[aprssig] High density timeslotting

Phillip B. Pacier ad6nh at arrl.net
Thu Jun 8 22:29:58 CDT 2006

This would be teriffic for something like the Baker to Vegas race... 
right now, I fire a beacon once every 3 seconds, and digipeat twice in 
channel.  We are likely going to stop digipeating in channel next year 
and have that handled by a 9600 baud UHF network.  But to be able to 
squeeze more trackers into a smaller slot is something that would be 
valuable to us.

See http://www.b2vtracking.com/slotfiles/ to see how we currently assign 
beacon slots.

Phil - AD6NH

scott at opentrac.org wrote:
> I'm thinking of trying an experiment, and I was wondering if anyone's
> attempted this before.
> Right now, timeslotting on the OpenTracker (and presumably the TT3 and
> anything else that supports it) isn't accurate to more than a second or two
> because of the way time data is presented in the incoming GPS data stream.
> The practical limit would seem to be about 2-3 seconds separation between
> trackers.
> With an accurate time reference, though, you could do a lot better than
> that.  Deluo's pin out info shows (or did the last time I was able to find
> it) a 1 pulse per second output, but it turns out that at least for the
> current generation of receivers it's actually a TTL version of the data
> output.  I'm not sure if that applies to the SiRF units as well.  The Garmin
> GPS18 does provide a real 1PPS output, and better yet, it's well-documented
> - something you can't say for ANY of Deluo's offerings.
> Assuming you're not running something with a horribly slow TXD (like a
> D700), getting two beacons per second should be pretty easy.  Three looks
> doable with decent transmitters, and four might be possible with good
> equipment and very short packets (200 ms for a 30-byte frame and < 50 ms
> TXD).
> The only hardware modification required for an OpenTracker should be an
> inverter circuit for the 1PPS pulse to make it into a negative edge to
> trigger the interrupt request.  Actually, even that might be optional, as
> long as you know exactly how long the pulse is.
> The firmware would need a bit of tweaking to synchronize the internal clock
> to the 1PPS signal, and some more to make sure that it stops acquiring fixes
> a second or two before it's due to transmit so it's not caught in the middle
> of an incoming fix.
> Digipeating would present an interesting challenge.  Obviously if the
> channel utilization is over 50%, you can't digipeat in-channel.  Are there
> any dual-port TNCs out there that can handle full duplex digipeating, and
> what transmitters would be suitable for 100% transmit duty cycle for
> cross-band operation?
> It might seem like an exotic application, but I've got a few users who have
> expressed interest in this sort of thing.  I think they're all for races or
> competitions of one sort or another - organized events where everything's
> coordinated in advance, with appropriate infrastructure.  Higher
> timeslotting density means more tracker users and/or a faster update rate
> without the added expense of multiple channels.
> Anyway, has this been tried before?  Should be interesting to experiment
> with.
> Scott
> N1VG
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