The Global Positioning System is a network of 24 satellites used for navigation. The U.S. Dept of Defense launched the first GPS satellite into orbit in 1978. By 1994, the network as we know it now was complete. Although the GPS system functioned pr
ior to all 24 satellites being operational, it's far more accurate with the full complement of satellites in orbit. There are occasionally more than 24 satellites in operation because the DoD continually launches replacements for older satellites. In the 1980s, the government made the GPS system available for civilian use.
GPS satellites circle the earth in a very precise orbit and transmit signals to earth. GPS receivers receive this signal and, by calculating the time it takes to receive a signal from at least three satellites, triangulates the receiver's location anywhere on earth. If the GPS device receives a signal from at least four satellites, it can determine the user's altitude in addition to latitude and longitude.
With a GPS device, you can usually determine your position to within a few meters. Using a GPS device in conjunction with a wardriving setup, you can map the location of discovered networks as your sniffer discovers them. Your best bet is to use a National Maritime Electronics Association (NMEA) compliant unit, with a serial connector. Many popular GPS units are NMEA-compliant, including some Garmin units, although they require you to enable that option (by default Garmin units use their own proprietary format).
Depending on the sniffing software you use, the GPS data recorded by the sniffer usually corresponds to the location of the strongest signal. You can export location data from your sniffer into mapping programs, such as Microsoft Map Point 2004, and create location maps.