[aprssig] Having trouble with APRS setup

[Stephen H. Smith]

John Denison wrote on 12/26/2004, 4:36 PM:

 > The GPS is working fine because I connected it to my pc and could see
 > the data displayed in hyperterminal every 2 seconds. The cable that
 > came with the GPS has a 9-pin female connector, and the TNC has a
 > 25-pin female connector. I used a 25-pin male to 9-pin female adapter,
 > and a 9-pin male to male gender changer to get the GPS and TNC connected.
 >
 > I configured the TNC exactly the same as the example shown in the
 > manual for GPS use. The baud rate is set to 4800 for both the GPS and
 > TNC. The GPS is set to output NMEA type data. The TNC's receive light
 > comes on as it normally would when a signal is heard. The TNC will key
 > up the radio in TERMINAL mode, but not in GPS mode. I used the
 > following parameters to set up my TNC:
 >
 > GPSINIT $PGRMO,GPGGA,1
 > GPSHEAD 1 $GPGGA
 > LTP 1 GPS via DIGI
 > BLT 1 EVERY 00:01:00
 > ABAUD 4800
 > INTFACE GPS



The TNC is intended to be connected "straight through" to the PC.
The GPS is intended to be connected "streight through" to the PC.

Therefore connecting the GPS to the TNC will require a CROSS-OVER cable 
or adapter (i.e. a "null modem") to work.   Specifically a DB-9 male to 
DB-9 female where pin 5 goes straight through, pin 3 goes to pin 2, and 
pin 2 goes to pin 3.



Assuming you get the TNC responding to the GPS, it is now discouraged to 
do this.  Transmitting RAW NMEA strings over the air (which is what this 
setup does) is terribly inefficient.   Almost one hundred characters 
have to  be sent over the air to do the same thing that the standard 
APRS format can do in about 30 or that Mic-E format can do in about 12.
The shorter packet bursts of APRS format or Mic-E are far preferrable 
because they:
1)  Reduce on-air transmit time and channel congestion.
2)  Dramatically increase the chance of successfully being received 
since the much shorter bursts are much less likely to get stepped on by 
another station, or get hit by mobile flutter or noise.
3)  Give you the flexibility of using the entire APRS symbol set.


At the once-per-minute transmit rate you are proposing, you would be 
occupying almost 10 percent of the entire available channel capacity on 
144.39 in your local area. A raw NMEA packet is about 6 seconds long.  6 
out of every 60 = 10%.   By contrast a Mic-E packet is only about 
ond-half to two-thirds of a second, meaning a channel usage of about 1%.


Instead of tying up an expensive TNC as a dumb mobile tracker, consider 
using a $30 Byonics TinyTrak (which is capable of encoding the efficient 
APRS or Mic-E formats) to mate your GPS to the radio.



Stephen H. Smith                wa8lmf (at) aol.com

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