[OZAPRS] HF Configuration.

Bob Bruninga bruninga at usna.edu
Thu Apr 12 22:11:16 EST 2007


> what you mean by a regular smart beacon rate? .. 
> ... the default settings... could see beacons 
> as often as once every 10 seconds... through 
> very winding terrain. Is this ok for HF ?

When we first began using HF APRS for vehicle tracking in 1992, we never
considered it a "tactical" frequency, but a "strategic" frequency for the
long haul traveler.  The idea was a place where a long haul traveler could
indicate his approximate progress along his journey and any change in
status.  Our goal was an hourly update if the band was open and at least
one update every four hour watch.

The point being, where is he? (what city or state?), not what street
address he is passing.  For this we chose a 10 minute beacon and status
rate because we had 20 vehicles.   At 3 seconds each, that was 20% channel
capacity which is about the maximum that an ALOHA network can support
reliably.

With the fits and spurts of HF propogation, it does not seem to make sense
to fill the channel with frequent detail exact positions which are
obsolete the minute they are transmitted, but instead stick to the
standard HAM radio net-cycle time of about 10 minutes.

Nothing against smart beaconing, but it makes no sense on HF.  The goal of
smart beaconing is to generate enough detail posits so that the best
possible detail track history can be obtained.  This makes sense on VHF
where propogation is quite homogenious, but with the long outages on HF,
it doesn't make sense to have little patches of detail with long gaps of
nothing.  

Of course, there is nothing wrong with collecting great detail when
available, except for one thing.  And that is equitably sharing the
channel with other users.  A user of HF that is only transmitting
strategically will not be happy if his relatively rare packet is stepped
on by a high rate tracker...

ALso, on HF, there is good value in CSMA, ie: listening first before
transmitting.  Some trackers only have an "energy" detector that probably
does not work at all on HF, where channel noise with AGC is the same as
signal power and the circuit either will never transmit, or it has to be
disabled, and then it will transmit right on top of someone else.

Anyway, just some thoughts as to the original concepts behind APRS on HF.

Bob Bruninga
WB4APR
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