GPS Must Account For Relativity

The incredible precision of your phone's map is a daily, practical demonstration of Einstein's theories of relativity. The system faces a cosmic tug-of-war between two different relativistic effects. First, due to special relativity, the satellites' high orbital speed of about 14,000 kilometers per hour actually causes their clocks to tick slightly slower than clocks on the ground. However, a much more powerful effect from general relativity is also at play. Orbiting far from Earth's surface, the satellites experience a weaker gravitational pull, which causes time to pass more quickly for them. The effect of weaker gravity overpowers the effect of high speed, causing a net gain of about 38 microseconds for every satellite, every single day.

While 38 millionths of a second sounds trivial, it is a catastrophic error for a system that locates you by measuring the travel time of signals moving at the speed of light. Left uncorrected, this tiny time drift would introduce positioning errors that accumulate at a rate of nearly 10 kilometers per day, making the entire GPS network (Review) useless within hours. When the system was first designed, some engineers were so skeptical that these corrections were necessary that the first satellites were launched with a switch to turn them off. The clocks immediately drifted exactly as physicists had predicted, providing a crucial, real-world validation of Einstein's work that we now rely on every time we ask for directions.