1. A system of satellites, computers, and receivers that is able to determine the latitude and longitude of a receiver on Earth by calculating the time difference for signals from different satellites to reach the receiver.
2. A satellite-based radio navigation system run by the U.S. Department of Defense.

It was designed so that signals from four satellites would be available at any point on the earth. By triangulation of these signals, a receiving unit can pinpoint its current location (longitude and latitude) as well as altitude to within 20 meters. The first GPS satellite was launched in 1978.