[MilCom] satellite comms?

[David I. Emery]

On Wed, Feb 14, 2007 at 09:31:45AM -0500, David T. Stark wrote:
> For those of us who are just joining the list, could the folks who are 
> discussing satcoms please indicate the mode?  i.e., Is 254.2 AM or FM?  I 
> have never had much luck with milsats and part of that is probably because 
> I'm never sure what mode to sit on.

	Normally transmissions through the military UHF transponders are
medium bitrate (4800-19.2 kbs) digital (FFSK/PSK/GMSK and encrypted) and
sound like other digital encrypted signals on regular fm scanners (more
or less open squelch noise).

	A few specially authorized users are allowed to use nbfm and
those transmissions are regular narrow FM with around 5 kHz or so
deviation.  They usually seem very loud and clear when one encounters
one.   They aren't all that common and mostly consist of test counts and
other attempts to establish communications which then go digital
encrypted, but once in a while an exercise or user gets authority to run
in the clear in FM and one hears hours of FM voice traffic.

	But there are two or three other kinds of signals on the
transponder downlinks:

	1.  Pirates (from Brazil and other places outside of US control)
who run a variety of modulations from really narrow FM to occasional SSB
and often operate off the center frequency of the transponder (up near
one edge or the other).   These guys are deliberately making use of US
military transponders illegally, but apparently little can be done to
catch and shut them down because they aren't in the US.

	2. Random ground based systems operating in the UHF military
band that get unintentionally repeated via the transponders because they
operate at one of the uplink frequencies  and radiate enough power to
get into the transponder.    These of course get repeated with whatever
signal format the original signal used.   There are two or three common
ones..

	One of those is AM UHF military aircraft communications - which
get repeated as an AM signal - often but not absolutely always rather
weak and somewhat variable in strength.   But hobbyists with enough
antenna gain and preamp sensitivity have been able to dig these signals
out of the mud for many years and sometimes have overheard interesting
exercises and even real actual combat missions.   The recent reports
of the Calgary Alberta signal on 254.2 are an example of this.

	Another very common uplink QRM problem, especially in years
past, has been eastern European and Russian UHF band FM-FDM-SSB
multiplexed multichannel telephone links in rural areas.    Apparently
some of these operate around the 290-315 uplink band and parts of their
wide spectrum signals get into some of the transponders.   These used to
be audible in SSB mode as regular trunk telephone calls (of course in
Russian or some eastern language) with ringing and signaling and all the
bells and whistles.    Not sure how many of these are still getting into
transponders these days...

	And a third QRM signal around the UHF milsats has been various
South American FM broadcast stations that put third harmonics into the
transponder or sometimes other inter modulation products or perhaps even
STL signals. This should not happen, but in some parts of the world
there is little effort to police spurious emissions from broadcasters.  
Certain transponders had almost intelligible FM broadcast audio running
on them when there was no traffic though the FM deviation was wider than
the transponder and the audio was never particularly clear.


-- 
   Dave Emery N1PRE,  die at dieconsulting.com  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in 
celebration of what could have been, but wasn't and is not to be now either."