[MilCom] 254.2 Again!?

[David I. Emery]

On Tue, Feb 13, 2007 at 01:57:59PM -0600, rob.berezowski at sasktel.sk.ca wrote:
> Jody in North Georgia can monitor the bird, but I  believe the satellite 
> is in an elliptical orbit, and she can only hear the satellite when it is 
> above the horizon for her. So the signal comes and goes. The same would 
> apply at your location in Boston. 

	The Skynet 4 satellite is in a pretty close to circular
geosynchronous orbit BUT at a slight (3.5 degree) inclination to the
equator.  This makes it take a figure 8 track in the sky with the upper
and lower loops northward and southward respectively for an observer in
the northern hemisphere.

	The satellite is parked around longitude 34 W which means it is
ALWAYS over the horizon in Georgia and Boston - this is not why the
signal is audible only some of the time there - but rather it is over
the horizon in Calgary, Alberta from where the uplink originates only
for a few hours a day on the upper loop of the figure 8.   When it is
below the horizon in Calgary little or no ATC traffic gets into the
transponder.

	It is likely true that the choice of 34 W to park the satellite
and the use of the 294 MHz ATC frequency in Canada were interrelated - I
suspect an effort was made to ensure that there was frequency
coordination and no routine ATC channels were on the satellite's input
anywhere that would be visible from the satellite.  Canada, after all,
is pretty friendly with the UK and would like coordinate to ensure this
(as the USA would also probably do).   And in fact if the satellite was
on the equator at 34 W Calgary would be over the horizon and not a
problem.

	What happens to a geosynchronous satellite for which stabilizing
fuel is not burned to keep it at nearly zero inclination is that its
inclination grows by a half a degree to a degree every year due to
various orbital  perturbations (the moon, sun, irregularities in the
shape of the earth etc). This likely meant that Skynet 4 started out not
hearing Calgary and as it has aged in orbit it hears more and more
Calgary during its northmost loop.

-- 
   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."