plane altimeter looks like an FRB
may2019
Intro
A number of pulsar observations at cband have
seen rfi that looks like an FRB. Data taken on 190416 illustrates
the problem.
The file used was puppi_58421_GRB130831A_0934_0007.fits from project
p3218.
Data was taken with puppi in search mode:
- 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
- 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 :).
processing:
x101/190524/cbrfi/p3391_cbandrfi.pro
rfi measurements
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