Why Your GPS Needs 4 Satellites, Not 3

Your phone always knows where you are — by catching its own clock in a lie.

Three satellites should pin you down, right? So why does your phone always need a fourth? The answer isn't about position — it's about time.

Each satellite broadcasts when it sent its signal. Your phone times the delay, multiplies by light speed, and gets its distance. Three satellites, three spheres — they cross at you. But only if your phone's clock matches the satellites' atomic clocks — which it can't. A millionth-second error becomes 300 metres, the wrong building. So GPS adds a fourth satellite: four equations, four unknowns — latitude, longitude, altitude, and the clock error — solved at once, fixing your clock in real time.

The satellites must also handle Einstein's relativity. Moving fast and sitting high, their clocks run faster than ours — about 38 microseconds a day. Without that correction, GPS would drift nearly 10 kilometres a day. Einstein didn't just change physics — he made Google Maps possible.

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