[evlatests] A Most Bizarre Event

[Rick Perley]

    ... and indeed this was the case.  The culprit was a GPS satellite, 
which went right through the beam.  Vivek has proven this by plotting 
the GPS orbits on our az-el coverage.  It is also clear the from spectra 
taken at that time -- subbands 4 and 10, which overlap the GPS regular 
transmission bands, are wiped out.  But all other subband spectra look 
normal. 

    Of more interest to me is how this mucked up all the SY table values 
in the subbands which were not affected ...



Barry Clark wrote:
> or if there was a satellite near the beam.
>
> Rick Perley wrote:
>>     The short L-band observation taken last night (around 7PM) was 
>> affected by an event of unknown origin which greatly modified the 
>> switched power table for one scan.
>>     There are four scans, two of the calibrator and two of a nearby 
>> (offset 1 degree or so) blank field.
>>     The first three scans are completely normal.  All switched power 
>> and visibility values appear normal.     The last scan (duration ~ 2 
>> minutes) was disrupted by some sort of event which greatly modified 
>> all switched power values, and the visibilities.
>>     The major characteristics are:
>>
>>     1) All antenna were affected equally.     2) All subbands were 
>> affected, but not in the same way.     3) Effects were seen in both 
>> PDif and PSum, but they are not the same.     4) The effect caused 
>> the monitored power to change *smoothly* during the 2 minute scan.  
>> The preceding scan (a few seconds earlier and only 1 degree away) is 
>> fine.     5) The effect is *much* stronger in RCP.  For some 
>> subbands, there is no effect at all in LCP.
>>     Most remarkable is how the PDif and PSum were affected 
>> differently, for different subbands.   In all cases, the PDif 
>> dropped, typically by a factor of four.  For one subband (4), PDif is 
>> actually negative!  But PSum dropped for 12 subbands, and rose for 
>> the other four.  The four subbands with an apparent power increase 
>> are:  4, 9, 10, and 15.  All antennas apparently behave in the same 
>> way.     It's hard to imagine how the switched power can drop by a 
>> factor of 4 while the summed power can rise by about the same factor.
>>     I've looked at the cross-power spectra, and there is certainly 
>> strong evidence of an unusual event, seen in some subbands, occuring 
>> during this time.  Some spectra are simply blank (presumably, all 
>> integer zero), and others are clearly messed up.  But other subbands 
>> look fine during this time -- but evidently are not, as judged by the 
>> system power monitors.
>>     The IAT time for this event was from 1:38 through 1:42 (or, 6:38 
>> to 6:42, PM, local time).  It would be useful for someone to look at 
>> other monitors to see if something very odd was occurring then.
>>
>>
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