The GPS (Global Positioning System) is an American system of satellites and receptors enabling users to know their exact location anytime and almost anywhere on Earth.
24 medium Earth orbit satellites (20,000 kilometers) transmit signals, picked up by user’s GPS receiver (user segment). The signals picked up by the GPS receivers are transformed and calculated in 3D coordinates, according to the World Geodetic System WGS84.
To locate his exact position on a map, the user must transform the WGS84 coordinates of the receiver into the national system of the country where he is located.
Also to be mentioned, the European Union is developing its own localization system : Galileo, to be independent from the American Army. This system should be operational in 2008.