GPS (Global Positioning System)
GPS is a global positioning system by satellites. One can construct a sphere around the satellite S1 that contains the desired location P. By proceeding the same way with the satellite S2, the intersection of the 2 spheres is a circle containing the point P. So, thanks to the third satellite S3 there are two possible positions. A fourth satellite would make it possible to determine one unique point: the exact position of the location.
