[evlatests] WIDAR RFI Tolerance test

[Dan Mertely]

Hi Rick.  What has changed "lately" to make the 1612 MHz
RA allocation more free of RFI is:

1) The new Glonass-M navigation satellites now use only
1598 - 1605 MHz.  They used to straddle the OH RA allocation,
with additional channels from 1613 - 1615 MHz.  By 2005 they
had all moved to channels at and below 1609.  Since then they
have not only limited themselves to channels at and below
1605, their new birds have special filters on-board to limit
out of band emissions & protect the RA band @ 1612 MHz.

2) IRIDIUM changed it's channelization and control scheme,
removing the very strong "ring-page" channels @ 1626 MHz.
The ring-page channels were very strong relative the the lower,
data channels, and were what gave our old VLA AGC such a fit.

-Mert


Rick Perley wrote:
>     The 'Golden 3C286 Dataset', taken on 28 July, has provided (as 
> reported earlier) some excellent images of the field of 3C286, with 
> dynamic ranges up ~200,000:1.  The observations were taken in RCP only, 
> with 11 antennas at L-band,with 4 subbands of 128 MHz width each, and 
> were about 6.5 hours long. 
> 
>     The imaging done earlier were utilized only a single sub-band -- the 
> one most free of RFI (indeed, almost entirely free).   I report here on 
> an imaging test done on sub-band #3 -- the one most heavily clobbered by 
> RFI. 
> 
>     The AIPS program SPFLG is very useful in perusing for RFI (except 
> that for this purpose, and ability to utilize a logarithmic transfer 
> function would be very useful!!!).  In displaying portions of subband 3 
> (which extends from 1556 to 1684 MHz), it was noted that (following 
> basic calibration) that the RFI which occupies nearly the entire 
> frequency range from the lower frequency edge through 1628 MHz has an 
> intensity of up to 5000 Jy, and is typically 1000 Jy.  I noted that 
> there is a narrow slot, of width 3 MHz, centered at 1613 MHz, which 
> showed no RFI at all -- this lies between the Glonass and Iridium 
> transmissions -- each of which are thousands of Janskys at times, and 
> are located about 5 MHz (ten channels) away on each side. 
> 
>     The test was then to determine the imaging performance within this 
> apparently narrow empty slot. 
> 
>     I extracted, with SPLIT, the 6 'empty' channels, applying Hanning 
> smoothing to damp down the Gibbs ringing from the adjacent RFI.  I did 
> not apply any bandpass solution.  This formed a single channel of width 
> 3 MHz, centered at 1613 MHz.  Perusal of these amplitudes showed a 
> nearly complete absence of any instabilities in amplitude or phase which 
> might be the signature of nearby RFI causing 'ringing' or 'echoes' in 
> the adjacent channels.  Near the middle of the 6.5 hour run is about a 
> half of of ratty data -- there is nothing special about the behavior of 
> the RFI signals at that time, so I attribute this to actual emission at 
> the frequency of the narrow slot.    I removed these on the basis of the 
> calibration solutions (via EDITA), and by CLIP on the final data 
> following calibration. 
> 
>     No 'heroic' measures were incorporated in the calibration -- I 
> simply started with a point source model, and iterated twice as the 
> background sources appeared.  (No symmetrization occurred).   As noted 
> above, EDITA was employed to remove antennas with deviant solutions -- 
> nearly all of these were in the short period of time noted above. 
> 
>     The final image is quite outstanding:  rms noise of 0.27 mJy, so a 
> DR of about 52,000:1.  The background 'noise' looks normal for the 
> amount of data actually gridded -- we are likely noise-limited.  This 
> noise is a factor of about 20 million below the peak RFI noted only 6 
> MHz away on each side (in frequency). 
> 
>     This little test strongly indicates the correlator is linear to a 
> very high degree -- but certainly does not constitute any proof of such. 
> 
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