The application of the GPS is very broad, and as the prices come down the number of uses is increasing. Portable GPS devices are used by fisherman and hikers to help them navigate in the wild. Many new cars are being equipped with GPS systems to help drivers with navigation. The military uses GPS to guide cruise missiles to pre-specified targets.

On 2 May 2000, the Department of Defense switched off selective availability (SA) which made GPS readings less accurate. They reserve the right to turn it back on during national crises and the like, but for the most part GPS devices are now about 10 times more accurate than they were when selective availability was turned on. A GPS receiver calculates its position by measuring the distance between itself and three or more GPS satellites. Measuring the time delay between transmission and reception of each GPS microwave signal gives the distance to each satellite, since the signal travels at a known speed. The signals also carry information about the satellites' location. By determining the position of, and distance to, at least three satellites, the receiver can compute its position using trilateration. Receivers typically do not have perfectly accurate clocks and therefore track one or more additional satellites to correct the receiver's clock error.