Ham Radio 2000 #2/Ham Radio 2000 - Volume 2.iso/HAMV2/PACKET/APRS805/README/SPACE.TXT

Text File | 1997-11-30 | 13KB | 223 lines

SPACE.TXT 8.0         USING APRS FOR SPACE COMMUNICATIONS
          0.6 APRtrak

MISSION STS-78 APRS SUCCESS!

    APRS experimentation was authorized via SAREX on STS-78 in June 1996.
Ham Radio activity was reported on 15 days of the mission with 20 voice
passes, 25 packet  passes and 11 school passes, or 75% of all passes.
A great effort on the part of the STS-78 crew!

    Eighteen APRS stations successfully digipeated their position via
SAREX and 2 others relayed STATUS, but no POSIT. A total of 65 APRS packets
were received.  Thirty Nine APRS stations attempted transmissions but
indications are that only about 10 APRS stations were making a serious
effort and trying every pass.    This compares favorably with statistics
for conventional SAREX ROBOT activity of 561 stations being heard a total
of 1350 times with only 146 getting a successful QSO number.  It is
estimated that thousands tried...


SEE ALSO: TRAKNET.txt for a proposal to use the 1200 baud PACSATS for a
worldwide amateur mobile status/position reporting network.

APRS AS A HIGHLY EFFECIENT BRIEF PROTOCOL:

     APRS is an ideal solution to the congestion normaly found on any
narrowband Amateur Satellite uplink channel.  Especially the high
visibility missions where many of the 2 million world wide amateurs
want to make a brief contact in a short period of time such as SAREX.

       The problem with SAREX 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 are required to constitute a successful contact.  Of an
estimated tens of thousands of uplink stations, only a few hundred are
successful.  Recognizing the stringent requirements for success using the
CONNECTED protocol, provision is also made to recognize those stations which
were successful in getting only one packet heard onboard the shuttle.
Almost three times as many stations are heard (one successful packet)
as are successful in the full two-way connected protocol.

     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
packlet 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.

GRID SQUARE POSITION REPORTING:  To convey more information than just
seeing station callsigns plotted via grid square on the map, provision
is made for stations to also include a special Station SYMBOL character
in their packet as well.  The format is ]$[ at the start of the packet
and will cause APRS to use the $ symbol character (see SYMBOLS.TXT).
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.


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

       To implement this experiment on any shuttle mission, the SAREX
TNC only needs to have DIGI ON.  No changes onboard the shuttle or other
spacecraft TNC would be required.  Stations worldwide can use APRS or
APRtrak to then watch successful uplink stations plotted in real time.
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-72.  During 3 midnight and later passes,
over 66 stations successfully uplinked position reports.  You can replay
this file using the FILE-REPLAY command and select the SPRE file.

DEMONSTRATION:  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.  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.  See the TRAKNET.txt file.

APRS and APRtrak use a special SPACE FORMAT which also configures them
for sending their GRID SQUARE Status 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 packet digipeated.
  * The alt-SETUP-MODES-AUTOspace command can be used to activate 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 packet at the decayed (15 minute) period (which
    will miss most 8 minute passes).
  * Your shortest packet will be your STATUS.  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 STATUS period to max to minimize
    QRM since you have already been successful!  After 10 minutes, the
    AUTOspace mode will reactivate for the next pass.


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 STATUS
   timer to minimum when the spacecraft is first heard.
*  Make your INPUT-MY-STATUS text as short as possible, or none at all.
*  Use XMT-STATUS command to force transmissions as needed.  Notice that
   your STATUS 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 10 degrees or so.
*  NOTE:  The SPACE mode only permits a single SYMBOL character, so only
   SYMBOLS from the PRIMARY APRS symbol table are usable.

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 STATUS 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 STATUS 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!

APRS POSITION REPORTING VIA THE 1200 BAUD PACSATS!

     Although any of the PACSATS can operate in digipeater mode, only
AO-16 and WO-18 usually have it turned on.  There are several items
that make these satellites very attractive to APRS:

   1)  They can hear ANY 5 watt or better FM XMTR on the uplink!
   2)  Uplink only requires an OMNI antenna with no pointing (mobile!)
   3)  ANY TAPR-2 compatible TNC (with an 89 cent mod) can be used on
       the UPLINK. (see TRAKNET.TXT).  The mod is a 7400 chip wired as
       an XOR between the XMT clock and the data.
   4)  For vehicle tracking, only a few downlink stations are needed,
       since they can digipeat the packets onto HF and VHF nets or
       be linked into the worldwide live APRS internet system...

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

APRTRAK AND APRS:  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.