The Global Positioning System (GPS) is a satellite navigation system that provides positioning, navigation, and timing services to civilian and military users. GPS is a US-owned utility maintained and operated by the US Space Force. GPS obtains geolocation and timing data by sending and receiving radio signals between GPS receivers and thirty-one satellites in medium earth orbit.
There are three other satellite navigation systems providing similar services:
- GLONASS—developed and operated by the Russian Federation
- Galileo—developed and operated by the European Union
- BeiDou—developed and operated by China
GPS consists of three segments:
- Space segment—satellites
- Control segment—ground stations
- User segment—receivers
GPS satellites carry atomic clocks that provide accurate timing information. The satellites emit radio signals containing this timing information measured by GPS receivers, which calculate the time difference between sending the signal and its reception. From this time difference, the receiver can determine its distance from the satellite.
By calculating the distance to three satellites, the receiver can determine its three-dimensional position on Earth. However, accurately computing the distance from each satellite requires another synchronized atomic clock. Taking measurements from a fourth satellite means each receiver does not need its own atomic clock. Therefore, GPS requires the receiver to receive signals from four satellites to accurately compute its latitude, longitude, altitude, and time.
Ground stations use radar to monitor, maintain, and control the positioning of each satellite as well to make adjustments to the satellite clocks.
Designed by the US military in the 1960s, the first experimental Block-I GPS satellite manufactured by Rockwell International was launched in February 1978. In December 1993, initial operational capability of the GPS system was achieved, with a complete constellation of twenty-four satellites in orbit by January 17, 1994.
President Bill Clinton issued a policy directive in 1996 declaring GPS to be a dual-use system for civilian as well as military use, establishing an interagency GPS executive board to manage it as a national asset.
GPS has a wide range of applications in numerous industries, including the following: