Intro

Setup:

• The gregorian dome was set to za 18 degrees. The azimuth swung from -90 to +270 degrees (and back) at .35 degrees/second.
• Data was recorded with the mock spectrometer
• 6 x 160 MHz bands were recorded  covering 1112 to 1760 MHz.
  
• spectra were dumped at 50 millisecond with 21 KHz resolution.
  
• A 10 second calOn,calOff was done before and after the first azimuth swing to convert from spectrometer counts to kelvins.

Processing the data:

• The calon,off was used to compute the scaling factor from spectrometer counts to kelvins.
  
• The average spectra was computed for the 10 second  calOn and calOff spectra.
• an attempt was made to exclude frequency channels with rfi by fitting to  calOn/Caloff and excluding freq channels residuals greater than 3 sigma.
• the spccntsToKelvins was computed by using the cal value for the center of each band and the mean value of (calon-caloff).
  
• outliers (from step 2) and the bandpass edges were excluded in the average.
  
• the same channels were used in the actual spectra when converting  spc counts to kelvins.
• A band pass correction (to remove the IF bandpass) was computed using the calon-caloff spectra
• a robust 13th order harmonic fit was done to calOn-caloff.
  
• let bpIF(frq) be the IF bandpass.
  
• assume Tsky,Tcal is constant in freq over the 160 MHz bands.. then:
• CalOn-calOff = (Tsky + Tcal)*bpIF - (Tsky)*bpIF  = Tcal*bpIF(Freq)
  
• the band pass correction is then Tcal*bpIF(freq)/mean(Tcal*bpIF) .. (where the average is over the non rfi channels).
• Each 50 millisecond spectra was then converted to kelvins  by multiplying by spcCntsToKelvins/bpc
• PolA and polB were then averaged together.
  
• To convert from kelvins to dbm:
• dbm= alog10(degK * resolutionBandWidth*Kboltz)*10 + 30 .. (Kboltz =1.3806e-23)
• The 6 x 160MHz spectra were interpolated to a single spectra covering the entire band.
  

The results:

The average and peak hold spectra:

• The average and peak hold spectra  in degrees Kelvin (.ps) (.pdf):
• The vertical axis is degrees Kelvin.
  
• The black trace is the first azimuth swing, the red trace is the second.
  
• top Frame: the average spectra over each azswing.
• The rfi above 1700 Mhz did not repeat in the 2nd swing. Looks to be time variable (which is why it didn't show up in the median spectra).
  
• 2nd Frame: the median spectra over each azswing.
  
• 3rd Frame: the peak hold spectra for the azimuth swings.
  
• This kept the highest value in each frequency channel (20Khz resolution) for a swing.
• Bottom Frame: rms/Mean  for each frequency channel over each  az swing.
• If we had just noise, the radiometer equation would give sqrt(1./(21Khz*2pol*.05 secs) = .021..
• the second red swing had a higher rms/mean than the first. I think this was caused by crossing more of the galactic anti center (ra=6hrs) during the second swing.
  
• The average and peak hold spectra in dbm (.ps) (.pdf)
• The average spectra in degK was converted to dbm using the 21Khz channel resolution.
• Black is the first 1st azswing, red is the 2nd.
  
• top Frame: The average spectra
• 2nd Frame: The median spectra over each az swing.
  
• bottom Frame: The peak hold spectra.

The total power vs azimuth

    The total power vs azimuth (.ps) (.pdf):

• The total power (1113-1758 MHz) was computed for each 50 millisecond sample.
• Black is the first az swing, red is the 2nd azswing.
  
• Top frame: the average temperature in deg K vs azimuth
• Bottom frame: The average power in dbm vs azimuth.
• This power is averaged over 50 milliseconds.
• The rapid changes are the various radars with their 12 second rotation period.
  
• When they point at AO, the signal goes up.
• The radars have a duty cycle of around 10%.
  
• Their peak radar power  (within the rf pulse) will be at least 10db higher than the plotted values.
• The strong rfi around 130 deg az in the first swing looks to have moved to around 110 deg az in the second.
• The peak value is around -89.5dbm
  
• the dynamic range is about 6.5 db (averaged over 50 milliseconds)
  

 Plotting some known rfi vs azimuth.

    The power in some known rfi bands was computed and plotted vs azimuth. This showed which frequencies were causing the changes in the total power.
The top frame is the average total power for a swing. The other frames show the power in a particular frequency band.

• Page 1 is az swing 1. Page 2 is az swing 2
• 2nd frame: the 4 frequencies used by the FAA carsr radar (more info)
• The 4 frequencies are plotted in different colors.
• The spikes are from the 12 second rotation of the radar.
• The carsr radars have around a 10% duty cycle.
  
• 3rd frame: puntaBorinquen radar (more info)
  
• This is the same type of radar (carsr) as the faa. it too has a 10% duty cycle.
• bottom frame: the punta salinas frequency agile radar (more info)
• It was running in ModeA.. using only 4 frequencies. 
• I only plotted 3 frequencies since one of them overlapped with gps.
• punta salinas  has a short pulse/ipp: 51/1500usecs =3.5% and a long pulse: 409/2600usec 15%.
• We are probably seeing the long pulses sticking out in the plots.
  

The radar bands total power vs azimuth (blowup in azimuth) (.ps)  (.pdf)

• The same radar plots with the azimuth range limited to 70 to 100 degrees azimuth.
• You can see the individual 12 second rotations.
  
• You'll notice that the punta salinas has a large blanking period as it points at AO.
  
• The faa and punta salinas have shorter blanking periods when they point at AO.

Total power vs azimuth for gps, iridium, and 1710-1750 Mhz (.ps) (.pdf)

• Page 1 is azswing 1, page 2 is azswing 2.
• 2nd Frame: total power for the 3 gps frequency bands (more info).
  
• The bump in total power at az=150 degrees was from a gps satellite.
  
• it was stronger in az swing 1, and weaker in azswing 2.
• The satellites are at 1/2 geostationary orbit, so their movement on the sky is much slower than the LEO satellites.
• I didn't check how close to the beam the satellite came (i could if there is interest).
  
• 3rd Frame: iridium satellite band (more info)
  
• the power was computed over 1617-1627 MHz. The official iridium band is 1616 to 1626.5 Mhz.
• The 1st azswing has a peak  around az=150 degrees (similar to gps).
• The 2nd azwing has a large bump at 90 to 120 degrees azimuth.
• The iridium satellites are about 780 Km above the earth. They only remain above the horizon for about 15 minutes (more info)
• the time between the measurement of the two bumps was at least 14 minutes, so the two bumps are probably coming from different  iridium satellites.
• The iridium satellites are making some of  the largest change in the total power for the 2 azimuth swings (in the 50 millisecond integration).
• Bottom Frame: power in the 1710 -1750 Mhz band
• I plotted 4 separate frequency ranges in the 1710 to 1750 Mhz band.
• 1710-1720 and 1730-1735 were very strong in az swing 1.
  
• They were not present in the 2nd azimuth swing.

• These are the same gps, iridium, 1710=1750Mhz plots with the azimuth range limited to az=100 to 180 degrees.
• In the 1st az swing, you can see the iridium satellite going through the sidelobes of the telescope.
  

Summary

• the total power vs azimuth was computed for the lband wide (1113-1757 MHz).
• the power varies by about 6 db in the 50 millisecond averaged spectra.
• The largest signals came from
•  the 1710-1720 rfi. This only occurred once during the two swings
• The iridium satellites. These occurred on each swing (from different satellites
• the gps satellites.
• The strength of a satellite signal will depend on how close the satellite comes to the AO beam and sidelobes (so these values are  not the largest possible)
• The faa radars have a 10% duty cycle.
  
• The values show for the 50 millisecond integrations probably need to be increased by at least 10db to get the peak power during the radar pulses.
  
• The aerostat radar balloon was not up during this test. When running it will have a value up to 10-15 db higher than the other radars.
• The values shown here depend on the geometry of the AO beam/sidelobes  and the location/timing of the rfi emitters.
  

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