[evlatests] High-Speed RFI!

[Dan Mertely]

Hi Rick.  No satellite transmitters are allowed in 4200-4400 MHz, see
below from the allocation chart:
-------------------------------------------
International Table:
4200-4400
AERONAUTICAL RADIONAVIGATION
5.438 5.439 5.440

United States Table:
4200-4400
AERONAUTICAL RADIONAVIGATION
5.440 US261

5.438 Use of the band 4200-4400 MHz by the aeronautical radionavigation 
service is reserved exclusively for radio altimeters installed on board 
aircraft and for the associated transponders on the ground. However, 
passive sensing in the Earth exploration-satellite and space research 
services may be authorized in this band on a secondary basis (no 
protection is provided by the radio altimeters).

5.439 Additional allocation: in Iran (Islamic Republic of) and Libyan 
Arab Jamahiriya, the band 4200-4400 MHz is also allocated to the fixed 
service on a secondary basis.

5.440 The standard frequency and time signal-satellite service may be 
authorized to use the frequency 4202 MHz for space-to-Earth 
transmissions and the frequency 6427 MHz for Earth-to-space 
transmissions. Such transmissions shall be confined within the limits of 
± 2 MHz of these frequencies, subject to agreement obtained under No. 9.21.

US261 The use of the band 4200-4400 MHz by the aeronautical 
radionavigation service is reserved exclusively for airborne radio 
altimeters. Experimental stations will not be authorized to develop 
equipment for operational use in this band other than equipment related 
to altimeter stations. However, passive sensing in the Earth-exploration 
satellite and space research services may be authorized in this band on 
a secondary basis (no protection is provided from the radio altimeters).
------------------------------------------
Could the apparent fast geographic sweep of the RFI be the
result of aliasing with our sampling?  The cause is more likely
to be an airborne altimeter from a fairly slow moving plane.
-Mert


On 10/3/2016 3:38 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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