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Re: [sarex] Walkie-Talkie MIR tests



MIRtest:  The one thing we are demonstrating this week with the MIR as
a UI digipeater, is the APRSat concept.  Besides APRS Map displays, the 
other keys to the success of terrestrial APRS is the use of "generic" 
digipeating, and ONE nationwide frequency.  Every APRS digipeater has 
the same ALIAS  of "WIDE" and all 500 of them are on ONE continent-wide 
freq.  Thus, Mobile users and travelers do not have to know anything 
about the network.  They just turn on the radio and transmit VIA as 
many WIDE's as needed to cover the area of interest.
 
Now, Imagine, if some of the AMSATS (MIR, ISS and Shuttle too) in orbit 
carried an APRS digipeater all with the same alias of APRSAT, also on 
one common UPLINK.  THen anyone on the planet, anywhere simply sends 
his message via APRSAT and he can communicate with anyone within 1500 
km.  THe more of these satellites the better.  ALL on the same freq and 
whatever orbits we can get.  Thus, for each satellite, we might get 6 
more passes each.  It is trivial to hit a satellite with a 5W HT.  In 
fact, even 0.5W would work on 2 meters.
 
I have submitted a Freq request to AMSAT for such an APRS uplink.  There
are both opportunites and problems here:
 
1) 2 meters is ideal since it takes much less power and has less
doppler than 70cm, but there are no 2m frequencies left (unless we
share with some exsiting satellites).  ALthough the gain to amateur
radio (in having our own "irridium" type HT worldwide coverage) is
phenominal, the politics is probably just too hard to get a 2m freq.
 
2) 70 cm could work as the uplink, since mobiles can afford the
additional 8 dB power, but it makes HT operation harder.  The Satelites
would use wide band receivers (30 KHz) so that they are immune to
doppler (another 3 dB link penalty)
 
3) The DOWNLINK must be on 2 meters tho, because of the 8 dB or more
advantage for the omni-Satelllite-to-omni-HT link.  Thus, a 1 watt 
spacecraft  (about a 5 inch or smaller NANOSAT) could do the job.  
Over the North American continent, the downlink would be WELCOME on the
existing continent wide 144.39 frequency.  Everyone could hear it, and
its low power would NOT interfere with other, even, emergency
communications during its brief passes.   But I dont know what to do
about Europe or the rest of the world.
 
4) OPerating on a 70cm uplink and a 2m downlink will NOT work with 
the existing all-in-one Kenwood APRS Data radio, because the built-in
TNC can only select one-band at a time.   Thus to make use of these
few thousand radios as shirt-pocket world-wide satellite communicators, 
the UPLINK must also be on 2 meters.  (The uplink cannot be on 144.39 
though, because the hundreds of users there already would be too much 
QRM for the mobiles and HT's to get through.
 
CONCLUSION?  There is much more to discuss here, but I hope you see the
point. Just think in terms of an AMATEUR RADIO version of the worldwide
satellite HT communications capability (one-line messages only).
If we could get this vision across, then maybe we could find a permament 2
meter UPLINK and downlink that would be supported by all AMSAT
organizations.  ALso, everyone could build their OWN satellites to
contribute without competition for frequencies since we can all share the
same one without problems!
 
INTERNET:  OH yes, all packets on the downlink would be distributed 
worldwide by the existing APRServe internet system, so that a packet
originated ANYWHERE can get to ANYWHERE in real time.  This is also
a facet of APRS that makes it so universal.  The terrestrial APRS
system does this very well in populated areas, but it can never cover
the 70% of the planed covered by water and the 70% of all land that
has low population density.  ANd it is THOSE areas where we need
mobile, shirt pocket satellite messaging capability!
 
de WB4APR, Bob




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