[evlatests] new RFI spikes at L-band

[Emmanuel Momjian]

Some of our low redshift neutral hydrogen observers will have a major 
setback when observing sources with sky frequencies between about 1385 
and 1405 MHz. The RFI sweep results taken last week show five RFI 
features (see attached; RR on left, and LL on right, average of all the 
available baselines in the data). The widths of these features span 
between about 1 and 2 MHz. For reference, in these RFI sweeps we target 
the north celestial pol. It is not clear when this started, but science 
data taken in May of this year (targeting a source at a declination near 
0 deg) have these too. However, the RFI sweep in June 2015 did not show 
them.

The majority of the baselines are showing these features with varying 
strengths. The analysis of the data is ongoing, while colleagues in the 
electronics division are starting to investigate the source of the problem.

For completeness, I append below Dan's initial assessment on this topic.

Emmanuel

On 09/08/2016 11:24 AM, Dan Mertely wrote:
> ... a little more precision on the top end of the sub-1400
> spectrum:
>
> 1392 and above should not be military, but we do see a lot of
> signals there.  I will try to get a DF to them & find out
> where they are coming from.  1395-1400 is allocated on a
> secondary basis to wireless medical telemetry (WMTS).  It is
> possible that someone at the Socorro Medical Center installed
> such a unit without the required approval process.  I'll check.
> -Mert
>
> On 9/8/2016 11:15 AM, Dan Mertely wrote:
>> Hi Rob.  The stuff between 1400 & 1427 MHz is probably internals,
>> except, of course, for the 1420 MHz signal.
>>
>> We know about the 1408 (128*11) birdie.  I'm not sure about how we
>> manage to generate the 1403 & 1399 (both years).  The 1420 is
>> probably galactic Hydrogen (HI) at 1420.406....
>>
>> Below that the spectrum is allocated to the military on a primary
>> basis.  We have some secondary, deferential rights provided we
>> work to frequency coordinate with them.  If we give them a few days
>> notice, they can reserve the 1350-1400 band for us.  Of course,
>> since we dynamically schedule most of our observations, that hasn't
>> been happening, except for the few, fixed-schedule observations I
>> see on Barry's monthly schedule.  If we want clear(er) observing,
>> we'll have to bend & schedule-in our observations, otherwise we
>> just have to live with it.
>>
>> The training use of UAVs at Ft Bliss and the WSMR has been on the
>> increase the past few years.  I don't know if that is why 1385 to
>> 1400 has gotten more busy, but it is likely.
>> -Mert
>>

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