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New AMSAT Mobile Sat Coms?



During the AMSAT-NA Symposium this weekend in Vicksburg, MS, we had
the opportunity to discuss new opportunities for mobile satellite
amateur radio communications using the existing on-orbit satellites.  
We have been experimenting with the use of APRS communications via
the 1200 Baud Pacsats for mobile 2m FM uplinks via AO-16 but during
the conference we also got to demonstrate hand-held reception of the 
9600 baud PACSAT downlink using only the Kenwood/TNC HT and handheld
1 foot 3 element beam antenna.
 
First, concerning the 1200 Baud uplinks:
 
Several AMSAT members expressed interest in dusting off their old
1200 Baud Pacsat stations and joining us on AO-16 for further testing
of Mobile APRS satellite communications.  We encouraged them to join us
on the AO16APRS Special Interest Group on TAPR (send subscribe message 
to LISTSERV@TAPR.ORG).  The concept here is that ANY TAPR-2 compatible 
TNC with a $3 mod can be used by ANY mobile station with a 2 meter FM 
radio to uplink APRS mobile comms to the satellite.  All we need are a 
few PACSAT downlink stations to digipeat these packets into the rest of 
the national APRS network.
 
Three volunteers may soon join us from Atlanta, Huntsville and middle 
Mississippi.  The only problem was the lack of any APRS infrastructure 
in Central MS to get the packets from that station into the rest of the
nationwide network.  But there were a lot of Mississippians at the
conference so we might see them put up some digipeaters soon.  If you
drive through, talk it up...
 
Concerning the new Kenwood HT with built in 9600 baud TNC:
 
It was fun to just point a 3 element beam skyward and capture the 
downlink.  We picked up a copy of WISP from Martha, and on the next pass 
(with help from Steve Bible to install it), we were receiving the 
continuous downlink stream of bulletins and messages.  Using the Kenwood 
HT/TNC, and Libretto laptop, we were even able to copy the downlink 
while walking around the pool!  As the satellite approached LOS below 
the building, we simply walked around the pool to the other side to pick 
up another minute of data.
 
Using only an average quality 3 element 435 MHz beam (about 1 foot long)
we were able to copy continuous downlink most of the time.  This is 
barefoot, with no preamp.  The signal could almost always be found with 
3 dots on the LCD meter and maybe 5 dots during passes above 20 degrees.  
Above 45 degrees we even saw 7 dots occassionally.  Signals could 
ususally be tracked all the way down to the horizon (but with a Handheld, 
you need to choose a good location to see that low).  Polarization was 
always Linear!  Although we had to rotate the polarization of the yagi 
to peak the signal during the pass, we could always rotate 90 degrees 
to a null.  Thus we never saw any energy in the other polarization.  
Occassionally we would see a complete null from the spacecraft lasting 
several seconds as there was no signal at all in any polarization.
 
You can only operate in receive-only mode because the Kenwood built-in
9600 baud TNC cannot operate full duplex crossband in this HT 
configuration.  But this is just fine with the existing PACSAT
Broadcast Protocol.  It does, however, open up some interesting
possibilities for exploring mobile applications using this receive
only downlink.  Here are some off-the-wall ideas:
 
  1) 1200 bd Pacsats give us EZ uplink from any 2m FM rig/TNC ($3 mod)
  2) 9600 Baud Pacsats give us an EZ downlink to the kenwood HT....
  3) We cannot UPLINK at 9600 baud, because we would have to transmit 
     in the blind (radio cannot operate full duplex cross band) and
     this would interfere with the Pacsat Protocol controlled uplink.
 
How can we take advantage of this in the mobile environment?  
 
Lest you be concerned, remember, that fewer than 1% of APRS operators 
will ever be interested in the Pacsats for APRS.  THis is because most 
populated areas of the USA already have a worldwide connected APRS 
infrastructure, so there is no incentive for anyone in those areas to 
use a satellite.  This PACSAT APRS capability is only useful for the 
serious distant traveler in the wilderness, or on the high seas...  
 
So, how can they use this capability?
 
Well, for one thing, they can use their Palm Pilot and Kenwood HT
for shirt pocket Satellite Downlink!
 
What about stuffing all one-line messages for these APRS travelers in 
a single conventional PACSAT message and then always haveing one ground 
station requesting that file.  THis way, all receive only stations, could
capture this one file and read any lines addressed to them!
Remember, in APRS, a message is usually a one-line'er...
 
Or since a directory entry contains about a 30 byte subject and 10 or
more bytes of Key words, you may get as much as a complete APRS 45
character message in only the Directory listing!  And the directory is 
frequently broadcast.  Thus, you can receive a short APRS (Kewnood
45 byte line) in receive only!
 
I need to fully digest the details of the Pacsat Protocol to see if 
there are any mechanisms to apply to the mobile receive only station
who only needs to receive a one line message occassionally.  I am
brand new to the Pacsats, and have a lot to learn...
 
In conclusion (Im still in the airport flying home...)...
 
During discussions, there seemed to be general interest in the TRAKNET
mobile satelllite communications concept and keen interest in the
Kewnood radio as a 9600 baud Pacsat Receive only application.  Also, no 
one seemed adverse to encouraging further development of mobile 
commmunications applications including APRS via any of the satellties.  
 
If you are interested, come join us on the AO16APRS SIG on TAPR.ORG.
Maybe we need to rename the SIG to something more general to the
mobile satellite user, not just AO-16...
 
de WB4APR, Bob

See my Traknet paper http://web.usna.navy.mil/~bruninga/traknet.html
See MIR APRS LIVE:  http://web.usna.navy.mil/~bruninga/mirex.html


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