[evlatests] Lunar-reflected terrestrial RFI

[Rick Perley]

I thought the attached plots might be of general interest.

As many are aware, Eric and I are hard at work enabling AIPS to make 
images from arrays employing linearly polarized feeds.  To do this, we 
are utilizing MeerKAT data taken of the moon last year, and observations 
taken of 3C286 this year.  Many interesting/intriguing/puzzling things 
have been uncovered along the way -- (which is a way of saying that this 
project is taking a lot more effort than we had original estimated ...)

One of these is the origin of a distinct bright spot in the lunar 
emission.  The attached plot 'MoonAt935Mhz.png' shows the lunar emission 
at 934.5 MHz:  Besides the large round disk showing the intrinsic lunar 
thermal emission, there is a fuzzy bright spot, smack dab in the 
center.  We are quite certain this represent reflected terrestrial 
emission -- dominated by RFI.  We see this at most frequencies between 
530 (the bottom end of the MeerKAT UHF band) and 960 MHz.  Above this, 
none is seen except near 1830 MHz.  (This is an undeconvolved image -- 
the MeerKAT array design makes this possible for low DR efforts like this).

For amusement, I made lunar images from 782 through 935 MHz, every 532 
KHz.  The attached plot EarthRFIfromMoon.png shows the brightness 
maximum (always at the central point) as a function of frequency.  The 
low, flat portions represent the lunar thermal emission, for which there 
is no bright central reflected spot.

This (presumably) reflected emission is highly polarized (elliptical -- 
neither linear nor circular).  I haven't had the courage yet to make 
detailed Q, U, and V images for the hundreds of channels I have 
available ...

I don't know the specific origin of these emission -- and invite anyone 
to figure this out!

Rick
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