Global Positioning System (GPS), officially named NAVSTAR GPS, is a fully functional Global Navigation Satellite System (GNSS) developed by the United States Department of Defense and accessible as a public good to civilians. It enables a GPS receiver to determine its location, speed, direction of speed, and time, by making use of a constellation of at least 24 medium Earth orbit satellites, managed by the United States Air Force 50th Space Wing, that transmit precise microwave signals. Maintenance of the GPS costs about US$750 million per year.
Three components make up the GPS: the space segment (SS), the control segment (CS), and the user segment (US).
The orbiting GPS satellites, also known as Space Vehicles (SV), comprise the SS. There were 24 SVs included in the GPS design, with 4 SVs assigned to each of the six circular orbital planes, each possessing a 55 degree inclination tilt relative to the Earth’s equator, and 60 degrees right ascension of the ascending node separate them. Each SV orbits at an altitude of about 20,200 kilometers and at an orbital radius of 26,000 km twice every sidereal day. Due to the specific arrangement of the orbits, at least six satellites are within line of sight from almost everywhere on the Earth’s surface at any given moment. Additional satellites have been added to provide redundant measurements and in effect, achieve better precision on GPS receiver calculations. There are now 31 SVs present in the SS as of September 2007.
The CS refers to the flight paths of the satellites. They are monitored by National Geospatial-Intelligence Agency (NGA) stations, as well as US Air Force stations in Colorado, Hawaii, Kwajalein, Ascension Island and Diego Garcia. The 2d Space Operations Squadron (2 SOPS) operates the master control station of the US Air Force Space Command, which is located at Schriever Air Force Base in Colorado Springs, Colorado. This station receives the tracking information from all other stations that are monitoring the flight paths.
Every GPS satellite is regularly contacted with a navigational update by 2 SOPS, created by a Kalman filter which uses various inputs such as those from ground monitoring stations and space weather information. The navigational updates are used to synchronize the atomic clocks on board the satellites to within one microsecond, and update the information about the satellites’ location and general system health, also known as almanac and ephemeris data, respectively. This is crucial to the system, because a GPS receiver is able to calculate its position by measuring the distance between itself and three or more GPS satellites, and then using trilateration. The receiver-to-satellite distance is measured by the time delay between transmission and reception of each GPS microwave signal.
The US refers to the GPS receiver of a user, which is composed of: a highly-stable clock, usually a crystal oscillator; an antenna, tuned to the frequencies transmitted by the satellites receiver-processors; and optionally a display for providing location and speed information to the user. Though originally only able to monitor four or five satellites simultaneously, a GPS receiver can now track anywhere between twelve and twenty of them.
The GPS is an important technology for both the military and civilians, making it a dual-use technology. It was originally developed for the military, where it aids in navigation, target tracking, missile and projectile guidance, reconnaissance and map-making, nuclear detonation detection, and search and rescue. For civilians, GPS provides an aid for navigation and land-surveying, and a means of calculating local velocity and orientation. It also serves as a time reference for several applications.