                    USING APRS FOR SPACE COMMUNICATIONS

NEW: 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.  To facilitate setting up APRS for best effeciency
through a space digipeater, there were several changes made in APRS 4.04/5.
  * Added alt-G command to put GridSquare in BCN and POSIT reports (this not
    only puts position info in your BCN, but also saves space and makes the
    position report shorter.  It also sets ^O to receive all OTHER packets.
  * Modified alt-S command so that it can be used to reset the DECAY timer
    for position reports to a short period during a satellite pass.  As for
    all APRS packets, however, the period will automatically begin to decay
    to double the period after every transmission.  This assures that stations
    minimize packet transmissions.
  * See OPERATING section for important tips on operating APRS via MIR.
  * In 4.05 both the BCN and POSIT are identical, so that other stations ALL
    BCNS logs do not fill with alternating BCN/POSITS from adjacent stns.

 
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 after)
                                                (1 Aug 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, I have created a
track history file that can be replayed using the Ctrl-R command.  Simply
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 TAGS using the Alt-T command to reduce
the clutter of callsigns on the display.  The replay can be speeded up or
slowed down by hitting the F or S keys.  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.


OPERATING TIPS VIA MIR/SAREX:

To maximize your chances of being seen via the MIR digipeater and to minimize
unnecessary APRS packets, the following recommended procedures should be used
during MIR passes.  Even under worst case scenarios, APRS stations will still
generate far fewer packets than other stations attempting to CONNECT to MIR!

*  Use ^V to set your VIA path to R0MIR
*  Use alt-G to force your grid square report into the beginning of your BCN
   and your POSIT.  The grid square posit only takes 8 characters vice 27.
   This command also does a ^O command to open up APRS to parse all other
   incomming packets instead of just APRS packets.  This way, your L-List will
   show all stations digipeated by MIR
*  make your BCN text as short as possible.
*  Use T command to force a transmission as desired - OR -
*  Use alt-S command 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.
*  After hitting any APRS command, hit alt-V so you can VIEW all packets on a
   full screen instead of on the normal APRS bottom line.
*  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 MIR 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 with MIR.  Of
course your chances are similarly diminished, but if enough APRS stations are
reporting via MIR, there will still be enough DX position reports getting
through to make it interesting.  If this idea catches on, then maybe all of
those other stations will STOP trying to CONNECT to MIR and join us!  That
would be a net REDUCTION in MIR 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!

WARNING:  Versions of APRS prior to 4.04 will only recognize a grid square
report surrounded with brackets [].  But future versions of APRS will use
@...] to enclose grid squares so that you can distinguish between a grid
square report comming from an on-line APRS station and one enclosed in
brackets from a passive TNC BText.

Due to these changes in the way that APRS processes grid square reports,
all versions of APRS after 4.04 will recognize both formats, but the new
format should not be transmitted until everyone has upgraded to at least
4.04.  For this reason, version 4.04 and all later versions have a built
in timer that will automatically begin using the new format after 1 August
1994.

This means that after 1 August, older versions of APRS will not recognize
APRS generated @...] grid squares.  They will still recognize the passive
[...] grid squares, however.  This will probably only effect users of APRS
via MIR or SAREX where grid squares will be used exclusively because of their
shorter length.

SAREX NOTE:  Until we get the SAREX folks to recognize the value of APRS
position reports, DO NOT BEACON TO SAREX!  If they ever turn the SAREX
digipeater on, the word will be distributed ASAP via APRS bulletins.

MIR ADDENDUM:  Well it was fun while it lasted.  Apparently some stations
that do not understand the benefit of APRS to the overall space/educational
mission, have sent messages to MIR telling them to shut down the digipeater
mode.  As of today, it has been off for at least a week.  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 do not know of a single person that has been
exposed to APRS that does not agree that it is an excellent application for
both SAREX and MIR.  But so far, after my articles in the AMSAT journal and
sending copies of APRS to Houston have been to no-avail.  I am NOT insisting
that APRS should be used on SAREX and MIR, but I am insisting that IT SHOULD
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!

