Hot Shareware 32/hot34.iso/ficheros/VUBBS/800TO814.ZIP/README/MIRTEST.TXT

Text File | 1998-03-27 | 15KB | 268 lines

                        APRS/MIR Test 11 March 1998
                           Bob Bruninga, WB4APR


On 11 March 1998 a special APRS/MIR test was conducted via the packet
system on the Space Station MIR.  The test was to show possible methods
for improving the visibility of MIREX communications to students and
schools.  The objectives were to:

   1) Demonstrate the use of a one-to-all protocol using UI frames
   for more effective use of the MIR downlink to multiple
   participating ground stations.

   2) Demonstrate the usefullness of spacecraft beaconing their
   GPS position a few times per footprint to identify their position.

   3) Use APRS map displays to display the moving position of the
   spacecraft and to display the locations of all participating
   ground stations.

   4) Demostrate the use of linked internet groundstations to
   distribute the live downlink collected from ground stations all
   over the US to users worldwide in real time.

BACKGROUND:  The Automatic Packet/Position Reporting System (APRS) is a
connectionless protocol using UI frames to communicate information
effeciently among a large group of stations in real time.  Each station
with information transmits, and all stations capture, sort and accumulate
the information on a variety of display pages or windows as follows:

   BULLETINS  Lists bulletins in sequence regardless of receipt order
   MESSAGES   Displays messages to and from your station
   TRAFFIC    Displays messages between other stations
   STATUS     Displays the current status of all stations
   POSITIONS  Displays the current position of all stations
   TELEMETRY  Displays any telelmetry packets
   HEARD LOG  Shows number of packets per station per hour, etc

Although most people associate APRS with GPS position reporting, a GPS
is not required unless a station is moving.  Moving stations
use a GPS to automatically update their position, instead of
requiring manual entry.   Otherwise, the primary advantage of APRS
is sharing information between the largest possible number of users
effeciently over a single shared channel.

MIR BULLETINS:  The MIR packet radio link is ideal for a one-to-all
protocol for distributing BULLETINS, ANNOUNCEMENTS and other information
to all users in the footprint.  Although MIR currently transmits a
few single line bulletins, these are sometimes lost among the
hundreds of other ACKS, REJECTS, and BUSY packets in the downlink
that sometime overshadow any transfer of information.  By using a
protocol like APRS to receive the MIR downlink, monitoring stations
can capture such properly formatted BULLETINS and ANNOUNCEMENTS of
interest to everyone and display them in a user friendly manner.

MIR POSITION TRACKING:  Since APRS has a full maping capability, it can
not only display the location of all ground stations, it can also
plot the moving position of a spacecraft with an onboard GPS.   But
since a hardware upgrade to add a GPS to Mir is not likely, the
same effect can be accomplished by a few strategicly placed MIR
tracking stations that transmit up a pseudo-GPS position to be
digipeated by MIR to all ground stations.  Three such stations could
provide this service whenever MIR is over the USA.  These stations
transmit only a single 1/2 sec packet every minute (1% of channel
capacity) and the result is an accurately moving MIR ICON on all
ground station maps.

PARTICIPATING STATION DISPLAY:  Since the MIR PBBS is a single user
system, it can usually only be accessed by one or two users per
footprint under good conditions.  Meanwhile, within the same footprint
are often dozens if not hundreds of users sending useless and
competing CONNECT-REQUEST packets which only reduce channel effeciency.
The constant stream of DISCONNECT-BUSY packets in the downlink
conveys the number of users participating in the pass, but carries
no useful information.  If instead of useless connect requests, these
same stations simply transmitted a single compressed position report
it would not only show who is participating in the pass, but also
show the distribution of these stations on the APRS map displays at
no additional loading on the channel.  The flavor conveyed to the
users in this case is a feeling of camaraderie as a participant with
others in a pass instead of individuals fiercely competing with each
other for the single user access.  This in itself is a better
attitude to convey to students and the map display is certainly more
visually appealing than a scrolling display of disjoint packets.


INTERNET INTERLINKED GROUNDSTATIONS:  With the worldwide connectivity
of the internet, the downlink packets from MIR can be received by
groundstations anywhere and made available to all users everywhere.
For over a year, the APRServe system has been providing that kind
of connectivity to APRS packets genearated nationwide.  During
the APRS/MIR test it allowed stations even out of the footprint to
observe the event.  Similarly for schools, it gives easy access
even to individuals and classes with no amateur radio equipment.

THE APRS/MIR TEST:  Since the Mir packet system has been operating well
the last few weeks, Dr. Larson of the MIREX group suggested the APRS
Mir test be conducted as soon as possilbe since precession was taking
Mir passes earlier every day and it would soon be out of view during
school hours.  He authorized the APRS/MIR test on only two orbits on
the 10th of March.  Unfortunately the MIR packet system went off the
air on these two orbits, so the test was extended to the next few
orbits over the USA.  The test was limited to the USA only because
it had the largest numbers of existing APRS ground stations ready to
test in sufficient numbers to fully load the system.  But the delay
further complicated matters since the next orbits over the USA were
between 0230 AM and 0400 AM local time.  As a result, the test was
extended for a full 5 orbits to allow testers to choose a pass and
still get some sleep.

With the short notice of the test, and since the more effecient SPACE
mode of APRS had not been used or tested since the STS-74 and 78
SAREX and SPRE Missions over two years ago, APRS users were told to
just operate normally on the MIR frequency but to shorten their
position comments where possible.  The SPACE mode compresses the
station's position report into his gridsquare to save 16 bytes per
packet.  Due to the typical 10 minute duration of a pass, they were
told to transmit their position reports once every 3 to 4 minutes
to get a reasonable probability of success per orbit.

To make Mir appear to move on all groundstation maps, three special
tracking-uplink stations beaconed the moving position of MIR via the
MIR digipeater.  One from California using the callsign MIR-6, one
from Michigan using MIR-8, and one in Maryland using MIR-3 to match
their callsign areas.  West coast stations would see the moving
MIR-6, midwest stations would see the incoming MIR-6 change to
a MIR-8 and then east coast stations would see the moving ICON on their
maps change to a MIR-3.

Although MIR was not programmed to transmit any special Bulletins,
several ground stations transmitted such BULLETIN, STATUS and
MESSAGE packets.  Over 65 separate such packets were successfuly captured
during the test.  On average these packets were transmitted
successfully twice (110 times).  The advantage of using the few
specially authorized uplink sites to digipeat bulletins instead of
MIR means that the bulletins may be updated instantly on the ground
and then digipeated to ALL users at once without requiring an upload
to the BBS nor hundreds of individual downloads.  Only bulletins
originated by the MIR crew would need to be entered by the crew.

Each of the participating stations transmitted their brief position
packet once every 3 to 4 minutes.  These packets were far less frequent
than the usual once-every-10-seconds of all of the usual CONNECT-
REQUEST packets normally which congest the uplink.  Each time one of
these packets was successfully digipeated by MIR, all user map displays
in the footprint would be updated with colorful Icons showing all
stations locations.  During the test 202 station position reports were
displayed averaging about 40 per pass.

To inject the downlink from Mir into the Internet, a few of the normal
APRS I-Gates tuned their radios from the normal APRS frequency to the
Mir downlink frequency.  These Mir packets were intermingled with the
normal stream of APRS packets into the APRServe Internet
system.  Although they would be seen on the main www.aprs.net maps
they would be hard to distinguish from the usual 1000 to 1200 or more
APRS stations on the air.  To provide a unique display of the APRS/Mir
packets alone, a special WEB page was designated to filter out only
the APRS/MIR packets and display them spearately to users.  During
the day of the event there were over 11,000 hits on the server system
representing a peak load of 150 simultaneous users and as many as
1000 users on the special MIREX page.

STATISTICS:  In raw numbers, the following table compares the APRS
packets during the test to the other packets observed on the downlink.
These statistics were mostly derived from observations on the
east coast (Maryland) plus the APRS packets logged elsewhere.

PASS  TOTAL   PBBS  PBBS  BUSY  R0MIR  R0MIR  APRS  APRS
      PKTS    PKTS  USERS REJ   PKTS   USERS  PKTS  USERS
----- ------- ----- ----- ----  -----  -----  ----  -----
0740z  229      88    3   18    30     1       89   44
0910z  153      46    3    7    14     0      102   41
1050z  122      47    3    5    11     0       75   51
1220z  170      96    3   15    35     4       72   39
1400z  188      94    5   12    41     3       53   27

Of the 202 APRS stations, 6 were associated with schools and 5 were
mobiles inclusing one Naval Academy boat, and one railroad car.
Individual APRS packets were also reported by one station
in each of Tiawan, South Africa, Australia, Japan and Hawaii.
Although APRS  users were encouraged to only send their position
report until the total load on the channel could be assessed, there
were still 65 Bulletins and Messages that were successfully relayed.
Since the test was conducted over multiple passes, the 202 successful
stations actually represent 104 different stations.

SUCCESS RATE:  Of these 104 different stations, all were running 10
watts or more, but two were successful at 5 and 7 watts.  Of the
ten stations reporting a lack of success, 3 were running only
1 watt, and two were running 4 and 9 watts.  One was only transmitting
once an hour, and one was transmitting once every 5 minutes.  Another
was using an inside antenna.  Two were running 10 watts and 50 watts
but beacon rate was unreported.  If you consider a 10 watt baseline
and proper setup with a 3 to 4 miunte rate, then the success rate
appears to be near 98%.  Although there may be many more stations that
did not report their lack of success, these numbers clearly show that
the channel was not saturated nor congestion limited.  In fact, during
this test a nominal 3 Mir PBBS users logged onto the BBS per pass, but
typically only one was successful at uploading and downloading any
traffic.  Actually this is rather typical on most passes during user
hours, but in the middle of the night, as in this test, these BBS
users would have expected a less congested channel.


LESSONS LEARNED:

1)  Due to the short lead time to the test, there was insufficent
time to educate all users to use the short Gridsquare or compressed
SPACE mode, and to remove unnecessary verbage from their position
reports.  For this reason there were many quite verbose packets.
A 30% improvement could be expected here.

2)  Similarly, the SPACE mode was not used.  This further reduces
bandwidth by automaticlly canceling all further transmission as
soon as one's own packet is successfuly digipeated.  With the 3 to 4
minute cycle time used and without the automatic cancelation on
success, there were probably double the number of packets
transmitted than were actually required.

3)  Non APRS stations were recommended to use the very efficient
grid square method of reporting position by placing their gridsquare
in the UNPROTO TOCALL or their packets.  Unfortunately, this does
not work on Kantronics TNC's which always send their BText UI frames
to the callsign of BEACON.  THus their packets conveyed no position
information.

4)  For the pseudo moving MIR-6, MIR-8, and MIR-3 uplinked posits, these
should have all used the same ICON name of just MIR.  Although the
numbered MIR Icons made it possible to easily see which uplink station
was responsible for the ICON, the changing calls meant that as the
MIR-6 and Mir-8 uplink stations passed out of range, these posits
were stuck on the map at their last uplinked position and stopped
moving.  By using the same Icon name of Mir, then the single Icon
would have continued to move as long as there was at least one
uplink station in range.

6)  The uplinked MIR positions were more static than dynamic since
only about 20 were recorded on all 5 passes.  At one per minute,
this shows only a 20 % success rate for the possible 100 pass minutes.
Higher power or tracking antennas may be required.

CONCLUSIONS:   The test was successful in meeting all of the original
objectives.  The short notice and early morning hours helped to reduce
the number of participants to about 104 stations.  We think this number
is representative of the nominal number of schools that could be
authorized to simultaneously participate in future such Mir experiments.
A total of 12 stations associated with schools and students were
reported.  One station even displayed 5 APRS stations while operating
with a whip antenna inside a motel room on travel.

     The test demonstrated the value of using a UI frame one-to-all
packet protocol to improve the delivery of information to all ground
stations.  Further, the test demonstrated the value of a few special
MIREX ground stations to uplink the moving Mir position reports and
to relay real-time MIREX bulletins and announcements that can be
received by all stations in the footprint including receive-only
school stations.  Finally, the test showed the value of multiply
internet connected ground stations for not only providing a continuum
of data from the downlink across the whole country, but also for
providing WEB access to students and schools outside of the footprint
or without amateur radio equipment.  You may see a replay of the event
by using APRS to replay the file MIRTEST.HST or by visiting
www.aprs.net/mirex.html.

All of the APRS stations want to thank the MIREX team and also those
normal Mir BBS users who were inconvenienced by this test, for this
opportunity to conduct this important experiment.