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Mobile APRS Satellite Communications



Mobile Satellite hopefuls,

Here are some very interesting thoughts evolving out of the recent 1200
baud AO16/LO19/IO26 APRS tests and the introduction of the new Kenwood
APRS 1200/9600 baud mobile radio.

 * For tracking mobile GPS positions, status and uplink messages, a 1200
   baud PACSAT will deliver *twice* the traffic as any 9600 baud bird.

 * Conversly, for delivering APRS message traffic *TO* the mobile user, a
   9600 baud transponder can deliver 50 to 100 times *more* traffic than a
   1200 baud PACSAT.

Here is why. The 1200 baud birds have 4 simultaneous uplink channels
raising the aloha channel effeciency from say 20% to near 80%.  Conversly
the 9600 baud satellite designs only have one receiver and the
througput of a 9600 baud APRS packet is not even twice as fast as a 1200
baud packet, because most of the packet timing is based on the
transmitter/recevier delays.  Combine these and the 1200 baud PACSAT
delivers TWICE the user data in this "uplink limited" ALOHA environment.

But for delivering messages *TO* mobiles, a continuously streaming 9600
baud downlink (with no T/R delays per packet) can deliver over 1200
messages every 2 minutes. THus providing a 4 or 5 deep redundant delivery
of at least one message to over 1000 mobiles per footprint per pass.

CONCLUSION:  This leads me to conclude that we should continue the efforts
to use the 1200 baud birds with the $2 mods to TNC's as the uplink and
tracking channel.  It also suggests that it is a terrible waste to
consider using a 9600 baud uplink in an ALOHA fashion.  The downlink would
be idle 90% of the time! (though it could be doing other things...)

THus, the use of 9600 baud transponders will be best  for bent-pipe
delivery of message traffic TO MOBILES from the "footprint" IGates.
Mobiles will receive thier message traffic whenever a 9600 baud APRS bird
flies over.  But users uplink whenever a 1200 baud bird approaches.

For LIVE TWO-WAY exchanges, the 1200 baud birds will be best, or the 9600
baud bird will require two receivers... One for the IGate stream, and the
other inneffeciently used for occassional user uplinks...

At least this is what it looks like from here (and today).. :-)

de WB4APR, Bob


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