Why must GPS account for Einstein’s relativity?

GPS relies on the travel time of radio signals. Satellites transmit precisely timed information, and a receiver uses signals from several satellites to estimate its position and its own clock offset. Because radio travels at the speed of light, even a tiny timing discrepancy matters.

Relativity says that clocks following different motions and experiencing different gravitational potentials need not accumulate equal elapsed times. Compared with an Earth-based reference, a GPS satellite’s motion contributes roughly 7 microseconds of delay per day. Its higher gravitational potential contributes roughly 45 microseconds of gain per day. A microsecond is a millionth of a second.

These effects oppose one another, leaving a nominal gain of about 38 microseconds per day. Multiplying one day's uncorrected offset by light speed gives the diagram's timing-range equivalent of about 11.39 kilometers. Over two days the offset would be about 76 microseconds, equivalent to about 22.8 kilometers. These distances show how strongly timing matters; they do not predict that a receiver's position must be wrong by those amounts.

GPS is engineered to account for these relativistic effects, along with further clock and signal corrections. The satellite clocks are not defective: the difference is part of how time works. Relativity becomes a practical ingredient in making a phone’s map useful.