APRS is an application of Packet Radio, and stands for Automatic Packet Reporting System. Using a single VHF frequency (144.390MHz in North America) Amateur Radio Stations can report their location, which is logged and recorded by a number of computers connected through the Internet. There are several web sites that will display current stations. This one is easy to use: ttp://www.wulfden.org/APRSQuery.shtml
APRS was developed by Amateur Radio Operator WB4APR. Here is a link to his page: http://eng.usna.navy.mil/~bruninga/aprs.html
There are a couple things besides a radio and a ham license that you need work APRS. A TNC (Terminal Node Controller) and a computer with appropriate software are also required. For mobile or portable stations a GPS is also required. The TNC is a type of Modem. It converts the character data into sound in the audio range. That sound is then trasmitted on the 144.390 MHz FM carrier. The data packets sound very much like the computer modem screeches that you hear when a dial up connection is initiating.
A computer with a sound card can do the work of a TNC, it just requires a couple pieces of software. They work together to convert the data packets, control the transmitting of the radio, and log heard stations. The first program is AGW Packet Engine. It is written by ham SV2AG, available here: http://www.elcom.gr/sv2agw/agwpe.htm. The other software is WinAPRS, which is avialble here: http://www.winaprs.org/.
Individuals who do not have an Amateur Radio license can still set up a monitoring station. AGW Packet Engine can be set up to acquire stations through an Internet port, but if you have a radio that can receive 144.390 MHz FM you can monitor the position of local stations directly. Here is an example setup:
The radio is an older model Radio Shack Scanner which has a RCA line out on the back panel. The grey cable connects to that. The other end of the grey cable goes to a home brew stereo mini jack splitter sitting on top of the scanner. The splitter routes the mono output to the tip (Right Channel) of the sound card microphone input. A normal stero mini jack cable can be used, but with the splitter AGW Packet Engine can be set up to receive from separate radios on each channel.. Below is a screenshot of the WinAPRS program:
