GPS Runs on Relativity

Every satellite in the GPS constellation carries atomic clocks, and every one of them runs fast — by about 38 microseconds a day — relative to clocks on the ground. General relativity predicts it: clocks tick faster where gravity is weaker, and slower when you move quickly, and at GPS altitude the two effects do not cancel.

Thirty-eight microseconds sounds like nothing. But position is computed from time-of-flight at the speed of light, so an uncorrected day of drift is roughly ten kilometers of position error. Your phone finds a coffee shop because engineers took Einstein literally. Relativity is not an exotic theory at the edge of physics; it is a maintenance requirement.

Photo: U.S. Air Force, via Wikimedia Commons (Public domain).