[evlatests] High-Speed RFI!

[Rick Perley]

     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.