The use of GPS devices has become widespread to meet military needs, or large public professionals: navigation, topography and time synchronization, etc.
A satellite positioning system also referred to as symbol « GNSS » (for Global Navigation Satellite System) is a set based on a constellation of satellites positioned in the space and for providing to a user, via a portable receiver small its 3D position, velocity and time.
This system is characterized by a category decametric accuracy, global coverage and compactness of terminals. Some systems increase and reliability of regional or global scope, free or paid, to further improve the reliability and accuracy available (DGPS or EGNOS).
The first satellite positioning system developed by the US to an exclusively military with TRANSIT in the 60s, then with the Global Positioning System (GPS), which became operational in 1995 and which sets out the operating principles set by system satellite navigation developed by other countries. The GPS system relies on a constellation of thirty satellites, permetant a user, located on any point on the globe, always have at least four satellites in range. The user terminal calculates its position by the signal emitted by each of these satellites.
The USSR following the United States is developing GLONASS took office in 1996 and who, after the breakup of the Soviet Union once again became operational in 2010. The European Union with the Galileo system and China with Beidou-2 (COMPASS) develop their own system that will be operational in 2020. Japan and India develop a system on their side providing only regional coverage.