[evlatests] XM Radio, and S-band compression

[Rick Perley]

    S-band observations are now being taken in earnest.  Tests done in 
the past  -- and observations taken until recently -- indicated that the 
presence of the DSR (Digital Satellite Radio) signals were not of 
significant concern outside the subband in which they lay (usually 
subband #3).  Careful perusal of the data taken recently is, however, 
forcing a reconsideration of this. 

    I have in hand, and have reduced, seven S-band observations, all of 
objects of declination near zero.   Review of the visibilities, and of 
the switched power data, strongly indicate that we are getting 
significant amplifier compression for many of these observations.  
Plotting the PDif data (which monitors amplifier gain) against elevation 
clearly shows that significant gain reduction (by factors of up to a 
few, but typically 1.5 to 3) occurs at intermediate elevations when the 
antennas are pointed to the SE or SW.  No effect has been seen below ~35 
degrees elevation,  nor above ~60 degrees.  All antennas, and both 
polarizations, are affected, about equally.  (Although the satellites 
broadcast in RCP, our sidelobes are elliptically polarized in general, 
so we get roughly the same power to each side).  (We have one example 
where only LCP was affected -- no obvious explanation is offered, other 
than a suspicion that the saturation occurs in the T304, and the input 
attenuators on that day may have been set too high in RCP).   And *all* 
subbands are affected -- this is a global amplifier compression problem. 

    The two XM satellites are located, approximately, at El= 50, Az = 
195 (HA ~ +1, dec = -4), and El = 42, Az = 140 (HA = -2, Dec = -8).   A 
rough guess is that we are seeing the compression when observing within 
about 20 degrees of either position.   A similar problem should occur 
when near the Sirius satellite system -- however, these are not 
geostationary, so predictions are much harder to make.  Roughly 
speaking, the three satellites are found at azimuths between 0 and 180, 
and at any elevation!  They transit 8 hours apart. 

    Clearly, this hypothesis needs to be confirmed.  As part of the 
early tests mentioned above, two antennas were outfitted with XM/Sirius 
blocking filters.  Unfortunately (and clearly, prematurely), these were 
removed in order to do some bench tests in the lab.  These will be 
re-inserted into two antennas tomorrow.  It is important to then make 
another S-band run of a source of near zero declination, to see if the 
compression effect is absent in those two antennas.