Why does GPS need Einstein’s relativity?
Accurate navigation depends on clocks that tick differently in orbit and on the ground.
Finding a location from time
A GPS receiver estimates its position by comparing the arrival times of signals from multiple satellites. Because radio signals move at the speed of light, very small timing errors translate into noticeable errors in distance. Accurate satellite clocks are therefore essential, not optional accessories.
Two effects pull in different directions
Special relativity predicts that a moving satellite clock runs slightly slower than a clock at rest in a comparable reference frame. General relativity predicts that a clock farther from Earth’s gravitational field runs faster than one on the surface. For GPS satellites, the gravitational effect is larger, so the combined correction is approximately 38 microseconds per day relative to clocks on Earth.
Theory becomes infrastructure
The GPS system accounts for these predictable differences so its timing remains useful. Without appropriate corrections, position estimates would quickly drift by kilometres. Relativity is sometimes introduced as abstract physics, but satellite navigation shows how it operates inside technology used by drivers, ships, aircraft and phones.
External references are provided for exploration; Curio Hour’s explanation is written separately.