Are you thinking about the GPS Navigation System?
http://en.wikipedia.org/wiki/Global_Positioning_System
in the case of the GPS system, there are at least 4 satellites are always visible from any position on the Earth at any given time (with up to 12 visible).
The satellites each carries an atomic clock that has an accuracy of roughly 1 nanosecond in a given amount of time.
GPS receiver determines its current heading and position by comparing the time signals it picks up from the GPS satellites in orbit. By triangulating on the known positions of the GPS satellites, the receiver can know its positional information with ultra high precision.
in order for this to work, the clock in the GPS satellites must be known to an accuracy within 20 to 30 nanoseconds. Now, because the satellites are constantly moving in relation to people on the ground and each other, there is a discrepancy between the clocks. Special and General theories of Relativity must be taken into account to make sure the clocks stay within acceptable synchronization tolerances.
when looking at a satellite in orbit from the ground, the satellites are in motion relative to the observer (or vice-versa) Special Relativity predicts that we should observe the satellites clocks ticking slower than ours on the ground.
http://www.astronomy.ohio-state.edu/.../Unit5/sr.html
Special Relativity predicts that the clocks on the satellites should fall behind clocks on the ground by roughly 7 microseconds per day. This is because they are ticking slower due to the effects of time dilation due to their relitive motion.
also
satellites in high orbit, like the GPS satellites where the Earth's gravitational field is less than it is at the Earth's surface, General Relativity is asserted and shows that clocks that are on the earths surface will tick away more slowly than the satellites the higher orbits further away from earths gravitational field. and the clocks in orbit will tick faster than the clocks on the ground.
http://www.astronomy.ohio-state.edu/...lackholes.html
when using General Relativity to calculate the clocks in GPS satellites, it is found that the clocks in the high orbit satellite should move ahead of ground-based clocks by ~45 microseconds per day.
when one combines the two relativistic effects where special relativity says that the clocks will tick slower and general relativity says that the clocks will tick faster than the ground based clocks, it is found that the clocks on-board the satellites should tick faster than identical clocks on the ground by about ~38 microseconds per day (45-7=38).
sounds small, but when a system that needs nanosecond information where 38 microseconds is 38,000 nanoseconds, If these effects are not taken into account properly, a navigational position based on the GPS satellite constellation would be in error after 2 minutes. The errors would accumulate to about 10 kilometers per day.
http://www.losangeles.af.mil/library...et.asp?id=5311
Link removed
http://en.wikipedia.org/wiki/Effects...ativity_on_GPS