Satellite L-band RFI at Parkes, Narrabri

From: <John.Reynolds_at_email.protected>
Date: Wed, 23 Jun 2010 11:30:20 +1000 (EST)

Hi all,

Since March we've been seeing some particularly pernicious RFI in the
1260-1340MHz band, which looks to have a satellite origin given that
we've seen it simultaneously at Parkes and Narrabri. I'm wondering if
anyone has seen or heard reports of anything similar at other
observatories.

So far this RFI has been seen only a few days each week, and usually in
daylight hours, Typically it consists of a pair of very strong signals
spaced apart by about 50MHz, e.g. 1260MHz and 1310MHz, and each about 3MHz
wide. In other manifestations there've been up to six signals (three
pairs), coming and going on timescales of hours. These signals are clearly
related and have a complex time and frequency modulation. They can be
incredibly strong - attachment #1 shows a measurement yesterday taken with
a spectrum analyser and a 6dBi horn antenna. With the trace on max hold we
recorded bursts in excess of -20dBm in a 1MHz resolution bandwidth, with
three peaks of emission at 1276MHz, 1290MHz and 1335MHz. Observing in this
band is impossible during such events. This is almost unbelievably strong
- one wonders about the safety of our receivers should this chance through
the 64m beam!

Attachments #2 and #3 were taken with our filterbank DFB3 in 'search'
(non-folding) mode and show something of the complexity of the modulation
and how the DFBs are being driven hard into compression, splattering power
across the whole band.

In addition to the frequency pair there is often emission at around
1247MHz and a number of lower frequencies that looks to be related, though
these don't seem to have the strongly impulsive character so inimical to
observing.

I also suspect that there is emission in the protected 20cm band.
Attachment #5 shows emission at 1415MHz which is clearly related to the
stronger peaks at (1275MHz and 1325MHz: not shown) but establishing that
this is a real signal and not an higher order product within our receiver
system (in this instance the Patriot 12m + DFB2) will take some doing.

The source of these emissions must surely be satellites (to be seen
simultaneously at Parkes and Narrabri on 17 June). One suspect is the
COMPASS system, the Chinese version of GPS believed to be in the process
of being deployed. There are 5 GSOs in this system and 20-30 MEOs with
12-hr orbits. However the advertised frequencies of this system don't
match those observed here. One of the Compass GSOs is clearly visible from
Parkes at about Az=351f, El=51d and its spectrum is consistent with
published information, with strong peaks at 1269MHz and 1208MHz: see
attachment #4. But the experience with GLONASS suggests the advertised
properties of these systems should perhaps be received with caution.

So in short, 20cm observing Parkes and Narrabri is being badly affected by
a new source of RFI for up to two or three days a week from morning till
late afternoon. The RFI may also be posing a risk to receiving equipment,
and may be spilling into the 20cm protected band. The culprit appears to
be a satellite or satellite system as yet not positively identified. In
particular it is not known whether the emissions are part of some
commissioning phase, or the tip of an impending operational iceberg. The
latter prospect is not a happy one.

Any relevant info welcome!

Cheers,
John R
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Received on 2010-06-23 11:31:48