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[aprssig] Raw Messages decypher?

Stephen H. Smith wa8lmf2 at aol.com
Sun Sep 23 22:16:16 UTC 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
  http://wa8lmf.net/DigiPaths





--

Stephen H. Smith    wa8lmf (at) aol.com
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