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A woman with a phone on a hill above a city at dusk, with four lines of light connecting her phone to four satellites above the curved Earth
A woman with a phone on a hill above a city at dusk, with four lines of light connecting her phone to four satellites above the curved Earth
Technology

How does GPS know where you are?

About 30 GPS satellites circle the Earth, each constantly sending out its position and the exact time from an atomic clock. Your phone listens to at least four of them and measures how long each signal took to arrive. From the delays it works out how far away each satellite is, and there is only one spot that fits them all: where you are.

1Satellites

Satellites broadcast the time

Each GPS satellite carries an extremely precise atomic clock. It sends out a constant radio signal saying where it is and exactly what time it was when the signal left.

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2Distance

Delay becomes distance

The signals travel at the speed of light, but still take a tiny moment to arrive. Your phone compares the time in the signal with when it arrived. A delay of one thousandth of a second means the satellite is about 300 km away.

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3Position

Four distances, one place

Knowing your distance to one satellite puts you somewhere on a huge sphere. Several spheres only cross at one point. Three are needed for your position, and a fourth to correct your phone’s far less precise clock.

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The answer

GPS is a stopwatch in space.

Time signalsdelaysyour position

Your phone only listens, it never sends anything to the satellites. The clocks are so precise that the system must correct for Einstein’s relativity: without it, positions would drift by several kilometers a day.

Common follow-up questions

Does GPS need internet or mobile data?

No. The satellite signals are free and work anywhere with a view of the sky. Your phone often uses Wi-Fi and cell towers too, to find you faster and work better indoors.

How accurate is phone GPS?

Usually within about 5 meters outdoors. Tall buildings, trees and indoor spaces block or bounce the signals and make it less accurate.

Can someone track me through GPS?

The satellites can’t, because your phone only receives. But apps on your phone can share the location they calculate, which is why it’s worth checking app permissions.

Is GPS the only system?

No. GPS is American. Europe has Galileo, Russia GLONASS and China BeiDou, and most modern phones use several of them at once.

Next questionHow do self-driving cars work?

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