[evlatests] High-Speed RFI!

[Dan Mertely]

I believe only "altimeters" are allowed in that band (from the
allocation chart).  By def "radar altimeters cannot see terrain
directly ahead of the aircraft, only that below it" (Wikipedia
article "Radar Altimeter").  They are fixed at nadir.

A scanning-type radar would be a Terrain-following radar (or TFR).
I don't think the allocations permit such radars in this band.

All that being said, I do wonder what would happen however, if an
"aircraft" were to make a fast, sharp climb ... it seems that the fixed
altimeter radar beam would then appear to sweep across the terrain
at a rapid clip.
-Mert

On 10/4/2016 10:34 AM, Rick Perley wrote:
> What about SARs?  Airborne side scanning radars?  They would sweep
> the beam, accounting for the apparent velocity ...
>
>      Rick
>
> On 10/04/2016 10:27 AM, Dan Mertely wrote:
>> 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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