[MilCom] 254.2 Activity

[David I. Emery]

On Fri, Feb 02, 2007 at 08:00:10AM -0500, Larry Van Horn, N5FPW wrote:
> > Skynet 4F stationed over the Atlantic does operate on 254.200, the same 
> > satellite also sends out data on 257.325.
> > Based on what folks are posting on the RadioMonitors list id have to say 
> > it is probably that bird.
> 
> As I told Jody in a private email offlist, I have looked at both the Skynet 
> 4D and 4F milsats which has downlinks on 254.200 MHz. And they have to be 
> ruled out since both of their orbital slots are way to far east for their 
> uplinks to pickup any transmissions from Calgary, Alberta, Canada. 
> According to the latest information I have from the SSC# catalog, Skynet 4d 
> is at 34 west and 4f is at 6 deg east. This puts Calgary way out of both 
> birds footprint. There are three other scenarios that make sense.

	FWIW, I just took a look from near Boston Mass at 254.2 with a
12 dbi crossed yagi surplus milsat beam aimed at around 30 W and I very
clearly see a standard 25-30 KHz wide transponder there with its noise
floor at least 10-15 db over the thermal noise (reads around -130 dbm in
a 1 KHz RBW).   It is notable that the center frequency of the passband
of this transponder is 5 kHz low (eg 254.195 rather than 254.200)

	I can definitely see some weak carriers maybe 5-6 db over the
average noise appear and disappear - definitely could be ATC, but these
are clearly not right on 254.195 and I think I saw at least two distinct
frequencies there.

	I need not remind this audience that the INPUT frequency to
whatever we are seeing is NOT 254.2 (unless this is anomalous
propagation) as the transponders are a form of linear repeater with
DIFFERENT input and output frequencies.

	US milsats use an offset of +33.6 MHz for the input to most of
their 25 kHz channels so 254.2 would be 287.8 for the USA bandplans, but
whether this is true of Skynet is not immediately at my fingertips.  And
some of the Molniya birds use other offsets too (41 MHz for example).   

	I guess I should run some coverage plots from 34 W.  Not clear
that Alberta is out of the envelope without confirming this with a plot.
A site near the edge of coverage would probably radiate more energy
toward the satellite from ground and aircraft based radios which mostly
aim energy at the horizon than one directly under the satellite where
the energy would have to go straight up.

	But unless someone knows the input to the Skynet transponder (eg the
offset used) I'd not conclude Calgery without a lot of confirmation that
it really was Calgery based on the ATC comms overheard and not just the
254.2 frequency.

-- 
   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."