[ao16aprs] Mobile 9600 Baud Satellite tests

[Bob Bruninga <bruninga@nadn.navy.mil>]

Several of us are experimenting with the concept of delivering Messages
to mobiles and Handhelds via 9600 baud satellites.  It is easy to do with
a small handheld beam.  But what we are trying to quantify is how long
during a pass we can receive data on a mobile OMNI antenna.  If such a
mobile can receive 2 minutes or more, then we have a good chance of
delivering hundreds of brief messages to mobiles a few times a day.

These tests are being conducted with the handheld Kenwood THD7 with built
in 9600 baud TNC. (anyone with this radio can do this easily).  S units
are referenced to the number of Segments displayed.

Test 1:   A quickly built 6.5 inch whip on a 2 foot groundplane and got S3
for several minutes past the center of the pass.  (but with periodic
many-second fades).  Past experience has recovered some data with this
poor of a signal (I couldn't find my HT-to-PC data cable)

Indoors I measured -115 dBm to get S3 and -110 dBm to get S4/S5.
But then I measured my 6.5 inch whip and got 2.5 SWR!  So I Trimmed it
down to 1.0 SWR for next pass...  Again I was late and witout data cable,
but got the following on KO25
  
   S5 for 20 secs  Probably at 30 deg
   S3 for 20 secs  Probably at 25 deg
   S5 for 20 secs  Probably at 22 deg
   S3 for 40 secs  Probably at 18 deg
   Then lost it. (below S3)

Thus, I could have received probably 2 minutes or so of usable data.

CONCEPT:  So if a satellite was constantly listing traffic on a 2 minute
or so cycle, then at least twice a day per satellite, a mobile could
capture his traffic.  These peaks in data only occur between about 20
to say 45 degrees and you can assume +5 or -5 doppler during these two
possibilities.  THus, you can park your car, tune +5 or -5 depending on
which horizon is better, and your car Mobile has a good chance of
capturing traffic for you. 

There are several things we need to analyze and collect data on:

1) Performance of each different satellite downlink
2) Mobile antenna designs optimized for gain in the 20 to 50 deg (or so?)
   elevation area and minimized elsewhere.  
3) Produce probability distribution curves over these optimum Elevation
   angles for these omni antennas

What data we receive is not important at this point, but the consistency
of reception is what we are trying to quantify... for any possible future
satellite downlink experiments...

Currently known downlinks:  UO22 = 435.120 +/- 9 KHz doppler
                            KO23 = 435.175 +/- 9 KHz doppler
                            KO25 = 436.500 +/- 9 KHz doppler
                            TO31 = 435.925 +/- 9 KHz doppler

de WB4APR, Bob


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