[evlatests] A most interesting RFI event

[rperley]

The long (16 hour) L-band holography run, done March 13/14, contains two 
extraordinary events which appear to be due to satellite borne 
radiation.  The characteristics are unusual ...

1) Only the RCP channels were affected.  LCP shows nearly no effect.
2) Both events took about 1 minute, with smooth changes throughout, and 
maximum effect exactly in the middle.
3) The correlated power, monitored total power, and the synchronous 
power were all affected.  The (digital) total power was 'beam-like' -- 
plotted versus time, we see nice sidelobes, and the inner main beam, 
superposed on a large negative offset.
4) Only the reference antennas were affected!  There were six of these, 
and all showed the effects about equally.  None of the target antennas 
saw anything at all.  These were 10 to 15 beamwidths away at the time 
(or, 5 to 7 degrees) of the events.
5) The perturbations were much stronger in the lower frequency SPW (1445 
MHz) than the high frequency one (1820 MHz), and the first event was 
much stronger than the later one.
6) The effects were no uniform in frequency -- the upper frequency part 
of the lower window (1381 -- 1509 MHz) was much more strongly affected.

 From this, I deduce:

1) An RCP-emitting signal passed through the main beam, at an angular 
rate of about 1 degree/minute.
2) The signal is strong enough to depress the gains of the entire front 
end.
3) The true signal frequency is likely in the 1500 -- 1600 MHz range.

For reference, here are the times and the pointing directions at the 
time of the incidents.  Perhaps somebody can figure out what hit us:

#1 event:  13 March 2021 at 18:33 IAT.  Az = 41, El = 17.

#2 event:  14 March 2021 at 05:30 IAT.  Az = 309, El = 46.

Rick