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MIR 5w ERP test results



Dear AMSAT-BB,
   Sorry that the timing of this report appears to be continuung to "beat
the dead horse", but my student finally finished summarizing the data.
And we just want to get it out the door...

------------------------------
THE TEST:   During the week of 2 June an impromptu test was  conducted
via the MIR Digipeater to demonstrate the capability  for an Amateur 
Radio hand-held Satellite messaging system.  Taking  advantage of the
thousands of Kenwood TH-D7 hand held radios with  built-in TNC and 
APRS, the objective was to see how many operators  were able to exchange
data using only the 5 watt radio and rubber duck antenna.
 
RESULTS:  The results were enthusiastic.  Almost everyone that tried
was eventually successful in digipeating at least one packet via  MIR on
each pass.  A total of 67 stations submitted reports  showing over 190
different stations were logged.  For this test, success was  measured by
the ability of a station to transmit his position, status and a short 
(few words) message.  Thus the ability  to use Amateur Satellites for
two-way messaging between handheld data radios was aptly demonstrated.
Using such simple UI Digipeaters in space could support hundreds of such
stations per footprint per pass. (several hundred over the USA alone).
 
Imagine the benefit to HAM radio and survivable communications
if we had a few such UI Digipeating Satellites in space!
 
APRS DATA RADIOS:  For readers unfamiliar with the Kenwood THD7
APRS radio, it has a built in TNC and user interface so that you
can capture and display the STATUS, POSITION and MESSAGES from
as many as 40 other stations.  It can send your own STATUS and
position as well as single line messages up to 45 characters in
length.  This communications uses the APRS Mic-Encoder format so
that these packets are typically less than a half second in length
at 1200 baud.  THey are transmitted in the unconnected or UI mode
so that no acks are requred.  When the HT does see its own packet
digipeated, it displays "MY PACKET" so you can tell that it
was successful.   This is very valuable at reducing QRM on the
uplink, since successful stations can then stop transmitting.
The TH-D7 can transmit two types of packets:
 
  POSITION/STATUS - contains your LAT/LONG, and map ICON.
                    ALso contains one of 7 pre-defined "comments"
                    Also may contain up to 20 chars of STATUS
  MESSAGES - Any text up to 45 characters in length. 
 
Note that every APRS station will receive and display your POSITION/
STATUS, but MESSAGES are only captured by the receipient.  For this
reason and to vastly improve your chances of success via a space 
digipeater, stations were encouraged to send their traffic addressed
as a BULLETIN to all, so that everyone could see the traffic.
 
POWER and ANTENNAS:  Although most people have not considered 5W
as a viable power level to operate MIR, there are many advantages
in the handheld scenario making it much more successful:
 
  1)  Lack of coax provides 3 dB average improvement on XMT & RCV
  2)  ABility to tilt antenna to match downlink polarity can gain
      3 to 10 dB over fixed base station antennas at some times.
  3)  All stations were using the same power and rubber duck antenna
      thus getting equal chance moderated only by the ever changing
      satellite geometry.  Sooner or later your geometry is optimum.
  4)  5w ERP was 10 to 20dB below other MIR PMS users
      so our tests did not appear to impact normal PMS usage by
      the normally stronger packet stations.
 
TYPICAL PASS:  Using the THD7, you typically would load your position 
and status and a brief message (bulletin) before the pass.  THen
on hearing the MIR packets, your display would begin flashing
the calls, locations and bulletins of other stations heard.  When
you heard a silent gap in packets, you could hit the BCON button
to force your own position/status packet.  If you get through, the HT
will display "MY PACKET".

Next if you have a specific text message to send you can then bring 
up the message list and press the TRANSMIT button to force your 
BULLETIN message if desired.  Again, if you see "MY PACKET" then you
know you were successful.  At the end of the pass or during it, you can
see who all you heard on the STATION list or the MESSAGE LIST...
 
STATISTICS Compared to overall PMS Channel Activity.
 
Since the MIR digipeater shares the same radio with the BBS, and
the BBS users are usualy running considerably higher power, then
the lower power HT stations were only successfull during dead time
between BBS packets.  To consider the statistics of all packets on
the downlink during a pass, the following data was recorded during
three passes over the USA (MD) and Europe.  Note, "DM" frames are "BUSY-
REJECT" packets which just waste air-time).  
 
EUROPEAN PASS:  32 of 33 stations trying to log on (280 DM frames(80%))
    BBS:   330 packets to transfer 27 useful lines of text = 8%   success
    CNCT:   17 packets to transfer  1 useful line  of text = 6%   success
    UI:      2 packets to transfer  2 useful lines of text = 100% success
 
USA PASS1: 4 of 15 users trying to log onto BBS (15 DM frames 19%)
    BBS:   54 pkts to transfer 10 useful lines of text = 18% success
    UI:    31 packets conveyed 26 useful text lines    = 84% success
 
USA PASS2:  4 of 16 users trying to use BBS (23 DM frames (15%)
    BBS:   98 pkts to convey 18 useful lines of text   = 18% success
    CNCT:  29 pkts to convey 1 text line               =  3% success
    UI:    28 pkts to convey 26 useful lines or text   = 93% success
 
The difference between the numbers of users I think are due to the USA
passes being during working hours and the European pas as being during
evening or weekend hours...  But notice the vast difference in "success"
if you simply consider "success" to be the conveyance of meaningful
infromation in a packet.

de WB4APR, Bob and ENS Chris Morgan






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