[evlatests] High-Speed RFI!

[Bryan Butler]

there's an active and passive component...


Dan Mertely wrote on 10/4/16 10:29 :
> (BTW: The SMAP L-band sensor is passive.)  -Mert
>
> On 10/3/2016 4:41 PM, Bryan Butler wrote:
>>
>> 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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