The receivers get signals from several satellites and calculate by their own their with these datas. The GPS calculates the position by triangulation.

Considering the lenght of a radio waves emited by a satellite to reach him, the receiver determines a sphere on which is inevitably his position. Distance speed nbr seconds.With a second satellite, the intersection of two spheres create a circle on which is the receiver.Finally with a third satellite , Going to Blogs Sap the receiver determine a single place on which it must be.The problem is that the receiver doesnt have the real time the exact atomic time that allows it to know where are really the satellites. So with three satellites the gps calculates his relative position to them, but, as it doesnt know where they are, it cannot determine his own position. Engineers consider that this problem is solved with the use of a fourth satellite and a model of the shape of the earth WGS84.In real life, the receivers use more than 4 satellite up to 14 at the same time Obviously, the more numerous are the satellite used, the more accurate are the location datas.