How does GPS work?

 The global positioning system (GPS) is made up of many satellites that are constantly transmitting to Earth very precise time signals produced with the aid of atomic clocks. Because the satellites are positioned at various distances from the receiver, it is possible to calculate the position of the receiver from the satellites' orbits and the time delays experienced in receiving the signals-provided the signals are received from at least four satellites. There is a problem, however. The GPS satellites travel through space at 14,000km/h. Because of their speed, the clocks on board the satellites-as predicted by Einstein's theory or relativity-run slower than clocks on Earth. There is also a second relativistic effect caused by the Earth's gravitational field being a little weaker in orbit than it is at ground level, and this causes the clocks on board the satellites to run a little faster than those on Earth. Unfortunately, these two effects do not cancel each other out, and the navigation system has to make precise allowances for the temporal distortions caused by gravity and motion. Since the divergence amounts to several kilometers a day, these corrections are essential if GPS is to get us to where we want to go.