SPACE.TXT 7.2         USING APRS FOR SPACE COMMUNICATIONS


BACKGROUND:  Since APRS has such tremendous potential in the  effective 
use of orbiting terrestrial style packet radio digipeaters in the
amateur satellite program, a special version of ARS called APRtrak has
been donated to AMSAT for use in the amateur satellite program.  See
APRtrak.txt.  APRS still retains a minimum SPACE mode too.  

       The problem with space based AX.25 FM digipeaters is the total 
saturation on the uplink channel which makes the use of a normal CONNECTED 
protocol impractical.  For the SAREX robot QSO mode, a total of five 
successive and successful packet transmissions were required to constitute 
a successful contact.  Of an estimated thousands of uplink stations, only 
about 250 were successful.  Recognizing the stringent requirements for 
success using the CONNECTED protocol, provision was also made to recognize 
those stations which were successful in getting only one packet heard 
onboard the shuttle.  Over 700 stations successfully completed single 
uplink packets.

     APRS takes advantage of this unconnected, one packet, mode to 
demonstrate successful uplinks to the shuttle.  In addition, however, it 
capitalizes on the most fascinating aspect of the amateur radio hobby, 
and that is the display on a map of the location of those stations.  
Historically, almost every aspect of HAM radio communications has as its 
root, the interest in the location of other stations.  Look at DX maps, 
countries worked, counties worked, grid squares, mobile chatter; everyone 
is quite interested in where other stations are.

     If, instead of every station attempting to CONNECT with the Space 
Shuttle, all stations simply inserted his/her 6 digit gridsquare into their 
TNC TO callsign via the SAREX callsign, then, everyone within the satellite 
footprint would not only see when he made a successful uplink, but also 
where he was.  It takes a total of 128 bytes for a successful SAREX QSO 
plus 92 bytes for every retry.  The APRS GridSquare BEACON only takes 26.  
This alone could provide an order of magnitude improvement in the number
of successful SAREX contacts.

Since the shuttle is a rapidly moving object, the locations of successful 
uplink stations will move progressively along the ground track.  The 
weakest successful stations will almost certainly be immediately below the 
spacecraft.  Stronger and more viable groundstations can show up further 
to the side of the ground track.  If there is a skew in the spacecraft 
antenna pattern, the pattern of successful uplink stations on the map will 
clearly make that evident.  APRS responds to both LAT/LONG and GridSquare 
formats.  The exact format of a minimum APRS GridSquare report is as 
follows:

                           WB4APR>FM19SX,W5RRR: 
    
     To implement this experiment on the next shuttle mission, it would only
take a single AMSAT news bulletin to ask all stations to insert their Grid
Square in their beacon text.  No changes onboard the shuttle or other space- 
craft TNC would be required.

Those stations that had APRS could then watch the successful uplink stations
plotted in real time.  Even without real time APRS, a replay of a captured 
text file containing all the successful uplink packets would still give an 
excellent map display after the fact.  Analysis of antenna pointing 
anomolies on every orbit could be accomplished with ease.  On future 
missions, the UI beacon frame might completely replace the current 
CONNECTED robot mode.  Without all of the connect requests, acks, and 
retries at least a ten fold increase in the number of successful uplinks 
would be realized, and the data exchanged would be more meaningful by a 
similar factor.

     To demonstrate the expected results of this experiment, replay the
SHUTTLE.HST file and watch the contacts appear as the shuttle moves across
the country.  You may enhance the demonstration by selecting to see only
the Shuttle, STS-99, or by turning off CALLS to reduce the clutter of
callsigns on the display.   Obviously, in this SHUTTLE.hst file, I assumed
that the Shuttle had its TNC connected to a GPS navigation receiver so
that it was also beaconing its position once per minute in the APRS format.

     This capability also demonstrates the practicality of using a space
AX.25 digipeater for routine position and status reporting.  Imagine a
constellation of three AX.25 digipeater satellites all on one FM channel.  
It would not matter what satellite was in view, or when.  Mobile and 
portable stations could beacon their position once every 5 minutes and be 
tracked nationwide!  Just using 1200 baud AFSK, up to 1000 stations could 
probably be supported just in the US and have a reasonable chance of 
getting a position report through at least once every 3 hours!  Going to 
9600 baud FSK would support almost 8000 users.

APRS uses a special SPACE mode to configure for sending its GRID SQUARE 
BEACON via a space digipeater:

  * The alt-SETUP-MODES-SPACE command places your Grid Square in the TO
    address of your TNC.  It also sets CONTROLS-OTHER on so that you can 
    see other packets.  It sets up a congratualtions BEEP-MSG when it sees 
    your BEACON digipeated.  And it also activates an ARMing routine which 
    will reset your packet timers to minimum if the spacecraft is heard,
    so that you send several BEACONS randomly once the spacecraft is heard.
  * Next you must set your UNPROTO path via the space digipeater
  * Your shortest packet will be your BEACON.  Although your lat/long 
    POSIT, MESSAGES and OBJECTS are still active, they are not encouraged.
    As usual, all packet periods will automatically begin to decay to 
    double the period after every transmission.  This assures that stations
    minimize packet transmissions.
  * Since only the SPACECRAFT will be digipeating, APRS will detect any
    of your packets that are digipeated and will announce your success
    with some BEEPS.  It also resets your BEACON period to max to minimize
    QRM since you have already been successful!



OPERATING TIPS VIA SPACECRAFT DIGIPEATERS:

First, be sure that APRS position reporting is being authorized via the
particular spacecraft.  To have a good chance of being seen via the SPACE 
digipeater and to minimize unnecessary QRM, use the following procedures.  
Even under worst case scenarios, APRS stations will still generate far 
fewer packets than other stations attempting to CONNECT to SAREX!

*  Use UNPROTO to set your VIA path to the Space DIgipeater (W5RRR)
*  Use alt-SETUP-MODES-SPACE as noted above. 
*  If unattended, APRS will listen for the digipeater specified in your
   UNPROTO path, and will reset your BEACON timer to minimum when the
   spacecraft is first heard.
*  Make your BEACON text as short as possible, or none at all.
*  Use XMT-BEACON command to force transmissions as needed 
*  Use the APRS VIEW screen so you can VIEW all packets on a full screen
*  Use your lowest 2m antenna (preferably on the ground).  This minimizes any
   QRM to your receiver from other local uplink stations, and also minimizes
   your QRM to them.  A ground level antenna should work perfectly well, since
   it can still see the sky, and the SPACECRAFT is so far away on the horizon
   and has such high doppler that you will NOT make it anyway at elevations
   below 20 degrees or so.


UNATTENDED OPERATION:  If you use a low antenna as described above and leave
APRS unattended, you will be transmitting your normal packets about once
every 15 minutes.  This is less than one-half of one percent (0.5%) of the
number of packets generated by other stations trying to connect to the
spacecraft.  If you have set space mode, then your system will listen for
the DIGIpeater shown in your UNPROTO path.  Once it hears it, it will reset
your BEACON timers to minimum and also set a random number of seconds up to 
12 before your first packet is transmitted.  As long as you continue to 
hear the digipeater callsign, your timers will stay at minimum and your
starting time to the first packet will continuously be reset to a random
number under 12.  Since APRS is on a 5 second timing cycle, you have a 5/12
or 42% chance of transmitting in each window as long as the digipeater
is being heard.  This gives you an average of about 1 packet per 10 seconds
which is still less than what a connected station would be doing...
If this idea catches on, then maybe all of those other stations will STOP 
trying to CONNECT to the spacecraft and join us!  That would be a net 
REDUCTION in QRM to on the uplink!

    Imagine the fun that the cosmonauts and astronauts will have if they
carry a lap-top computer so they can see everyone on their maps!

NOTE:  Even if you only see a GRIDSQUARE from a station, you can tell if
he is running APRS by the @ vice [ at the start of the report.  APRS uses
this distinction so it can tell who is an on-line APRS station and one
which is only coming from a passive TNC BText.


    I am NOT insisting that APRS should be used on SAREX, but I AM
insisting that IT SHOULD at least BE CONSIDERED as a viable and useful
addition!  The net effect would be FEWER packets on the uplink, and more
meaningful packets on the downlink!  I am writing a version called APRtrak
to donate to AMSAT so that I have no monitary interest in this facet of
SPACE communications.  

