Launched in 1978, the Global Positioning System (GPS) is the only functional global navigational satellite system. It consists of 30 satellites in orbit around the Earth at approximately 20 200km, which transmit signals allowing receivers to determine their location, speed and direction.

The GPS system can be divided into 3 main parts: The user segment (US) consisting of the user's GPS receiver; the control segment (CS) consisting of US Air Force bases; and the space segment (SS) which consists of the 24 orbiting satellites distributed over 6 orbits, and 6 spare.

GPS receivers come in a variety of formats including car navigation, hand held device for hikers or trekkers, devices embedded into watches and other specialty uses.

The GPS technology uses a complex combination of constancy of the speed of light, the equivalence principle, the Sagnac effect, time dilation, gravitational frequency shifts, and relativity of synchronization to combat many of the physical effects inherent in the system.

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