GPS Satellites Must Correct for Relativity

The pinpoint accuracy of the Global Positioning System in your phone or car relies on a fascinating quirk of physics predicted by Albert Einstein over a century ago. Two competing effects from his theories of relativity are constantly at play. According to general relativity, time moves faster in weaker gravitational fields. Since GPS satellites orbit far above the Earth, their onboard atomic clocks tick slightly faster than ours on the surface. At the same time, special relativity dictates that time slows down for objects moving at high speeds. The satellites' immense orbital velocity of nearly 14,000 kilometers per hour causes their clocks to tick slightly slower from our perspective.

These two relativistic effects don't cancel each other out. The influence of weaker gravity is the more powerful of the two, causing the satellite clocks to gain a net total of about 38 microseconds every day compared to clocks on the ground. While that sounds insignificant, GPS works by measuring the time it takes for a signal to travel from a satellite to your receiver. A tiny timing error translates into a massive location error. That 38-microsecond daily drift would cause navigational errors to accumulate rapidly, making your position inaccurate by about 10 kilometers after just one day.

To counteract this, engineers built a clever, pre-emptive fix directly into the system. The atomic clocks on board GPS satellites are manufactured to run slightly slower while on Earth. Once they are launched into their high-speed, lower-gravity orbit, the combined effects of relativity speed them up to the perfect rate. This constant, invisible correction is a remarkable, everyday proof of Einstein's theories and is the only reason GPS can guide you precisely to your destination.