[evlatests] RFI in S-band -- a summary

[Rick Perley]

    Recent intensive observing at S-band has greatly improved our 
knowledge of the RFI environment.  The situation is sufficiently complex 
that a proper memo, with figures etc., will be needed.  Here I give a 
short summary. 

    It appears that all the significant RFI power comes from 
satellites.  (There are of course ground-based contributions, but their 
power input is negligible in comparison).   I break the contributions 
into two groups:

    1) An 'omni-present' contribution, seen in all directions, although 
with considerable variation in power.  These are concentrated into two 
frequency bands:  2180 -- 2200 MHz, and 2310 -- 2350 MHz.  The latter is 
from the DSR satellites (Digital Satellite Radio), both XM and Sirius.  
The Sirius system comprises three satellites in highly elliptical orbits 
with a 24-hour period.  (XM is geosynchronous -- see below).   At least 
two are visible at any one time (the apogee is above North America).  I 
don't yet know the origin of the first-listed contribution, but its 
omnidirectional characteristics suggest a system similar to that of the 
Sirius system. 
    2) A contribution from the 'Clarke Belt' -- the zone of 
geosynchronous satellites.  There are dozens and dozens (perhaps 
hundreds?) of these, broadcasting in many bands (including C, Ku, and 
K).  Prominent among this group are the two XM s satellites, plus a 
string of S/C band downlink satellites, approximately one for every 
degree along the Clarke Belt. 

    RFI power from the geosynchronous satellites is only a serious 
problem (defined by sufficient power to compress the analog electronics) 
when pointing close to them.  How close?  This is not yet well 
established, and clearly will depend on which satellite and which band.  
But one thing is clear -- observations at S (and presumably C) band, for 
declinations within a few degrees of -5, are *severely* compromised, and 
may be impossible. 
    In contrast, for observations, say, 20 degrees away from the 
geosynchronous belt, the only significant issues are the two relatively 
narrow frequency bands given in (1) above.  An open question is whether 
we should attempt to block, by passband filters, these two frequency 
bands. 
    The S/C band downlinks noted above are quite a nuisance -- these 
downlinks are continuous from 3.6 to 4.2 GHz, and if the antennas are 
anywhere close to the geosynchronous belt, completely overwhelm the 
system power.   At some (still not clear) distance from the belt, the 
contributed power is negligible (as a fraction of the total), but the 
visibility data from the target source may -- or may not -- be 
significantly compromised. 

    There is much to learn yet before clear decisions can be made on how 
best to deal with this hostile environment.  As shown by the Condon et 
al. result (the confusion-dominated deep survey), excellent results can 
be  obtained in S-band.