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[aprssig] WRAP-UP MISSOULA MARATHON APRS SUPPORT

Bob Bruninga bruninga at usna.edu
Tue Jul 10 16:19:18 UTC 2012


Great report!  Here are some Kibitzing thoughts which may or may not
apply...

> There were 5000+ runners/walkers/handicappers participating....
> trackers... following three lead males and female runners, 
> and two "tail end charlie" trackers 
> two Byonics MT-AIO "yellow box" units 
> a Byonics MT-RTG... pace car, 
> three Kenwood D7/D72's, and an 
> Arduino tracker ... on a bob-trailer
> ... we set them for 30 second updates

I count 7 trackers going at 30 second rate which is really pushing the Aloha
limit on a 1200 baud channel with digipeats. You may have lost a lot of
packets due to collisions...

> ... although there were some gaps in coverage...

Have you considered that what might appear as a loss in coverage is really
only a loss due to collisions.  If the channel is oversubscribed, then only
the strongest always get through.  So, if there is a distant or low area
where the signals are always a few dB lower than the other trackers, then
those packets are always going to be lost due to collisions.  Whereas if the
TX rate were less, allowing for the 80% silent time of a good ALOHA channel,
then the reliability of those weak spots might dramatically improve.

Remember, the IDEAL APRS channel is SILENT for 80% of the time if you want
the best and most reliable throughput.

Of course, an 80% silent channel can either be due to a PROPER designed
ALOHA rate or a completely saturated rate where 80% of all packets are being
lost due to collisions.  To the one person monitoring, unless he really
knows what to listen for, cannot distinguish the two cases.

IGNORE me if this does not apply to your event.  It just gives me a chance
to stump about generalities...

Bob, WB4APR




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