Global Positioning System (GPS)

Atomic clocks are the basis of the GPS navigation system. The atomic clock uses an atomic resonance frequency standard as its timekeeping element. It is very accurate but expensive. So it only use at the ground stations and on-board the GPS satellites. User only use crystal clock.

Atomic clock normally use Rubidium atoms or Cesium atoms or Hydrogen-maser atoms.

All GPS satellites have several atomic clocks. The signal that is sent out is a random sequence, each part of which is different from every other, called pseudo-random code. This random sequence is repeated continuously.

All GPS receivers know this sequence and repeat it internally. Therefore, satellites and the receivers must be in synch. The receiver picks up the satellite’s transmission and compares the incoming signal to its own internal signal. By comparing how much the satellite signal is lagging, the travel time becomes known.