SPACE.TXT 7.6f         USING APRS FOR SPACE COMMUNICATIONS
          0.6 APRtrak

BULLETIN!:  SAREX on STS-78 is available for digipeating APRS packets.  
Also WO-18 is officially available for APRS position reports.  Other 
PacSats with DIGI on could be used as well (AO-16 and ITAMSAT), if
there is no objection...

NEW in APRS76f and APRtrak 0.6:  Now instead of showing on the map as
just a grid square, your Station SYMBOL character will also be transmitted.
The format is ]$[ at the start of the packet and will cause APRS to 
use $ as the symbol character.  This format will also force APRS to
interpret the TO address as a grid square, even if it is not in SPACE
or MScatter mode.

BACKGROUND:  Since APRS has great potential in the  effective use of 
orbiting 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.  It is a stripped down version of APRS 
with added Spacecraft tracking capabilities.  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 
a few hundred 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.  Almost three times as many 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 SAREX
on the 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.  

FORMATS:  APRS and APRtrak respond to both the conventional LAT/LONG 
APRS POSITION reports and to BEACONS with included Grid-Squares.  The 
exact format of a minimum APRS GridSquare BEACON is as follows.  
Obviously the GRID-IN-TO format is the shortest and preferred.
These formats convey both your POSIT and your BText comments in your
APRS BEACON:
                                                    comments
     GRID-IN-TO FORMAT:         WB4APR>FM19SX,W5RRR:Hi!...
                                WB4APR>FM19SX,W5RRR:]$[Hi!...
                                                    ^^^ Symbol indicator
                                                        See SYMBOLS.txt

     GRID-IN-BEACON FORMAT:     WB4APR>APRS,W5RRR:@FM19sx] Hi!...
                                                  @ means APRS is online
                                                  / means TNC BEACON only

     To implement this experiment on any shuttle mission, the SAREX
TNC only needs to have DIGI ON and the word needs to get out to everyone
to insert their Grid Square in their UNPROTO or BText command.  No changes 
onboard the shuttle or other spacecraft TNC would be required.

Those stations that had APRS or APRtrak could then watch 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, a many fold increase in the number of successful uplinks might
be realized, and the data exchanged would be more meaningful by a 
similar factor.

SPRE EXPERIMENT:  The first APRS experiment was during the Uiversity of
Maryland SPRE mission on STS-74.  During 3 midnight and later passes,
over 66 stations successfully uplinked position reports.  YOu can replay
this file using the FILE-REPLAY command.  To demonstrate the expected 
results of a SAREX flight, 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 and APRtrak use a special SPACE FORMAT which also configures them 
for sending their GRID SQUARE BEACON via a space digipeater:

  * First, you must set your UNPROTO path via the space digipeater
  * Next, use the alt-SETUP-FORMATS-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.  
  * The alt-SETUP-MODES-AUTOspace command can be used to activates an 
    AUTOmatic routine which will reset your packet timers to minimum if 
    the spacecraft is heard.  Otherwise your station will continue to only
    send your posit BEACON at the decayed (15 minute) period.
  * 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:

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 fewer 
packets than other stations attempting to CONNECT to SAREX.

*  Use UNPROTO to set your VIA path to the Space DIgipeater (W5RRR-1)
*  Select alt-SETUP-FORMATS-SPACE as noted above. 
*  Select alt-SETUP-MODES-AUTOspace if you want APRS to reset your BEACON 
   timer to minimum when the spacecraft is first heard.
*  Make your alt-BEACON text as short as possible, or none at all.
*  Use XMT-BEACON command to force transmissions as needed.  Notice tha
   your BEACON contains your compressed grid square posit as well.  DO
   NOT send your full length APRS POSITION packets unless you are mobile.
*  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 
   QRM to your receiver from other local uplink stations, and also 
   minimizes your QRM to them.  A ground level antenna is perfectly 
   adequate, 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.


AUTOMATIC OPERATION:  In AUTOspace mode, your station will transmit 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 AUTOspace MODE, then 
APRS will listen for the DIGIpeater shown in your UNPROTO path.  Once 
it hears it, it will reset your BEACON timer 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 BEACON timer 
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!

   I hope that other users of SAREX will try APRtrak and realize the
reward of a successful digipeated position report.  The net effect would 
be FEWER packets on the uplink, and more meaningful packets on the 
downlink!  I wrote APRtrak and gave it to AMSAT so that I have no 
monitary interest in this facet of SPACE communications. 


     APRS POSITION REPORTING VIA WEBERSAT OSCAR-18

     WEBERSAT has beeen placed in digipeater mode and AMSAT enthusiasts 
have been encouraged to use it.  There are 3 significant items that make 
this very useful as an APRS position reporting satellite:

   1)  It can use ANY 25 watt FM XMTR on the uplink!
   2)  Uplink only requires an OMNI antenna with no pointing (mobile!)
   3)  Rumor says a Manchester encoded modem can be added to MANY
       TNC's with only a single XOR gate and a little surgery!
   4)  For vehicle tracking, only a few downlink stations are needed, 
       since they can digipeat the packets onto HF and VHF nets

Receiving the BPSK downlink takes a separate BPSK satellite modem, but
many hams already have these...

