[evlatests] High-Speed RFI!

[Barry Clark]

The speed from the time lag measured this way is actually the
pattern speed divided by the cosine of the angle between the
baseline and the pattern's path.  So not necessarily incompatible
even with a subsonic aircraft.

On 10/04/2016 01:14 AM, Michiel Brentjens wrote:
> I hope this is something that is scanning and that 2000 mph is the speed
> of the beam across the VLA and not the transmitter's speed. What are the
> phases in the cross-correlations of neighboring antennas doing? Do they
> vary rapidly too? If the transmitter is moving at that speed, this is
> very interesting indeed. 2000 mph is definitely sub-orbital. It is close
> to mach 2.6. Therefore, it is likely *not*:
>
> - an F-35 (~mach 1.6)
> - an F-22 (~mach 1.8)
> - an F/A-18 (~mach 1.8)
>
> The only planes that can do this that I know of are the SR-71
> (decommissioned) and the Mig-25 (somewhat unlikely...). Perhaps it's
> "the next fast thing" out of Groom Lake?
>
> Anyway, have fun tracking down this one, and see you next week,
>
> Michiel
>
>
> Bryan Butler writes:
>
>> 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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