The Global Positioning System (GPS) is a U.S. space-based global navigation satellite system. It provides reliable positioning, navigation, and timing services to worldwide users on a continuous basis in all weather, day and night, anywhere on or near the Earth which has an unobstructed view of four or more GPS satellites.
GPS is made up of three segments: Space, Control and User. The Space Segment is composed of 24 to 32 satellites in Medium Earth Orbit and also includes the boosters required to launch them into orbit. The Control Segment is composed of a Master Control Station, an Alternate Master Control Station, and a host of dedicated and shared Ground Antennas and Monitor Stations. The User Segment is composed of hundreds of thousands of U.S. and allied military users of the secure GPS Precise Positioning Service, and tens of millions of civil, commercial and scientific users of the Standard Positioning Service (see GPS navigation devices). GPS satellites broadcast signals from space that GPS receivers use to provide three-dimensional location (latitude, longitude, and altitude) plus precise time.
GPS has become a widely used aid to navigation worldwide, and a useful tool for map-making, land surveying, commerce, scientific uses, tracking and surveillance, and hobbies such as geocaching and waymarking. Also, the precise time reference is used in many applications including the scientific study of earthquakes and as a time synchronization source for cellular network protocols.
GPS has become a mainstay of transportation systems worldwide, providing navigation for aviation, ground, and maritime operations. Disaster relief and emergency services depend upon GPS for location and timing capabilities in their life-saving missions. The accurate timing that GPS provides facilitates everyday activities such as banking, mobile phone operations, and even the control of power grids. Farmers, surveyors, geologists and countless others perform their work more efficiently, safely, economically, and accurately using the free and open GPS signals.
History
The design of GPS is based partly on similar ground-based radio navigation systems, such as LORAN and the Decca Navigatordeveloped in the early 1940s, and used during World War II. In 1956 Friedwardt Winterberg[1] proposed a test of General Relativity using accurate atomic clocks placed in orbit in artificial satellites. To achieve accuracy requirements, GPS uses principles of general relativity to correct the satellites' atomic clocks. Additional inspiration for the GPS came when the Soviet Union launched the first man-made satellite, Sputnik in 1957. A team of U.S. scientists led by Dr. Richard B. Kershner were monitoring Sputnik's radio transmissions. They discovered that, because of the Doppler effect, the frequency of the signal being transmitted by Sputnik was higher as the satellite approached, and lower as it continued away from them. They realized that since they knew their exact location on the globe, they could pinpoint where the satellite was along its orbit by measuring the Doppler distortion (see Transit (satellite)).
| Block | Launch Period | Satellite launches | Currently in orbit and healthy | |||
|---|---|---|---|---|---|---|
| Suc- cess | Fail- ure | In prep- aration | Plan- ned | |||
| I | 1978–1985 | 10 | 1 | 0 | 0 | 0 |
| II | 1989–1990 | 9 | 0 | 0 | 0 | 0 |
| IIA | 1990–1997 | 19 | 0 | 0 | 0 | 11 of the 19 launched |
| IIR | 1997–2004 | 12 | 1 | 0 | 0 | 12 of the 13 launched |
| IIR-M | 2005–2009 | 8 | 0 | 0 | 0 | 7 of the 8 launched |
| IIF | 2010–2011 | 0 | 0 | 10 | 0 | 0 |
| IIIA | 2014–? | 0 | 0 | 0 | 12 | 0 |
| IIIB | 0 | 0 | 0 | 8 | 0 | |
| IIIC | 0 | 0 | 0 | 16 | 0 | |
| Total | 58 | 2 | 10 | 36 | 30 | |
| (Last update: 29 December 2009) PRN 01 from Block IIR-M is unhealthy | ||||||
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