. This is a pretty lousy oscillator.
17. 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.
18. On 10may01 we went out to route 10 overlook.. no results. Next was the arecibo airport. We got a good signal coming from the east.
19. 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.
Some plots of the signal are:
• Figure 1 shows 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.
• Base band sampling 1 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.
• Side bands 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.
• Image 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.
• The power levels 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).
• Birdie direction from 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 by hand.
Discussion:
• 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:
(Aexp(iwt)+Bexp(i(w+delta)*t)^2= AAexp(i2wt)+ ABexp(i(2wt delta)) + BB*exp(i(2w+2delta)t)
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.
If the video is a 2nd harmonic, then the audio below the video harmonic could be 3*video-audio.
• 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 receiver.
• 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.
processing:x101/010427/rfifigs.pro, x101/010425/doitri.pro,x101/010504/doit.pro

        Summary:

• birdies at: 1287.5, 1299.84,1300.,1399.83, 1400.,1411.52,1412.5.
• Every 2 minutes, azimuth dependent.
• clk:27Mhz. looks like +/- 12.5 Mhz above/below the 1300,1400Mhz birdies

• 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.
• the plot 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.

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!!)

rfi measurements home: