GPS works thanks to a system of satellites positioned in orbits about 20,000 km up (there are currently 32 satellites in orbit). It's run by the U.S. military. When you use a GPS receiver, you're receiving a signal from (at minimum) four satellites to get a fix: the signal from three satellites is used to triangulate on your position, and the signal of a fourth satellite, to provide a time correction. Where the satellites are relative to you, is determined by how long it takes a signal to travel between you and the satellites, and for the whole thing to work, the system has to use extremely precise clocks.

GPS can accurately determine position to around 30 centimeters, anywhere on Earth (barring physical barriers to radio signals, or electronic interference) but that's only if the satellite atomic clocks, and the more precise atomic clocks on the ground that correct them, are providing accurate time. The whole system is simple in principle, but timekeeping accuracy is everything. A nanosecond (one billionth of a second) error means a position error of about a foot, which means a one second error puts you off by a billion feet: 189,394 miles, which is around 5/8 of the way to the Moon. At that level of sensitivity to clock precision, GPS has to compensate for effects described by Einstein's theory of relativity – clocks moving with respect to each other, will see each other's clocks as ticking at different rates, and clocks experiencing different forces of gravity will have the same problem.

Thanks to relativistic effects, to a clock on the ground, GPS satellite clocks look like they're running 38 microseconds faster, which produces a cumulative error of 10km per day, so if you do find your mother-in-law's house accurately with GPS, you can thank Albert Einstein – and the ultra-precise atomic clocks that keep the whole system in sync.