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In 1983, President Reagan makes available the use of GPS system for the civil aviation to improve flight safety after a plane crash mainly caused by a problem of localization. In 1990, the US military opened using GPS to civilians. The system became fully operational in 1995 with 24 satellites orbiting the Earth. With this system, the US military can located all units with high accuracies. Positioning is performed by calculating the time it takes for a radio signal to run the distance from the satellites to the GPS receiver, with great precision. In 1998, a program was launched to publicize the use of GPS. In 1999, the first GPS phone is marketed. From the 2000s, the technology has continued to improve. Thus, GPS and devices equipped by this system are becoming smaller and cheaper. The system of satellites is continually improved and allows location more accurate. Today, the Air Force is responsible for many satellites for the good operation system and continues to perform their satellite performances.[Mar12].
The GPS system consists of a network of 24 satellites managed by the US Defense Department [Ort02]. This department is in charge of their moves and information they emit. These satellites are orbiting the earth at a distance of approximately 12,000 miles[Mar06]. They follow a specific path around the earth and emit a unique signal. These paths are determined so that there are always five satellites above a GPS receiver. GPS Receiver is the name that should be used instead of GPS. GPS is the name of the global system. When a GPS receiver is turned on, it tries to establish a connection with at least four satellites. "The more satellites a receiver can lock onto signals, the better the accuracy" [Ort02]. When it receives a satellite signal, the GPS calculates the distance between the satellite and itself that allowed it to determine its exact location.
To know the distance, the GPS needs to be synchronized to the satellite and to have exactly the same time with an extreme precision.
