1357.2 rfi

    A2335 (gass) reported rfi at 1357.28 MHz on 14mar11. They did not see it on their 10mar11 run. Their setup was:

• lbw linear pol with 1320 hipass filter installed.
  
• interim cor 25 MHz bw 1024 channels with 1 sec integrations.
• 300 second on,off processing.

The first plot shows a2335 14mar11 data (.ps) (.pdf):

• processing:
• the rfi total power was computed by averaging  two channels about 1357.28.
• A baseline of 10 channels (1356.9 to 1257.13) was divided into the rfi tp average.
• Top: the rfi total power vs azimuth.
• The onsource and off source are plotted separately. PolA and polB is also separated.
• Middle: a blowup around strongest rfi (az=33). This is close to distomat 1 location.
• PolA on and off source (which trace the same az, za) overlap each other. This implies that the rfi is a function of azimuth (and is not coming from inside the dome).
• Different on/offs that cover the same azimuth have different rfi amplitudes.. so the rfi is changing slowly with time.
• Bottom: the spectrum where the rfi was strongest. It gets to about 20% of Tsys.

During the day of 14mar11 azimuth swings:  clockwise (CW) and then counter clockwise (CCW) were done to check the azimuth dependence of the 1357.2 MHz rfi.
The setup was:

• lbw receiver with 1100-1800 MHz filter.
  
• az 270->630->270 at .35 deg/sec. The dome was at 15 degrees za.
• The mock spectrometer used 8192 channels across 172MHz (20 KHz channel width) and 1 second sampling.

The data processing was:

• fit a harmonic function to the median bandpass of each azimuth swing. Use this to remove the bandpass from each azswing.

Dynamic spectra of the azswings (about the 1357.2 MHz rfi) (.gif):

• The left image is the CW az swing. The right image is the CCW az swing.
• the horizontal scale (frequency) is blown up (1356.8 to 1357.8 MHz).
• The vertical scale is azimuth position (.35 deg/sec).
  
• the horizontal steps in the dynamic spectra show the rfi moving by a single channel (20 KHz).

The plots show the rfi amplitude vs az, time , and frequency (.ps) (.pdf):

• Top: A blowup of the average spectra for an azimuth swing (black:CW, red:CCW). PolA and polB have been averaged.
• The average rfi is about 15% of tsys.
• The CW spin saw rfi in 1357.12 - 1357.19
• The CCW spin had rfi  1257.08-1357.20
  
• middle: 1357.2 rfi strength vs azimuth for the 2 az swings.
• The CW and CCW peaks do not overlay one another (unlike the on,offs of the a2335 data). the same azimuth positions were separated by about 20 minutes for the two swings so the rfi is probably changing in time.
  
• Bottom: 1357.2 rfi vs time.
• Black in CW swing, Red is CCW swing.

Summary:

• The a2335 shows that the rfi is coming from outside the dome (since it repeats in on,offs).
• When the rfi appeared:
• 10mar11:
  
• morning (till 7:00 am):
• a2335,a2331 did not see it.
• evening:
• a2010 not present at 23:35, was present 23:55 AST.
  
• 12mar11:x111 saw birdie 16:07
• 14mar11:a2335 saw birdie.
  
• So it looks like the rfi appeared about 24:45 (ast) 10mar11 (thursday evening)
  
• The rfi was seen to range from 1357.08 to 1257.3 MHz.
  
• during the az swings the rfi drifted by about 100 KHz in 15 minutes
• The rfi is also changing slowly in time.
• We turned the new distomat off, and the rfi did not go away.
• During the az swings the alfa motor, and cameras were turned off.
  

processing: x101/110314/a2335_rfi1357.pro, lbwazswing.pro