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[aprssig] APRS DF reporting

scott at opentrac.org scott at opentrac.org
Tue Nov 21 19:56:21 UTC 2006


> to adapt it to this task, but I now think it might be simpler 
> for me to just
> write my own code. Also looked at the APRS spec and the AX.25 
> spec. Aside
> from the CRC calculation routine ( which I suppose I can 
> "rob" from Bob
> Ball's ( et. al. ) code, worst case ) I think I understand 
> how to do it all.

You might read over my page at <http://n1vg.net/packet> for some of that.
The FCS algorithm in particular - I had a hard time myself finding the right
one.  Too many bad examples out there.

If you do implement it from scratch, please do us all a favor and don't
transmit raw NMEA strings for position reports.  We've got too many dumb
devices wasting bandwidth that way already.

My offer stands for the OpenTracker option - it'd cost under $10 in parts to
add it to your board if you're so inclined.  And the code is open source,
under the BSD license.

>  The PicoDopp DF outputs a standard Agrello message string at 
> 4800 baud, (
> quality fixed = 7 ) with an occasional GPS message tossed in, 

Ok, I found a description of the format.  It's going to look something like
this, right?

%143/7

That should be easy enough to recognize automatically.  I'm not sure what I
ought to report in the APRS string for number of hits or range.  I suppose
range could be a user-selectable parameter based on empirical knowledge of
the system's sensitivity, but it doesn't seem to make much sense when you
don't know the transmitter power.

Accounting for the interleaved NMEA data might be harder.  Hmm.. maybe I'll
just merge the Agrelo parser into the NMEA parser.  Might make things
simpler.  I might even be able to fit the code in the OpenTracker without
dropping other features, if I can find a few bytes of RAM to hold the
bearing.

I'd still like some real-world Agrelo data, from ANY sort of DF unit, if
anyone's got some or could capture some easily.

Scott
N1VG







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