[aprssig] Raw Messages decypher?
Stephen H. Smith
wa8lmf2 at aol.com
Sun Sep 23 17:16:16 CDT 2007
Bernard Tyers wrote:
> Hi all,
> Can anyone point me towards a good explanation of the raw packets in
> the attached URL? http://ei8fdb.org/aprs/xastir_messages.txt
> I understand most of whats listed here, namely the q constructs, the
> n-N paradigm (but I am still trying to understand the WIDEn-N paths!
> Thats for another mail), but the main parts I dont understand come
> directly after '>' eg "US1XWP", "US2PUR"
> I know its to do with the path, but, I am slightly confused.
> Apologies for the possibly simple question.
These are Mic-E-encoded compressed position reports. Normally the
"destination address" of an APRS packet consists of a string starting
with AP and then up to 4 more arbitrary alpha characters. These extra
characters usually encode the program or hardware device that created
the packets. (In your own samples, "APK101" indicates the transmission
originated from a Kenwood. )
In the Mic-E format, in order to get the shortest possible position
transmission, part of the position data is encoded into the
weird-looking destination address. If you watch as such a mobile
moves around, you will see that the "destination" keeps changing.
(The Mic-E format is the default and only format that the Kenwood APRS
radios use for transmitting positions. You normally only see the APKxxx
prefix in non-position packets such as messages and status reports, when
the destination address is beinging "hijacked" for position data.)
"APRS 101" Explanation of APRS Path Selection & Digipeating
Stephen H. Smith wa8lmf (at) aol.com
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