How GPS Finds You From 20,000 Kilometres Away

A satellite 20,000 kilometres above you knows where you're standing — within a few metres, using nothing but a clock.

No cable, no camera, no sensor — just a radio wave and a clock. So how does your phone turn that into a dot on a map?

Each satellite broadcasts the exact time it sent its signal, from an atomic clock accurate to a second in millions of years. That signal travels at the speed of light — 299,792,458 metres per second. Your phone measures the tiny delay, about 67 milliseconds, and multiplies by light speed for the distance. One satellite gives a sphere; two, a circle; three, two points; a fourth locks the exact spot — latitude, longitude, altitude. It's called trilateration.

Without Einstein's relativity, your GPS would be wrong by 11 kilometres a day. The satellites move so fast and so high that time runs faster for them — so the correction is baked into every fix. That's what stops your maps app sending you into a lake.

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