GPS
Non-contact measurement

The GPS (Global Positioning System) uses transmissions from twenty-four satellites to determine position. These satellites are regularly distributed over six circular orbits inclined at 63 degrees to the equatorial plane, at an altitude of 20,200 kilometers, giving them a period of revolution of twelve hours. This space architecture guarantees the permanent visibility of at least six satellites at all points on the globe.

Each satellite has an atomic clock and transmits signals coded on two L-band frequencies with a relative stability of 10 ^–13 . These frequencies are...