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[aprssig] Re: The best resolution of position from APRS

Stephen H. Smith wa8lmf2 at aol.com
Wed Jan 4 08:19:44 UTC 2006


vk4tec at tech-software.net wrote:
> So try this
>
> Put your TNC into converse mode, you know when you hit enter it transmits.
>
> But instead hook up your GPS receiver.
>
> In theory every line of NMEA data has a return, so the TNC should just
> UNPROTO it out
>
> Quick and nasty APRS
>
>   
This is exactly how the original APRS trackers worked 10 years or so 
ago.  GPS receivers plugged into the serial ports of pre-APRS-aware 
TNCs.   You had to slow down the NMEA reporting rate from the GPS and 
turn off all but one NMEA sentence; else the TNC would try to beacon 
multiple GPS strings every second.  

The first generation of APRS-aware TNC firmware (TAPR Ver 1.1.9 for 
TNC2s like MFJs and early Kantronics) provided for constantly updating 
several beacon comment fields in the TNC by capturing specified GPS 
strings at whatever repetition rate the GPS spewed out.   The TNC could 
then be set to beacon these strings at reasonable rates with the usual 
BEACON EVERY n commands.    Consequently, you could now use most GPS 
receivers as-is in the default mode of multiple sentences output every 
second.

There is an APRS standard for these dumb NMEA trackers.  Set the 
destination address to GPSxyz (instead of the customary APnnnn). The 
three characters in positions x,y, and z determine the APRS symbol to be 
displayed.

The main disadvantage of the raw NMEA approach is wasted air time on 
crowded channels.  The raw NMEA string is about 3 times as long as a 
standard APRS format posit, and about 10 times as long as a Mic-E format 
posit.    Besides wasting air time, the posit is less likely to get 
received since the longer string is much more likely to be hit by a 
mobile fade, noise, or get stepped on by another station. 








Stephen H. Smith    wa8lmf (at) aol.com
EchoLink Node:      14400    [Think bottom of the 2M band]
Home Page:          http://wa8lmf.com

"APRS 101"  Explanation of APRS Path Selection & Digipeating
  http://webs.lanset.com/wa8lmf/DigiPaths

Updated APRS Symbol Chart
  http://webs.lanset.com/wa8lmf/miscinfo/APRS_Symbol_Chart.pdf   


New/Updated "Rev G" APRS     http://webs.lanset.com/wa8lmf/aprs
Symbols Set for UI-View,
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