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New! Satellite Digital Messaging



This weekend, coincident with the Dayton Hamvention 2000, ushered in a new
milestone in personal wireless  world wide amateur satellite message
communications.  Following several months of on-air testing, and the
convergence of several new announcements, mobile-to-mobile and handheld
amateur satellite message communications are now a reality!
 
At Dayton, Kenwood revealed the 9600 baud upgrade to their existing
1200 baud APRS handheld,  Alinco revealed their new 1200/9600 baud data
radio, and the South African SUNSAT team announced the availability of
their satellite for APRS type UI messaging when not in use for scheduled
voice passes.
 
Using the built in 9600 baud TNC's in the Kenwood and ALinco  data radios
and the effecient APRS messaging protocol, it is  possbile for simple
mobile or even handheld stations with  simple antennas to communicate
briefly via SUNSAT or any satellite  that is enabled for UI digipeating.
 
ALthough the recent growth in FM voice satelites has sparked
excitement throughout a new generation of satellite enthusiasts,
the short, single channel voice operations are highly competitive
and difficult to share equitably among a large number of users.
 
But with one-line APRS messaging, each packet taking less  than a second,
there is potential for far more stations to share the satellite during
each pass while also getting short but meaningful communications through
the bird.
                                                       
In addition, being in digital form, any station receiving these packets
can link them into the worldwide APRServe internet system for
real-time delivery back to terrestrial RF anywhere on the planet.
 
The following example says it all.  During the Dayton Hamvention Sunday
morning SUNSAT pass, I was trying to demo 9600 baud handheld reception 
in the fleamarket area using nothing but my HT and whip antenna.  Hearing
nothing but 60-over S9 interemod, I gave up and put the HT in my pants
pocket and continued my shopping.  A few minutes later I heard the
tale-tale beep from my HT indicating a new message.

Thinking it was just another message from my buddies at the
hamfest, I casually pulled it from my pocket and was surprised
to see that it was a BULLETIN from SUNSAT!  It was one of the handful 
of Bulletins that SUNSAT downlinks and it was announcing that this
capability was now open for general use.  There could not have been
a better and more fulfilling conclusion to our previous testing than this
serendipitous reception.
                                    
Thus, an Amateur Satellite Message was delivered to a non-optimized
in-attentive handheld user using EXISTING amateur radio equipment.  What
makes this possible is the 6 dB stronger downlink transmitter and 9 dB
better 2 meter path loss  compared to the other 9600 baud UHF downlink
PACSATS.
 
Although it might appear that the return path on UHF from the HT 
back to the satellite would be impossilbe due to the additional 9 dB
path loss, the presumption is that an HT user with an outgoing
message would be aware of the pass and would use a handheld
beam to make up for the difference.  Thus, we have worldwide amateur
satellite HT message capability anywhere.  (Note, due to the real
time digipeating of these messages, there must be groundstation
somewhere within the same satellite footprint, about 2000 miles
to send/capture these packets.)

But even without beams on the uplink OR downlink, we can still do two-way
messaging to handhelds.... Here's how...

Some other non-published testing via a 9600 baud satellite has
taken advantage of the 9dB 2 meter uplink advantage and has demonstrated
reliable UPLINK of packets from HT's with omni whip antennas and
non-tracking, un-attentive style message delivery.  THus, if one of the
existing mode J PACSATS were enabled for UI APRS style messaging, then the
combined two bird constellation of SUNSAT making message delivery and the
PACSAT relaying return traffic, gives us a worldwide two way shirt-pocket
amateur satellite messaging capability!
 
The real excitement is that we are doing this now, today.  It requires 
NO new satellites and NO software mods to the existing birds.  Two way
messaging is possible HT-to-HT and mobile-to-mobile.  Further enhancements
linking this in to the terrestrial APRS worldwide internet linked
infrastructure that then gives global real-time  delievery is simply a
matter of adding simple omni antenna receivers at a few of the existing
APRS IGates and a little software.
 
For details of this exciting new mobile satellite capability and the
years of UI digipeating testing leading up to it, see:

http://web.usna.navy.mil/~bruninga/astars.html

We expect this site to change rapidly as more experimentation is conducted
on this mode of communications.  SO check back in a week or so when we
condense all this new info...

We hope this realtime-messaging capability will provide a  new lifeline
for amateur stations on-the-go,  in the wilderness, at sea and anywhere
else that is beyond the reach of terrestrial amateur communications.

de WB4APR, Bob Bruninga

Satellite Ground Station, US Naval Academy,  Annapolis, MD


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