KC4YOZ Mobile APRS Tracker Implementation

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GPS and TNC Hardware Interfacing

  1. The DB9F connector (not included on the Garmin adapter cable) is to allow testing of the GPS using a computer or terminal. The connector should be wired in the DCE configuration (GPS talks on pin 2 - RXD) for this to work.

  2. Build your DB9-to-DB25 cable so that the DB25 is wired in the DTE configuration (GPS talks on pin 2 - TXD in this case) so the TNC (a DCE device) can hear it. DO NOT USE a standard 9-to-25 adapter cable for this!

  3. The Serial Data In (SDI) line on the GPS cable is not connected. The Garmin GPS-45XL needs no initialization string to start sending data; it just spits out a serial stream at 4800 BPS.

  4. Note that the GPS is always energized, so the TNC is supplied with current data whenever it is on.

TNC Software Configuration Steps

The KPC-3 v.6.0 GPS Enhancement introduced four "LText" buffers, each with the capability to store a particular string and beacon it out on its own schedule to its own UNPROTO address. The 'GPSHEAD' command sets up the search string header, 'LTP' sets the UNPROTO address and 'BLT' sets the beacon schedule. Whenever the TNC sees the string header in the serial stream, it updates the buffer.

The instructions below were lifted from the Kansas City APRS Group web page, and I also sprinkled in a few of my own steps. Configure your terminal software and TNC to the same parms used by your GPS -- usually 4800/N/8/1. If you have a new out-of-the-box KPC-3, follow the manufacturer's instructions to set up the baud rate (it's real easy).

NB - If you're following along in the KPC-3 manual, you will see references to the "LTRACK" buffer... ignore LTRACK as we are not going to use it here.

  1. Configure your GPS to send NMEA-0183 v.2 data formats.

  2. Set up a little Escape Message to remind you of how to get out of GPS mode...

    "GPSINIT Hit CTRL-C three times to exit GPS Mode."

    'GPSINIT' is used to send an initialization message to the GPS receiver to get it primed; as noted earlier, the Garmin doesn't need it. If you ever have to pull the TNC and hook it up to a computer to change a setting (and you will), you'll thank yourself for doing this....

  3. Set up the LText string headers..



  4. Set the beacon timings...

    "BLT 1 EVERY 00:01:00 START 00:00:01"

    "BLT 2 EVERY 00:01:00 START 00:00:31"

    This example will alternate the two beacons every 30 seconds.

  5. Set the UNPROTO paths...




    See the SYMBOLS.TXT file in your APRS documentation for an explanation of how to use the GPSxx addresses.

  6. "MON OFF"


  8. "CMSG DISC"

  9. "CTEXT Stand-Alone Tracker.. No Operator.. Disconnecting."

  10. Your BTEXT can be used for your name and your voice channel...

    "BTEXT YOZ-Man Tracker.. voice on 443.85+ PL 100.0"

  11. "B E 00:10:00"

  12. Now set GPS Mode on.

    "INT GPS"

Programming is completed. Power off the TNC. Power it back on and make sure your reminder message works. Now take it out to the mobile and hook it up!

General Hints & Kinks

In the process of switching to 144.39 I had decided to take the entire hardware setup apart and re-build it, documenting along the way. What I found after a year (or so) of service caused me to re-do some things a little differently, and here is that list.

  1. Solder your connections. The wires are going to be exposed to temperature extremes and the tape you use will likely deteriorate. I soldered everything and used heat-shrink tubing to keep everything insulated and together.

  2. Screw the connectors together and make sure they can't vibrate apart.

  3. Lock the radio particularly if it's in the trunk or prone to getting bumped.

  4. Monitor your local APRS net for a few days before you do the programming; see what the other folks with trackers are doing with their beacon text and timings, etc.

  5. Have fun. Go wild and make it an installation you'll be proud of.

Hey, ya need to e-mail me a question or comment?

Ya want to find me or my car on APRS?

73 de KC4YOZ.