[evlatests] High-Speed RFI!
Dan Mertely
dmertely at nrao.edu
Tue Oct 4 12:29:09 EDT 2016
(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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>>
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