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TCP-group 1988


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Re: WA4DSY modems



Henry,

Three members of this list are very closely associated with AMSAT: Bob
McGwier, N4HY, Tom Clark, W3IWI and myself. We have long hoped that
AMSAT satellites could fill in the backbone between the relatively
isolated islands of packet radio activity that now exist.

Some initial experiments were done back in the heydey of Oscar-10. The
modulation technique was noncoherent FSK: Bell 202 modem tones into SSB
transmitters through the Mode B transponder. As you might expect, this
didn't work well. The bigger stations obtained only marginal results.

However, there will be a (hopefully) big difference when Phase 3-C goes
up.  Many people now have PSK demodulators originally designed for Fuji
Oscar-12. These modems are considerably better than what had been tried
on AO-10. N4HY has been hard at work building "software modems" for the
TMS-320 digital signal processor, and I expect these implementations
will work even better than the (hardware) FO-12 PSK modems.

If the links are at least equal to AO-10 (and we have good reason to
believe they'll be *better*) then we have a very good shot at doing some
serious packeting through the transponder on Phase 3-C. And there's also
RUDAK, a regenerative packet repeater with a 400 baud downlink. Although
I have every reason to expect RUDAK to perform as advertised, I don't
think a single 400 bps channel will be very exciting to those of us who
are already itching to go a lot faster than 1200 baud.

But don't underestimate the effort involved. It still takes several
radios, a GaAsFET preamp, two antennas and a tracking computer (assuming
you want to run automatically) to get a reliable satellite path. It
won't be as easy or as cheap as dialing up PC Pursuit, but it will be
fun!

Phil







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