SPACE.TXT 6.7b         USING APRS FOR SPACE COMMUNICATIONS


BACKGROUND:  The Automatic Packet Reporting System could be a solution to the
effective use of orbiting terrestrial style packet radio digipeaters in the
amateur satellite program.  To date there have been three AX.25 1200 baud FM
transponders flown in space.  The first was on the Space Shuttle STS-35, the
second was on the space station MIR, and the third has been via the FM
transponder mode of AO-21.   The problem with a space based digipeater 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 were encouraged to simply insert his/her position as the first few
characters of his beacon text, everyone within the satellite footprint would
not only see when he made a successful uplink, but also where he was.  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 an APRS position report is as follows:

   Beacon Text:     !DDMM.HHN/DDDMM.HHW/CQ comments etc.....
   For example:     !3959.11N/07629.12W/Naval Academy Radio Club

   Grid Square:     [FM18xf] Naval Academy Radio Club...
                    @FM18xf] comments... etc    (format used by  APRS
                                                (to indicate APRS stn)

     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 POSIT
in their beacon text.  No changes onboard the shuttle or MIR 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 five 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 based
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.

    I saw my first APRS packet via the MIR digipeater from W6PNC on 5 April
and then saw first plotted position from N4YHD on 6 April plus other BCNS from
N2JNT, KB8DP and KE4EER.  APRS has several features to facilitate its use via
a space digipeater:

  * The alt-SETUP-MODES-GRIDSQ command puts just your GridSquare in your
    BCN and POSIT for brevity.  It also sets CONTROLS-OTHER on so that
    you can see other packets.
  * All Beacons and Posit periods will automatically begin to decay to double
    the period after every transmission.  This assures that stations
    minimize packet transmissions.
  * In GRIDSQ mode, both the BCN and POSIT are identical, so that other
    stations ALL BCNS logs do not fill with alternating BCN/POSITS from
    adjacent stns.
  * 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 POSIT period to max to minimize
    QRM since you have already been successful!



OPERATING TIPS VIA MIR/SAREX:

First, be sure that APRS position reporting is being authorized via the
particular spacecraft.  Remember that 99% of HAMS (that dont understand
the advantage of occassional UI frames in this application) are ready to
crucify anyone that BEACONS on a packet frequency.  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 MIR or SAREX!

*  Use UNPROTO to set your VIA path to R0MIR
*  Use alt-SETUP-MODES-GRIDSQ to make your posit as short as possible.
   This command also opens up APRS to parse all OTHER packets instead
   of just APRS packets.  This way, your L-List will show all stations.
*  Make your BCN text as short as possible, or none at all.
*  X command to force a transmission as desired - OR -
*  Use alt-SETUP-POSRATE to reset your POSIT period decay timer.  APRS will
   send an immediate packet and then double the period to the next one and
   so forth.  This prevents unattended stations from accidentally cluttering
   the freq.
*  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 only one BCN and POSIT 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.  Of course your chances are similarly diminished, but its only
fair for unattended stations not to QRM manned stations. 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 MIR/SAREX QRM!

    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 comming from a passive TNC BText.


MIR ADDENDUM:  Well it was fun while it lasted.  Apparently some stations
that did not understand the benefit of APRS to the overall space/educational
mission, sent messages to MIR telling them to shut down the digipeater
mode.   For about the last 6 months of 1994, MIR had digipeat off.  Somehow
we have got to educate the AMSAT community to the advantages of using short
APRS gridsquare reports for providing good instantaneous visual feedback
to MIR and SAREX users alike.  I have not met anyone that fully
understands APRS that does not agree that this application could be
beneficial for the mission of SAREX and MIR. 

    I am NOT insisting that APRS should be used on SAREX and MIR, 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!

