Sidebar from QST September 1996, p.58
The Automatic Packet Reporting System (APRS) was created by Bob Bruninga, WB4APR. APRS exploits the ability of a TNC to transmit beacon packets that carry short strings of alphanumeric characters. A beacon is an unconnected packet. You can think of unconnected packets as "broadcasts." The information is sent to no one in particular and can be received by anyone. An unconnected packet can be relayed through a node or digipeater if you "tell" your TNC to do so.
By taking data from a GPS receiver and incorporating it into beacon packets transmitted by a TNC, you can tell everyone on the network exactly where that GPS receiver is located. Any stations equipped with APRS software will display the position of the receiver on a computer-generated map. If the receiver moves (let's say it's in an automobile), its position on the map changes with every update.
Although APRS' mapping capability was developed to display the movement of hand-held GPS receivers, most features evolved from earlier efforts to support real-time packet communication at special events. Any person in the network, upon determining where an object is located, can move his cursor and mark the object 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!
Let's say you're monitoring the movements of rafts during a river race in which each rafter carries a 2-meter FM transceiver, a TNC and a GPS receiver. If your station picks up a transmission from any raft along the river, it will automatically relay the information to everyone else. So, everyone's maps are continually updated with the latest positions of the rafts.
You don't need to own a GPS receiver to enjoy APRS. All you need is the APRS software and your normal packet TNC. Just determine your latitude and longitude as best you can. Look it up in an atlas, or borrow a friend's GPS receiver just long enough to determine the position of your station. After you feed the information to the software, your TNC will regularly announce your position to anyone else who is monitoring. You can even use APRS to exchange bulletins and enjoy live conversations with others on the network.
Most APRS activity is on 2 meters, with 144.39 MHz being the popular frequency. If you do purchase a GPS receiver, you'll need a TNC with APRS firmware.
The APRS software is distributed as shareware and may be copied for any amateur application. The software includes maps for most areas of the US. You can also edit and add more detail to the maps. APRS software is available on many ham-oriented BBSs as well as the World Wide Web.