Paris (France) - Sure your digital wristwatch is accurate, but that’s nothing compared to the on-board clocks of the Galileo navigation satellites. Europe’s competitor to the US GPS system, boasts several active and backup clocks powered by hydrogen and rubidium. The European Space Agency claims these clocks are one thousand million times more accurate than your average wristwatch.
Each satellite will have four clocks, two primary hydrogen masers and two rubidium-powered clocks. One hydrogen clock is always running as the primary navigation clock while the other is kept as the cold spare. Similarly, one rubidium clock is kept as an always on backup to the hydrogen clock, with the other rubidium clock as a cold spare.
Satellites must keep the precise time down to the nanosecond because ground units are constantly querying for their coordinates. In one nanosecond light and radio waves travel approximately .3 meters.
As a comparison, the US GPS satellites use a combination of Cesium and Rubidium clocks.