[amsat-bb] Re: Fixed-uplink tuning (was: Eagle Launch Campaign)

[Paul Williamson <kb5mu@xxxxxxxxx>]

At 7:59 PM +0000 7/31/04, Mike Hooles wrote:
>P.S. If you want to get those of us using IC706s etc when mobile, onto FO29
>and successors, what about AMSAT nominating a specific single uplink
>frequency for us to use. Then we can tune our receivers around the nominal
>downlink  frequency accordingly and allow for the doppler shift ourselves.
>I'm sure there is a 'spare' 20kHz that we could use for this. It's worth an
>experiment anyway.,

In this scenario, the various users' downlinks in a single QSO would 
be scattered all over about a 6 kHz band as received at each station, 
because Doppler is different at different locations. It would be 
awfully hard to carry on a conversation with that kind of frequency 
split, especially mobile.

Assume Mode J, 2m up and 70cm down, in low orbit (like FO-29). That 
works out to about plus or minus 3 kHz of Doppler on the uplink, and 
plus or minus 9 kHz on the downlink. From the point of view of a 
single ground station in receive mode, every signal gets the same 
downlink Doppler shift at any given moment, so the +/-9 kHz can be 
dialed out "easily" as it varies smoothly through the pass at that 
groundstation. However, each user's signal has a different uplink 
Doppler shift, anywhere from +3 kHz for a station at AOS to -3 kHz 
for a station at LOS (of a high-ish elevation pass). This frequency 
distribution at the satellite would be repeated on the downlink, so 
the operator would have to tune madly around a 6 kHz band on every 
"over".

There'd be no way to know when all the listeners were finished 
tuning, so each speaker would have to assume that the first few 
seconds of his transmission were missed. That problem could be 
"solved" by starting each transmission with a count to 10 for tuning 
purposes. That'd be painful but perhaps tolerable, if it solved the 
problem. But it doesn't, because there could easily be TWO operators 
transmitting on different ends of the 6 kHz scatter, and each 
listener would be listening to only one. It would take very careful 
operating practices to prevent this from devolving into chaos.

The good news is that it wouldn't take 20 kHz out of the transponder, 
just 6 kHz. The bad news: it would still be nearly impossible to have 
a mobile QSO without crashing your vehicle.

73  -Paul
kb5mu@amsat.org
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