[evlatests] High-Speed RFI!

[Barry Clark]

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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