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[aprssig] The APRS AX.25 Frame

Joseph M. Durnal joseph.durnal at gmail.com
Tue Sep 20 19:09:29 UTC 2011

I'm trying to explain an APRS packet to many non-APRS users and I want
to get it right, but I've confused myself

Here is a basic packet

NE3R>APX200,WIDE1-1,WIDE2-1:=3925.63N/07726.00WxPHG3000Frederick ARC APRS Demo

The AX.25 frame is defined
Flag(1) Dest(7) Src(7) digi(0-56) ctrl(1) PID(1) info(1-256) FCS(2) flag(1)

I'm assuming that there are bits in the frame that aren't translated
to text on the TNC, what confuses me is the source, I obviously see
the source on the TNC, but it comes before the destination.  I'm also
not sure what defines the : between the path and position.

My explanation will look like this

"NE3R>" = The source of the packet
"APX200," = The destination, in practical application, it isn't really
the destination, but it identifies the APRS software generating the
"WIDE1-1,WIDE2-1" = the packet path, 3 hops in this case, once
dilapidated looks like N3KTX-2*,WIDE1*,KV3B-1*,WIDE2*
":" = not sure exactly what this is, found between the path and the
rest of the APRS info field
"=3925.63N/07726.00WxPHG3000Frederick ARC APRS Demo" = the APRS
information, which I'll break down further below
"=" = APRS datatype, this one represents an APRS position without a
time stamp from a messaging capable station
"3925.63N/07726.00W" = the position data, but the / is important to
the APRS symbol definition
"x" = the second part of the symbol, combined with /, the x displays
an X for XAPRS/Xastir (linux software), symbols for cars />, trucks
/k, houses /-, etc can be used
"PHG3000" = optional power height & gain info, important for
digipeaters and fixed stations (your software should calculate this)
"Frederick ARC APRS Demo" = A free-form comment

I have to keep it simple, but I want it to be informative as well.
I'll probably add a few links, like the symbols definition.

Thanks & 73 de Joseph Durnal NE3R

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