[evlatests] A most interesting RFI event

[rperley]

A short note to point out there's no conflict between Walter's 
identification of the event as due to a GPS satellite, and Mert's 
non-detection.

The explanation is that the W8mon is on ea28, which was one of the 
'target' antennas in the recent holography run.  The GPS passage event 
was seen only in the 'reference' antennas.  At that time, the 'target' 
antennas were approximately 10 degrees away -- including ea28, which 
houses the W8mon.

No doubt there were similar passages of GPS satellites through the 
'target antennas'.  I found the 'event' in the reference antennas only 
because I was looking at the gain solutions.

Rick

On 2021-04-09 11:54, Dan Mertely via evlatests wrote:
> Hi Rick.  I looked carefully at the W8mon plot for 20210313, 18:33 IAT
> (11:33 MST) (attached, annotated) and do not see any significant RFI
> events around that time.  Note that the time scale is compressed by
> about 1 hour in 24, but at midday the time axis should be within 1/2
> hour of correct.  Nothing shows near the end of the daily plot either,
> whereas you detected a notable event at "14 March 2021 at 05:30 IAT"
> (22:30 MST).
> 
> I do see a short, wide-band event at around 09:00 MST (16:00 UTC), but
> no significant detections near the 2 times you recorded.
> 
> 
> As a comparison, I've also included the 24 hour plot for the prior
> day, 20210312, in which you can see some notable GPS hits at the time
> indicated as 13:50 MST, and especially 16:30 MST.  No such significant
> GPS hits can be seen on the 24 hour plot for 20210313 however.
> 
> For those on the list who are not familiar with the W8mon L-band
> monitor, it is a analog, hardware tap off of the RCP output channel of
> the L-band cryogenic receiver on (currently) ea28 on pad W8.  Since we
> have excess gain over the spectrum analyzer's noise floor, the
> background noise level seen on the plots corresponds closely to the
> instantaneous noise temp of the cryogenic receiver (around 35 K).
> 
> I don't know what could be the cause of the RFI you noticed on the
> 13th, the W8mon did not see any unusual activity at those times.
> -Mert
> 
> 
> On 4/8/2021 10:35 AM, rperley via evlatests wrote:
>> 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
>> 
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