The Global Positioning System (GPS) has been operational since 1995. The main link in the system is a constellation of 31 satellites positioned on six orbital planes such that any point on the Earth’s surface is covered at all times by at least 6 satellites. These satellites emit on two frequencies a time signal generated by the atomic clocks on board. Satellites also emit their ephemeris (position and speed at the time of emission as well as prediction of positions and speeds for the following hours). The measurement principle is close to triangulation. Knowing the time of transmission and the reception time of the signals by a receiver and postulating a model of speed of the electromagnetic waves between the satellites and the receiver, it is possible to calculate the position of the receiver on the terrestrial surface. This system allows position measurements with an accuracy of a few meters for acquisitions lasting one second.
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