How to Build APRS on the cheap
Long-time packeteer Bob Bruninga, WB4APR, developed the Automatic Packet Reporting System (APRS), which allows packet radio to track real-time events. It deviates markedly from the usual message- and text-transfer activity. Instead, APRS concentrates on the graphic display of station and object locations and movements.
For example, if you know the latitude and longitude of your station, you can add this information to the beacon transmissions sent by your packet TNC. Any monitoring station that's equipped with APRS software will translate the data and display your location on a computer-generated map. Taking this idea a step further, if you own a portable Global Positioning System (GPS) receiver, you have precise position information at your fingertips. Connect the GPS receiver to your TNC and you can transmit your location information even as you're moving! When any person in an APRS network determines where you're located, he can move his cursor and mark your position on his map screen. This action is then transmitted to all screens in the network, so everyone gains, at a glance, the combined knowledge of all network participants. In other words, everyone knows where you are. The map screen retains this information for future reference. This means that moving objects can be dead reckoned to their current locations with one keystroke--based on their previous positions. With a small GPS receiver, a TNC and a hand-held transceiver stuffed in a cigar box, almost any object can be tracked by packet stations running APRS software. You can place these boxes on bicycles for a marathon event, and, of course, in automobiles. This system is an excellent tool for triangulating the location of a hidden transmitter or jammer.