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How the Global Positioning System (GPS) works

The GPS system is made of a constellation of 24 satellites orbiting at an average altitude of 20,000 kms, scattered on 6 orbital planes equaly spaced. As a result, at least three satellites are always visible at anytime and from anyplace on earth. Each satellite emits a coded signal which contains essential informations like its position and the exact timing of the signal emission to earth. Therefore, nothing but a simple receiver is needed to measure the elapsed time between emission and reception of the signal. The satellite to station distance is simply deduced from this travel time. Three different measurements made on three different satellites give the three distances needed to determine the three coordinates of the station position : latitude, longitude, and altitude. This type of measurements is known as pseudo-range measurements in GPS jargon. Every satellite emits two types of pseudo-ranges : A precise code (P code) which enables a position precision of around 10 meters and a coarse code (C/A code) which allows a precison of around 100 meters. The precise code is encrypted to deny the precise positioning to anyone else than the american military. Therefore, the only precision available to civilian applications based on the pseudo-ranges measurements is this of the C/A code.