Intro

• Standard lbw puppi setup with rf center freq set to 1680MHz.
• this maps 1380MHz rf to 1200MHz IF (which is the center of the 800 to 1600 puppi  band).
  
• Puppi used search mode with 2048 channels, 800MHz bw, 40 Usec dumps.

To debug the problem i looked at file

puppi_59023_J1843+05_0341_0001.fits

Plotting the average spectra and total power:

    Each row in the fits file had 4096 spectra (40Usec each). I computed the average spectra for 500 rows (83.8 seconds of data).

The first plots shows the average data (.ps) (.pdf)

•  Top: Average spectra (for 84 seconds).
• The colored dashed lines show the frequencies of various radars (more info)
  
• blue: faa radar freq: 1257.59,1252.41,1349.59,1344.41 MHz
• Green: punta borinquen radar (Pb) freq: 1274.58,1269.41,1332.59,1327.41 MHz
• Red : punta salinas radar (Ps) freq: 1228.50,1243.50,1279.50,1294.50 MHz
  
• purple: Aerostat radar (aero) freq: 1261.25,1246.2 MHz
• I've also labeled
  
• gpsL1 : 1575
• gpsL2 : 1227.6 (this is close to a punta salinas freq)
• I failed to label a few more gns satellites (gpsL4,beidou, galileo, glonass).
• Bottom: total power vs time
• the total power (1120 to 1750 MHz) was computed for each .17 second row of data and then plotted vs time.
• the data was normalized to the median value.

    The Average spectra has lots of small  spikes in it.

Dynamic spectra of .17 sec spectra

• 500 .167 second spectra were used.
• The median spectra was used to flatten the image
• Colored lines flag the radar frequencies.
• there are stonger bursts of rfi every 12 seconds when the rotating radars point at AO.
• eg (green lines at 1328, and 1332 punta borinquen radar at 4 and 16 seconds).
• during the rest of the 12 second rotation ao still sees some rfi.
• There are some wide (in freq) interference with a drifting (in time) phase.
• These are gns satellites after normalizing the image to the median value.
  
• 25mhz bw area around 1575.
• This is a gps satellite's L1 frequency
• 1175 MHz. this could be gps L5
• 1228: probably gps L2
• 1207:this could be galileo or beidou
• 1267: beidou or galileo
• There are horizontal stripes  of black.
• These are caused when the if/lo and or backend goes into compression
• the median correction causes them to be black since most of the time there is no compression.
  
• These can occur when a radar points at AO (every 12 seconds).
  
• If you look at the black  trace at 48 seconds you'll see that it lines up with the the yellow aero stat radar pulse.
• A large number of the short interference glitches occur in these black lines (though not all)
• when the system goes into compression, it can create intermods of  signals within the band.
• Most of the narrow vertical lines have a spacing of about 3MHz.
  

Total power vs time

• Each plot has been normalized to the median value.
• The   radar plots   use a single channel from the 800 MHz spectra (390KHz bw).
• the radars transmit at more that 1 frequency. I've only plotted one freq from each radar.
  
• Top: total power vs time (1120-1750 MHz) using the  40usec samples (although there aren't enough pixels to show the complete resolution)
• The values that go below 1. are when the system compresses (eg 78 seconds)
• Some of the blank areas are when a particular radar blanks in our direction.
• 2nd: Aerostat radar (1262.25 MHz)
• The wide blanking region is when the aerostat points at AO (eg 20 to 22 secs)
• The narrower blanking region is when the aerostat points at the San Juan airport.
• The total power compression corresponds to peaks in the aerostat signal
• 3rd: Punta Borinqeun radar (remy) at 1332.59 MHz
• This has a much narrower blanking region (eg at 50 secs).  It blanks for about +/- 2 degrees of rotation.
• There is a large sidelobe 6db above the blanked main beam 12.6 degrees from ao boresight. This is a known problem with PB dish.
• 4th: FAA radar (1349.59 MHz)
• The Punta borinquen and FAA are similar carsr radars (using different sets of freq).
• The faa blanking is also +/- 2deg
• the off ao boresight level is more constant the the PB radar.
• Bottom: Punta salinas radar (1294.5 MHz)
• this has a larger blanking area when it points at AO.
  

• The deepest compression occurs around 78 seconds
• The longest compression occurs 47 to 49 seconds
• The compression will turn out to be from the aerostat radar.
• It does not occur around the blanking region when the aerostat points at AO
• It must occur at different sidelobes of the radar
• It occurs more that once in a 12  second rotation.
  

Dynamic spectra blowup at deepest compression

• The 4096 spectra were smoothed in time by 6 ( 240usecs resolution) to fit on the screen.
• The horizontal compression lines line up with the aerostat pulses
  
• Looking at the aerostat total power vs time, this period was about 3 seconds  away from when the aerostat pointed at AO.
  
• The colored lines show the radar frequencies (ignore the turquoise noise line :).
  
• The dark horizontal lines (where the compression occurs) line up in time with the individual aerostat ipps (occurring about every  3ms).
• The bursts every 90 milliseconds at 1620MHz is an iridium satellite (or a ground user).

A second dynamic spectra blowup using 9 milliseconds of time about 78.973 secs was made (.png)

• This has 215  40usec spectra. I've blown the vertical scale up by a factor of 3 (pixel replication).
• The colored lines mark the different radars
• The aero rf pulses are:
  
• 160 Usecs at 1262 usecs folllowed by 160 Usecs at 1246 MHz (4 40usec sec spectra followed by 4 40usec spectra).
  
• Each pulse is a chirped signal of 1MHz bw.
• there are 7 time intervals for the pulses that then repeat: (1371,3504,3076,2809,2903,3289, and 3676) usecs
  
• The compressed regions clearly occur during the aerostat pulse.
• The compression starts right after the 1262 Pulse start and ends toward the end of the 1246 mhz pulse.
• During the band compression  there are a series of pulses across the entire band. These pulses are not there outside of the band compression

7 40 usec spectra from within the 78.9725 sec compression  were plotted (.ps) (.pdf)

• Each spectrum is plotted using a different color.
• the spikes go full scale (8 bits)
• The spikes are also present in the 1000 to 1150MHz part of the band.
• The 1-2 GHz IF1 filter cuts off power in this part of the band.
• So the spikes are not coming from the AO IF/LO.  They must be coming from the puppi backend.
• The most likely source of the spikes are:
• Clipping at the puppi A/D converter
• clipping in the xilinx butterfly stages of the polyphase filter bank (18 bit fixed point  arithmetic).
  
• The final fft is done in the gpus (which doesn't have this problem since they use floating point)
  
• either of these can generate lots of spikes (my guess is the second).

The Global Navigation satellites

    The row averaged (.167) dynamic spectra showed wide rfi at some gns bands:

• gps:l5: 1175, l2:1228,l1: 1575
• beidou: 1207, 1268
• galileo: 1260-1300
• (more info)
  

plots were made of the various satellite constellations positions relative to AO on 23jun20 1:00

Plot  description:

• Each frame of a page holds 8 satellite (each plotted in a different color)
• The y axis is the ao Za for the satellite vs time.
• The x axis is the satellite azimuth relative to ao (this is the source azimuth, not the feed azimuth).
• the right col lists the satellite name, yymmdd hh:mm:ss time for the minimum za for the pass
• the * plot the orbit. it is stepped in 2deg of orbit phase
• the square symbol is the first point of the pass.
  

During the 89 secs of the dynamic spectra the telescope was at  az=346, za=13 (looking south).
Satellitesthat could have caused what we saw:

• gps10
• galileo gsat0205
  

SUMMARY:

• The if/lo system was being compressed by the aerostat radar
  
• It is not normally up in the air at 1:00 am
• spikes across the entire band were being generated during the aerostat compression.
• They were being created after the if/lo filters.. so they must have been created in puppi.
• Either the A/D converter or the butterfly stages of the polyphase filter bank.
• The wideband (in freq) continuous rfi was coming from gns satellites.
• The lband wide system has had some problems, but the 2degK increase in Tsys during this period did not cause any of these problems.
• I didn't look into any dropped packets by puppi.
  

• The aerostat is normally 20db higher than any other radar since it is floating 5000 Ft up in the sky, looking down at the ground (no terrain shielding)
• For ground based radars, a plane flying by can give large signals (it gets around the ground shielding). In the case of the aerostat i don't think this is the case.. (so we can't just say this was a plane flying by).
• experience has shown that the signal level you get from a particular radar can vary with az and za. (by many db).
  
• If the system is being overdriven, then the only thing you can do is add more attenenuation (before the piece that is compressing).
• This could be the rf attenuators (before the first mixer) if it is the first mixer saturating.
• the if attenuator (upstairs after the first mixer and before the fiber optics) if the fiber optics is saturating.
• 2nd if2 gain or the puppi attenuators .. if the problem is with puppi.
• doing this can cause the system temperature to go up a little (eg.. too much attenuation before the hot fiber optic tx..). But an increase in tsys may be better than no data :).
• since puppi was causing the spikes, increasing the if2  or puppi attenuation would have helped in this case.
  

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