So... you have an APRS mapping display running on the computer at home, and you would like to keep track of the kids when they borrow your car (I'm just going to the Mall...). You need a "Tracker"! Just kidding...
"Tracker" is the word we use to describe the combination of a GPS unit, a TNC, and a transmitter. A GPS (Global Positioning System) unit receives signals from the constellation of 23 satellites out in space, put there by the US military. The signal coming from the satellites is basically an ID/Time stamp. Each satellite has a specific ID number, and continuously sends the time of day (in UTC format).
There are many GPS receivers available. Make sure when you select one that it has a serial output connector, and that it sends the data out in NMEA format at 4800 baud (this is the industry standard). I use a Garmin GPS II, and a Delorme Tripmate, both work just fine.
The GPS unit listens for the signals from the Satelites, and performs a simple triangulation to find your current location. The GPS unit then outputs the data as a latitude and longitude in a standard format recommended by the National Marine Electronic Association (NMEA). Please see the NMEA Data section. The NMEA format also specifies that the serial data "Sentences" come out at 4800 baud.
You will need a TNC that accepts NMEA data, and can process this data for APRS use. Most NEW TNCs have APRS support built in these days, but check it out. My current TNC of choice for making a Tracker is the TinyTrakII. Kantronics is a major supporter of APRS, and makes some good TNC's See the Kantronics section. Timewave has purchased AEA, and the new EPROMs for some AEA TNCs support APRS too. Contact Timewave.
The TNC accepts the NMEA Sentences (usually $GPRMC and $GPGGA, but others could be used), and reformats the data, adding your callsign, converting it to the APRS standard format. The TNC then sends the data, now as an audio signal, to the radio, and keys the transmitter via the PTT (push to talk) connection.
The radio transmits the data on the APRS frequency of 144.390MHz. Most TNC's can accept audio back from the radio to "listen" to see if the frequency is in use before transmitting. This avoids two or more stations transmitting at the same time, a "Double" or "Collision", which results in garbled, scrambled data at the receiver or home location.
You can use about any 2 meter HAM radio you like. For special events and ARES work, many of us just use an HT and a good mag mount antenna. APRS packet is done at 1200 baud in Milwaukee, and most everywhere else in the US, so a new 9600 baud "data radio" is not needed. For a more permanent application, the old spare mobile rig, or a swapfest special is a perfect choice.
My first Tracker was a Garmin GPSII, a KPC3+ and an ADI HT. This worked very well, but was a "spaghetti fest" of wires across the back seat of the car, and for events (using it in someone else's car) it was not the greatest or cleanest setup. I then got a Tripmate GPS, which has a built in antenna that works very well, and a Paccomm Pico TNC. This reduced the physical size of the Tracker system, but was still 3 separate boxes on the back seat of the car, and a lot of interconnecting cables. Next I acquired a APRS Engineering MIM TNC, and built it directly into the Tripmate, So I was down to 2 pieces (GPS / TNC, and the HT), and one cable between them.
My current Trackers all run TinyTrakII's
from Byon Garrabrant, N6BG of Las Vegas, NV.
I like these the best, check them out.
Please see the individual sections in Hardware for particular setup and wiring information.
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Jan. 21, 2002