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Re: Amateur radio module on commercial Geo Sat?



>Just one wee little question.  How would TDMA work, when there's stations
>spread across a wide area (with vastly different, and generally large
>propagation delays), without being either horrendously slow, or stations
>accidentally falling out of their timeslots, due to propagation delays?

>That's one of the limitations of GSM mobile phones in rural areas here,
>incidentally.

You are indeed correct that this is a serious problem with TDMA in
terrestrial cellular with overlapping cells and omni mobile
antennas. That's one of the reasons Qualcomm invented IS-95 CDMA.

Things are different in a single-beam satellite environment with
directional antennas on the ground.  You don't have significant
interference from other satellites on the downlink or from users of
other satellites on the uplink.  Each user on a given satellite
adjusts their uplink timing so that everybody arrives at the right
time at the satellite. Once that happens, it doesn't matter that there
are different propagation delays to the various stations on the
downlink, as all the signals coming out of the satellite are delayed
together by the different propagation delays.

My thinking is that the satellite would periodically inject a frame of
telemetry into the transponder, and this frame would serve as the
timing reference for the uplink transmissions. The individual stations
would adjust for the uplink delay from the satellite orbital elements
and the known station location. The satellite could broadcast its own
orbital elements as part of the telemetry message, just as GPS does.
The ground station location could be entered manually by the user, or
determined automatically from a GPS receiver. The GPS receiver would
also provide time for the accurate propagation of the orbital
elements.

BTW, the Globalstar satellite system does use CDMA rather than
TDMA. Each satellite has multiple, overlapping beams and the ground
antennas are pretty much omnidirectional, so here TDMA would have the
differential delay problems you describe.

Phil



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