Intro

• 800 MHz bw
• 4500MHz cfr
• 512 freq channels
• full stokes
• 40.96 usecond sampling

Processing the data

    The fits file contains:

• each spectra is 40.96 useconds
• there are 16834 spectra in a row of the fits file (.671 seconds
• i used 113 rows of the fits file (75.832 seconds) to look at the data.

    The processing steps were:

• Input a row of data
• compute polA,PolB from I and Q
• average the 16384 spectra for polA and for polB
• For the row corresponding to 60 seconds into the file, keep the spectra at 40.96 resolution for a higher time resolution image. 
  

The images:

• .67 second spectral time resolution images
  
• polA image
  
• polB image
  
• the pixels have been blown up by a factor of 2 in freq, and 5 in time
• The image has been flattened by the median spectrum over the 75 seconds, so the images shows the variations from the median.
• I've labeled the microwave link and radio Altimeter areas in red
• The red horizontal line at 60 secs is where the high res time data was taken (for the images below)
  
• RFI:
• 4100-4175 - micro wave links
• 3700-4200 is allocated to fixed links (fixed microwave  services .. title 47, part 101)
• We see usage up to 4175... (more info)
• 4235-4335 - aeronautical radio navigation
• planes use radio altimeters for height above terrain when they are under about 5000 feet.
• This rfi should only be present when a plane in passing near to AO (unfortunately this occurs a lot since planes land in san juan).
• A description of the altimeter method (.pdf)
• 4400-4940 - (federal table is fixed mobile)
• 4500 - 4800 (non federal table fixed-satellite (space to earth)
• Note sure where the rfi in the image is coming from.
• It may be intermods in our system, or illegal internet providers that are far away (say ponce) who are supposed to be in 5150-5800 MHz)
• 860 usec time resolution images
  
• One row at 60 seconds into the file was used to make images.
• to fit the 16384 40.96 spectra on the screen, we needed to average by 21 spectra giving 860 usec resolution.
• polA image
• polB image
• You can see the signal sweeping back and forth in frequency (although it looks like the chirp rate may be changing in time?)
• 120 usec time resolution images.
• this image uses spectra averaged to 123 usecs covering 8.3 milliseconds of time.
• polA image
• polB image
• You can see that there are two separate signals that are being chirped

Summary:

• the airplane radio altimeters are chirped signals that cover 4230 to 4330 MHz (at least in this case).
• the chirped signal can show up as FRB'S (at least the high freq to low freq portion of the sweep).
• This rfi should only be present when an airplane using an altimeter is in the vicinity of AO (of course I'm not defining "vicinity here :).