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Protocols (was: Re: Re: Discovery Project: Which Mission Next?)



The current PB protocol suite uses unconnected AX.25 packets,
sent to broadcast addresses. The equivalent in the TCP/IP
protocol suite is multicast UDP.

We could have fun with this. The original implementation works
well, but surely we could apply what we've learned in the
interim and do better.

A satellite implementation might listen for some initial
acquisition signal, and then poll the satellite with IGMP
to find out what's available. And, of course, listen for
other group reports, issued in response to other ground
station queries. I recently implemented a new version of
IGMP at work, so I'm inordinately familiar with RFC 1112
and RFC 2236 at the moment. :-)

Compute power is a non-issue. I recently lusted after an
embedded board that contains a 200 MHz StrongARM processor
with up to 64 MB of RAM and 16 MB of flash, and all sorts
of other juicy peripherals. Credit card size? No. How clunky!
It's half the size of a credit card. Have a look at
http://www.brightstareng.com/arm/index.html

Current digital satellites always transmit, and if they have
nothing else to send, they send HDLC frames. A TCP/IP-
based satellite would need to do something like this, or
take some other approach for ground stations to acquire
frequency offset and bit sync fast enough to receive useful
information. The commercial digital satellites I'm most
familiar with take a simple but effective approach: Man-
chester-encoded BPSK, with a partially-suppressed carrier.
So it's easy to recover the bit clock, and it's easy to
track Doppler too.

As always, I plead with people: let's develop our tech-
nology ourselves, rather than letting others develop it
for us. I applaud the postings about Predict, and have
played with its (UDP-based) socket interface myself...

Laura Halliday VE7LDH     "Que les nuages soient notre
Grid: CN89mg               pied a terre..."
                                   - Hospital/Shafte
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