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APRS Satellite



We have been doing conceptual studies for a small nanosatellite
for relaying APRS status/position packets from stations in remote 
areas and off shore.  Since 144.39 is allocated continent wide (North
America) for APRS, the temptation is still high for operating on that
frequency. Thus mobiles need not configure separately for terrestrial or
the space link when they wander outside of the existing terrestrial
network.

The problem has been designing the system to optimize for the mobiles
while mitigating the interferring signals from all the fixed assets on the
channel.  Today, in looking at a map showing the location of all thousands
of active users, we realized that it is easier than we thought.

Since 90% of the north american population is already covered by the APRS
infrastructure, but only 20% of the land, then we only need satellite
coverage in the remote 80%.  This is also the same 80% where only 10% of
the population is!  Thus, where we need satellite coverage, is the same
location where the interference and congestion is least!  

Thus, by having a down looking antenna with reduced sensitivity on the
horizon, the satelite will hear the remote mobiles easily with enough gain
to capture the FM receiver even in the presence of dozens of interferring
signals on the horizon much more distant.  The downlink on 144.39 is easy,
since there are dozens of APRS Internet gateway stations already listening
and so even in the presence of a 60% busy channel, at least ONE IGATE
somewhere will hear each packet.  All it takes is one, and then the rest
of the world will see it via the worldwide linked APRS system.

Also, the remote mobile will be hearing NOTHING else on the channel since 
he is far from any other stations.  Thus he has a 100% chance at hearing
the downlink too!

Thus, a flying simplex digipeater on 144.39 is still a perfectly viable
space satellite application for North America and surrounding off-shore
areas.  The satellite would be silent over the rest of the world where
144.39 is not available in order to abide by international agreements.

Hummh....
de WB4APR, Bob

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