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TDMA (was Presidents Letter)



....... Is it expected to be able to use the shack PC
> >for the necessary timing and, if so, what will be the
> >master clock source?
(REPLY)
> The key is accurate timing. Why not use GPS? Many appli-
> cations use GPS for distributed sub-microsecond timing.
> So could we. We'd have to fiddle a bit to compensate for
> transit-time delays, but that's not difficult.

As far as I know, most commercial satellite TDMA systems in use today, use a
clock source generated by a master reference station. This can be any
designated station in the footprint. Each remote station calibrates it's
local timing using a "ranging" routine on a special ranging frequency or in
an extra wide time slot. With a geostationary satellite, this is very easy.
With a moving satellite things are a little more difficult as the timing
offset will be moving with the satellite. In this case, it makes allot more
sense to put the master reference "station" on board the satellite.
Approximate timing could be derived from using ephemeris data to calculate
slant range to the satellite. Once frame acquisition occurs, timing offset
could be refined by synchronizing to "start of frame" headers generated by
the on board master reference station.  Of course, accurate absolute time,
as provided by a GPS would help the process, but would not be essential. If
the data rates are not too high, I would think current common PC's should be
able to handle this as long as they could process information while being
interrupted at the TDMA frame rate.

Just my thoughts on the subject, I'm sure there are others....

Howie AB2S

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