GPS time correction

The pull of gravity causes clock to run slower than outer space based on relativity theory.  Satellites orbiting at 400km altitude still have 90% of gravity pull as on surface and so clock aboard runs slightly faster.  At the same time, the satellites are moving fast sideway that prevent them from falling down.  This high speed movement slows down the clock.

The reduction of gravitational pull causes the satellite clock to run faster than Earth by 45 micorsecond a day.  The movement of the satellites on the other hands slow down their clock by about 7 msec a day.  This gives a net effect of 38msec a day slower for clock in satellites.

GPS positioning works by measuring the time it takes for a signal from the device on the ground to bounce off a satellite and returns.  As each msec delay is translate to about 300 meters.  The time difference, if not correct, will cause positioning problem of 10km each day.