[evlatests] High-Speed RFI!

[Bryan Butler]

if he was talking about SMAP, that's L-band.  i don't know of an 
orbiting NASA satellite doing radar at C-band.  not that i know 
everything that's up there!  there are two ESA C-band SARs - Sentinel-1 
and Sentinel-2 that might fit the bill, but they don't go over every day...

	-bryan


Barry Clark wrote on 10/3/16 16:22 :
> One of the tour participants at the openhouse Saturday said NASA
> has a downlooking radar satellite that probably passes, rather
> rapidly, over the VLA about noon or a little later every day.
>
> On 10/03/2016 03:38 PM, Rick Perley wrote:
>>      In calibrating astronomical data, mostly taken last year, a
>> remarkable type of RFI has been seen.
>>
>>      Observations were made in all four configurations at C-band. Over
>> one year elapsed between the first and last of these observations.
>>
>>      The effect was noted in the gains, starting with the
>> D-configuration data.  In this configuration, it was noted that all 16
>> spectral windows had a reduced amplitude, for all antennas, both
>> polarizations, by about a factor of two.  The effect lasted 30 seconds,
>> after which all gains were again normal.   Only one event was seen.
>>
>>      Investigation (via 'SPFLG') showed that the cause was a strong RFI
>> signal, located within SPW3 (spanning 4232 -- 4360 MHz).  The data in
>> this SPW had very large and random values, presumably due to overflow in
>> the accumulators.   All other SPWs retained their coherency, but the
>> amplitudes were reduced.  Phases were unaffected.
>> The signal was strong enough that the entire 2 GHz bandpass was
>> compressed enough to lower the gains by a factor of up to two.
>>      There were two C-band IF tunings in this experiment -- the other
>> one (6 -- 8 GHz) showed no effect, indicating the the compression is in
>> the IFs, not in the receiver.
>>
>>      But much more interesting (to me, at least) is that the data from
>> the C, B, and A configurations also showed similar compression.  But the
>> much larger spatial scale of these configurations clearly show that the
>> range of the RFI effect is localized, and moving at high speed.
>>
>>      The evidence is clearest for the A configuration.  There were four
>> 'events' during this run (which was taken in July 2015).  For all four,
>> the effect was localized to a a subset of the array.  For all four, only
>> the antennas on one or two arms were affected.  In all cases, the
>> strength of the compression varied along the antennas of the arm
>> affected -- usually with the end-most antenna the most strongly affected.
>>      The timing of the 'events' gives us a pretty good estimate of the
>> velocity.  The peak saturation for one of these four events showed a 30
>> second lag between the end antenna of the east arm and a middle antenna
>> of the north arm (and with the peak progressively later for antennas
>> along the east arm).  That translates to ~ 2000 mph!
>>
>>      Might this be some sort of satellite imaging radar?   It's clearly
>> highly focused, and moving quite quickly.
>>
>>
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