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Re: July 2001 President's Letter

>How will the control station be selected? What is the estimated cost
>of the peripheral equipment that will be required to synchronize
>everyone's uplink signals? Is it expected to be able to use the shack
>PC for the necessary timing and, if so, what will be the master clock

The TDMA project is my idea so I guess I should comment here.

My thinking is that there would be no central control
station. Contention resolution would be handled in a distributed
fashion. You (actually, your computer) simply picks an idle time slot
and starts transmitting. As long as it hears its own signal come back,
it knows it has captured the channel and can continue transmitting.

A big issue here is the ratio of the round trip delay to the time that
each station will "own" a time slot. Contention resolution protocols
tend to be inefficient when that ratio is high.

As for the timing reference, my plan is to have the spacecraft inject
its own telemetry at a periodic rate using the same burst format as
the user stations. This burst serves as the timing reference for all
the other slots in a TDMA frame. Ground stations will have to adjust
for the round trip propagation delay, but this is easy if they know
where they are and have a current set of elements.

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