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[aprssig] Symbol Protocol Help

Stephen H. Smith wa8lmf2 at aol.com
Fri Apr 25 16:27:55 UTC 2008


Ev Tupis wrote:
>
> Btw...I couldn't find any statement that restricted
> GPS/SYM/SPCxyz use to raw NMEA posits only (though
> there's a lot of documentation and I may have missed
> it).
>
>   

The GPSxyz construction is essentially an kludged expedient for dumb 
trackers implemented with ordinary TNCs.  In such a tracker, the raw GPS 
NMEA data is just dumped into the TNC serial port and transmitted as-is. 

Since the lat/long data is not being reformatted into normal APRS 
format,  there is no way to insert the normal APRS symbol control codes 
into the packet payload.    Instead, the symbol control codes are 
formatted into a fake "destination call sign" entered into the TNC's 
UNPROTO field.  Since this IS a callsign field, it can only accept 
letters, numbers and the hyphen character for SSIDs.    

All the other punctuation marks used as symbol control codes in normal 
APRS format are illegal in the destination call field, so the GPSxyz 
codes do not have a one-to-one correspondence to the normal APRS-format 
symbol control code format.


The GPSxyz code tables are mainly of academic interest now, since 
practically no one today would be (or should be!)  using the long-winded 
raw NMEA-format dumb tracker instead of something like a TinyTrak or 
Opentrack which can create normal APRS format packets including standard 
symbol codes. 





--

Stephen H. Smith    wa8lmf (at) aol.com
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