How it works
There are around 30 satellites in orbit, each carrying a precise atomic clock. The receiver in your phone picks up signals from at least four satellites and calculates its position from the differences in their arrival times. An error of one millionth of a second would put you 300 metres off.
Einstein in your phone
Because of the theory of relativity, the clocks on the satellites run differently from clocks on Earth. The difference is about 38 microseconds a day. Without correction, your position would drift by kilometres every day. GPS is everyday proof of Einstein's theory.
Not just navigation
Precise time from GPS synchronises mobile networks, banks and power grids. Jamming the GPS signal is therefore a security threat, and in recent years it has become common, especially near the borders of war zones.