lband wide: high time resolution power
levels
05mar19
Intro
Azimuth swings were done with lband wide on
22feb19 to measure the power levels at the input of lbw the dewar (more info).
The az swings sampled different directions searching for the
strongest value. The test dumped spectra at 50 milliseconds.
The radars are pulsed signals (300 usec rf pulse,
about 3000 usecs ipp). The 50 millisecond integration time averaged
over the 10% duty cycle of the radars. An estimate was made that the
radar levels needed to be increased by 10db to get the peak power
level.
On 05mar19 high time resolution data was taken
with lband wide to try and sample the individual radar rf pulses
(and test the 10db assumption). The telescope remained stationary at
az=84,za=18 degrees during the test. This location looked to
be a spot where the 22feb19 data showed a strong faa radar response.
The average spectra , total power vs time, total
power vs time for some radars is also plotted. A final plot shows
the power level input to lbw that caused the system to go into
compression.
The aerostat radar balloon was not up during this test.
Setup:
- The telescoped was stationary at az=84,za=18 degrees.
- Data was recorded with the mock spectrometer
- 6 x 160 MHz bands were recorded covering 1114 to 1755
MHz.
- spectra were dumped at 64 useconds with 625 KHz
resolution.
- A 5 second calOn,calOff was done to convert from
spectrometer counts to kelvins.
- data was taken for 37 seconds (a little more that 3 12 sec
radar rotations).
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 5 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 64 usecond spectra was then converted to kelvins by
multiplying by spcCntsToKelvins/bpc
- PolA and polB were then averaged together (note: the first 3
bands had trouble with polB so only polA was used).
- 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.
- Top Frame The black trace is the average spectra. The red
trace is the median spectra
- 2nd Frame: the peak hold spectra for the 37 seconds (using
the 64 usec spectra).
- This kept the highest value in each frequency channel
(625Khz resolution) for the 37 seconds.
- Bottom Frame: rms/Mean for each frequency channel for
the 37 seconds.
- If we had just noise, the radiometer equation would give
sqrt(1./(625Khz*2pol*64e-6secs) = .11
- You can see a jump around 1380 MHz. below that only polA
was used.
- The average and peak hold
spectra in dbm (.ps) (.pdf)
- The average spectra in degK was converted to dbm using the
625Khz channel resolution.
- Top Frame The black trace is the average spectra. The red
trace is the median spectra
- bottom Frame: peak hold spectra.
The total power vs time
The
total power vs time (.pdf)
- The total power (1114-1755 MHz) was computed for each 64
usecond spectra.
- the 37 seconds is a little more than 3 12 second radar
rotations.
- the carsr radars pulses are:
- 117 usecs freq 1, 117 usec freq 2, 20sec freq , 20 usec
freq4.
- the 64usec spectra should sample this correctly.
- power ranges from about -96dbm to -83 dbm
Plotting some radar bands vs time.
The power in some radar bands was plotted to
see which was affecting the total power the most.
The radar bands total
power vs time (.pdf) (6.5Mbytes)
- Only 13 seconds was used for the plots (a little more
than 1 radar rotation).
- Top frame: lbw total power
- 2nd frame: the 4 frequencies used by the FAA carsr radar (more
info)
- The 4 frequencies are plotted in different colors.
- You can see when the faa radar blanks when it points toward
AO.
- The two edges of the blanking are the same height, so
their blanking direction is aligned with the AO direction.
- 3rd frame: puntaBorinquen radar (more info)
- This is the same type of radar (carsr) as the faa.
- The strongest peak in lbw total power is coming from the
sidelobe .5 seconds (15 degrees) after the center of the beam.
- this sidelobe is a known problem with the punta borinquen
dish.
- It is 10db higher than the blanked beam. (should we
encourage them to fix it??)
- 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 punta salinas blanking period is much wider than the
other two radars.
The radar bands total
power vs time (blowup around the strongest total power) (.ps)
(.pdf)
- This has the same radar plots as above, with the time range
limited to 6 to 7 seconds.. during the strongest total
power spike.
- You can see that this is coming from the punta borinquen
sidelobe.
Plot showing the onset of
compression in the if/lo or backend (.pdf)
- Page 1 : total power vs time for 13 seconds.
- top: the lbw total power.
- bottom:the total power in the 1390 - 1420 MHz band during
this same time period.
- The negative going spikes show the system going into
compression.
- The strongest compression is around 6.5 seconds (the punta
borinquen signal).
- there is also some compression around 12 seconds (the faa
radar pulse).
- Page 2: .15 second blowup around the strongest compression
- Top: lbw total power vs time.
- The spikes are the punta borinquen rf pulse (every ipp).
- The red lines show where the system went into compression.
- Bottom: The 1390-1420MHz total power
- You see the radar pulses sending the system into
compression.
- The red dashed line is close to the onset of compression.
- The radar pulse for this time had about -86.7dbm.
- We don't know what part of the system went into compression.
- the 1400 MHz band also has the radar close to it, so it
could have caused this to happen in say the digitizers.
- I also check the total power at 1700 MHz. It also showed
compression from the punta borinquen pulse.
- The 1700 MHz part of the band has gone through a separate
IF filter than the pb radar.
- So the compression is probably not in the digitizers.
Summary
- the total power vs time was computed for the lband wide
(1114-1755 MHz).
- spectra were dumped at a 64 usec rate.
- the power varies by about 13 db in the 64 usec averaged
spectra.
- Sitting at az=84, za=18, the strongest signal came from the
punta borinquen radar sidelobe.
- looking at the 1390-1420 MHz total power, the system went into
compression around -86.7 dbm
- This was probably before the mock mixers/digitizers
- the compression was also seen at 1700 MHz.
- The 1700 MHz band uses the 1750 IF filter.
- this filter should cut down the radar signal by at least a
few db..
- There is no guarantee that the az,za used will have given the
strongest radar signal.
- The maximum levels will be at least this strong.
processing: x101/190305/lbwhightmrespro, docalonoff.pro
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