[aprssig] Why does...

[Stephen H. Smith]

Earl Needham wrote on 1/18/2005, 11:20 PM:

 > Why does
 > 
KD5XB-9>APRS,MELROZ*,WIDE5-4,qAo,W5ALL-1:$GPRMC,043322,A,3410.6052,N,10321.2030,W,032.0,228.6,190105,,*0F 

 >
 > show as a house on Findu but as a little red car (as it should) in
 > Ui-View?
 >


There are three ways to cause a particular symbol to be displayed:
1)    By appending a particular SSID to the originating callsign.
2)    By fiddling with the destination callsign (instead of "APRS" you 
use GPSxy)
3)    By placing the symbol select codes in the position report itself.



You are transmitting a raw unprocessed NMEA string directly out of the 
GPS receiver which makes it impossible to embed the symbol codes within 
the posit.

Findu ignores the archaic callsign SSID mode of specifying the 
displayable symbol and defaults to a house when it can't find any symbol 
info in the packet.  Thus you get the "wrong" results

UI-View DOES interpret the callsign SSID if it can't find any other 
symbol info in the packet.  Thus you get the intended results for SSID = 9.



Given your constant comments about how long and unreliable your digi 
path has to be to reach an igate,  you would enjoy a FAR higher success 
rate (and be able to control the symbol better) if you used a more 
compact transmission format instead of the horribly inefficient 
(needlessly long and air-time-wasting) raw NMEA.

The standard APRS format (about 1/2 to 1/3rd the length of the raw NMEA) 
or the really-compressed Mic-E format (about 1/10th the length of raw 
NMEA) yield FAR shorter packets that are FAR more likely to get through 
3-4-5 hops successfully.  [The shorter packets are far less likely to 
get clobbered by other stations or zapped by pops of noise or mobile 
fading.)

If you are using a "dumb" TNC that can't do anything but blindly echo 
the raw NMEA output of a GPS receiver onto the air, perhaps you should 
consider a TinyTrack.  It CAN do the short formats, AND can store two 
different path settings that are selectable in the field with a toggle 
switch.





Stephen H. Smith                wa8lmf (at) aol.com

Home Page:                      http://wa8lmf.com

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