Every GPS position fix is secretly a time measurement. Satellites about 20,200 km overhead broadcast their location and the precise atomic-clock time of transmission; your phone measures how long each signal took to arrive and converts delay into distance — light covers about 30 centimetres per nanosecond. Intersect distances from four satellites and you have latitude, longitude, altitude, and, as a bonus, the exact time. Positioning to a few metres therefore demands timing to a few nanoseconds.
Last updated: July 22, 2026
At that precision, Einstein stops being philosophy and becomes engineering. Special relativity says moving clocks tick slowly: the satellites orbit at about 14,000 km/h, which slows their clocks by roughly 7 microseconds per day relative to the ground. General relativity says clocks deeper in a gravitational field tick slowly: at 20,200 km up, the satellites sit in weaker gravity than we do, which speeds their clocks by about 45 microseconds per day. The effects oppose and don’t cancel — the net result is satellite clocks running about 38 microseconds per day fast.
Thirty-eight microseconds sounds negligible until you multiply by the speed of light: it corresponds to more than 11 kilometres of ranging error, accumulating every single day. A GPS that ignored relativity would drift from usable to useless between breakfast and dinner. So the system doesn’t ignore it: satellite clocks are deliberately tuned before launch to tick slightly slow, so that in orbit, relativity speeds them into agreement with Earth-bound time, and receivers apply further corrections for each satellite’s slightly elliptical orbit.
The satellites carry the corrections in hardware and broadcast time so reliably that GPS has become civilization’s free clock as much as its map. Power grids synchronize their phase measurements with it, financial exchanges timestamp trades by it, and cell networks coordinate handoffs with it — which is why GPS jamming worries engineers far beyond navigation.
it’s a satisfying inversion: theories about the strangeness of time, published in 1905 and 1915 as pure physics, now quietly correct the clock in every pocket. The synchronized result — atomic time, relativity included — is what ultimately feeds the live clocks at ellatime.com.
GPS is no longer alone up there. Russia’s GLONASS, Europe’s Galileo, and China’s BeiDou each fly their own atomic clocks with their own relativistic corrections, and a modern phone listens to all of them at once, often tracking dozens of satellites. Because a receiver solves for time as its fourth unknown alongside the three of position, every fix delivers a by-product almost nobody notices: your phone momentarily knows the time to within nanoseconds — atomic precision, free, from space — before it settles back to displaying the humble minute.
The dependence runs deep enough that governments treat timing as critical infrastructure. GPS-denied environments — tunnels, jammed battlefields, spoofed harbors — have pushed development of chip-scale atomic clocks small enough for a circuit board, holding time steady for hours without satellites. Financial regulators in the US and EU now mandate traceable timestamps to within specified fractions of a second, and exchanges buy time feeds anchored to national laboratories precisely so a satellite outage can’t scramble the sequence of the world’s trades.
Related on Ellatime: How Atomic Clocks Work: The Machines That Define the Second · NTP: How Your Phone Knows the Exact Time · What Is a Leap Second — and Why It's Being Retired
Further reading: GPS.gov — official U.S. GPS information
Editorial Note: This article was researched and written by the Ellatime editorial team. We fact-check our content and update it regularly. For questions or corrections, contact us.
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