How GPS uses time: turning nanoseconds into your location
GPS works by measuring time. Each satellite carries atomic clocks and broadcasts exactly when its signal left. Your phone measures when the signals arrive; because radio travels at the speed of light, travel time × speed of light = distance. With signals from at least four satellites, the receiver works out where it is and what time it is. That is why GPS is also the world's biggest clock-distribution system.
Time becomes distance
GPS satellites orbit about 20,200 km up, so a signal takes roughly 67 to 86 milliseconds to reach you, depending on whether the satellite is overhead or near the horizon. Since light covers about 300,000 km per second, tiny timing errors become distance errors:
| Timing error | Distance light travels |
|---|---|
| 1 millisecond | about 300 km |
| 1 microsecond | about 300 m |
| 100 nanoseconds | about 30 m |
| 10 nanoseconds | about 3 m |
| 1 nanosecond | about 30 cm |
Try it: time error → distance
These are range errors along one line of sight; the final position error also depends on how the satellites are spread across the sky.
Why it takes four satellites
One distance puts you somewhere on a huge sphere around a satellite; more satellites narrow it down. But your phone's clock is an ordinary quartz clock, not an atomic one, so it doesn't know the exact time. That makes four unknowns: three for position (x, y, z) and one for your clock's error. Four satellites give four equations, so the receiver solves for its position and the correct time at once. Extra satellites improve accuracy and help spot bad signals.
Einstein makes GPS work
GPS satellites move fast and sit higher in Earth's gravity, and both change how fast their clocks tick compared with clocks on the ground:
| Effect | Satellite clock vs. ground, per day |
|---|---|
| Weaker gravity (general relativity) | about +45 microseconds (faster) |
| Orbital speed (special relativity) | about −7 microseconds (slower) |
| Net | about +38 microseconds per day |
Uncorrected, 38 microseconds a day of clock error would add up to roughly 11 km of ranging error every day. So the satellite clocks are deliberately tuned slightly slow before launch, and receivers apply a further small correction for each satellite's elliptical orbit. See relativity and time dilation.
GPS time is not UTC
GPS keeps its own continuous time scale that began on January 6, 1980 and never adds leap seconds, so it is now 18 seconds ahead of UTC. The satellites broadcast that offset so receivers can show normal UTC and local time. GPS counts time in weeks and seconds; see the live GPS time and week number. Ground control (run by the US Space Force) monitors every satellite's clock and orbit and uploads corrections.
How accurate is it?
- Time: the US government commits to GPS time transfer within 30 nanoseconds, 95% of the time.
- Position: a GPS-enabled smartphone is typically accurate to about 4.9 m (16 ft) under open sky; buildings, trees and reflections make it worse.
- Surveying: professional receivers using the radio carrier wave and correction stations reach centimeter accuracy.
Who uses GPS just for the time
Many systems don't care where they are; they need the exact time. GPS timing receivers help synchronize:
- Cell networks, so base stations transmit in step.
- Power grids, where devices called phasor measurement units time-stamp readings across hundreds of miles.
- Financial markets and data centers, which time-stamp transactions and logs; network protocols such as NTP and PTP spread the time from a GPS-fed master clock. (This site's clocks use internet time, the same idea; see what is NTP.)
- Aviation: see how time runs air travel.
When GPS goes wrong
GPS signals are very weak by the time they reach the ground, so they can be jammed (drowned out) or spoofed (faked). Spoofing is dangerous for timing because a receiver can keep reporting a confident but wrong time. Critical systems therefore keep a good local clock that can "hold over" during an outage, cross-check several sources, and use more than one satellite system. In April 2019, some older devices showed wrong dates after the GPS week rollover; the next is in November 2038.
Not just GPS
GPS is the US system. Europe runs Galileo, Russia GLONASS and China BeiDou; together they're called GNSS. Most modern phones use several at once, each with its own time scale that receivers convert between.
Questions people ask
How does GPS use time?
Satellites broadcast precisely timed signals from atomic clocks. A receiver measures how long each signal took to arrive, multiplies by the speed of light to get distances, and combines four or more to find its position and the exact time.
Why does GPS need relativity?
Satellite clocks gain about 38 microseconds a day relative to ground clocks (45 faster from weaker gravity, 7 slower from speed). Uncorrected, that would cause roughly 11 km of error per day.
Does my phone have an atomic clock?
No. It uses an ordinary quartz clock and solves for its clock error using a fourth satellite.
Is GPS time the same as UTC?
No. GPS time doesn't add leap seconds, so it is 18 seconds ahead of UTC. Receivers convert it.
How accurate is GPS time?
The US government commits to time transfer within 30 nanoseconds of UTC 95% of the time; specialized timing receivers often do better.
Sources
- GPS.gov: GPS performance (time transfer ≤30 ns, 95%)
- GPS.gov: GPS accuracy (smartphones about 4.9 m)
- GPS.gov: space segment
- GPS.gov: timing applications
- IERS Bulletin C 72 (July 2026): no leap second end of 2026
- Wikipedia: error analysis for GPS (relativity)
Last updated October 22, 2026. Found an error? Tell us.