GPS, or Global Positioning System, is a satellite-based navigation system. GPS works via a system of satellites and receiving devices used to compute positions on the Earth. The basis of GPS is “triangulation” from satellites. To “triangulate,” a GPS receiver measures distance using the travel time of radio signals. To measure travel time, GPS needs very accurate timing which it achieves with some tricks. Along with distance, you need to know exactly where the satellites are in space. High orbits and careful monitoring are the secret. Finally you must correct for any delays the signal experiences as it travels through the atmosphere.