GPS and Relativity

As reported by GPSWorld: An educational video by the Perimeter Institute of Theoretical Physics shows how GPS, a navigational tool that can pinpoint your location to within a few meters, incorporates a number of effects from Einstein’s theory of relativity.

Special relativity effects include the speed of the GPS satellites (14,000km/hr), which can cause the atomic clocks on-board the satellites to register time about 7 microseconds/day slower than on earth.  General relativistic effects include the reduced gravity environment that the satellites inhabit at about 20,000km above the earth's surface which causes the clocks to run about 45 microseconds a day faster than they do on earth.  The overall effect is a +38 microseconds/day increase in measured time which is then compensated for at the satellites.