In satellite navigation the measurement of distance is reduced to a measurement of time. The satellite signals spread at the velocity of light. A clock that goes wrong by a millisecond only per day leads the navigation therefore astray by almost 300 kilometres.

The receiver does thus need an own atomic clock, or it uses the bearings of a fourth satellite in order to determine the own position within the four-dimensional 'spacetime'. This way, it synchronizes the internal clock to the on-board clock of the satellite.