How Your Phone Finds You Using CLOCKS 20,000 km in Space

Your phone can pin your location to a few meters, without ever sending a signal to space.

So if your phone only listens and never talks back, how do satellites that far away find you?

Each GPS satellite, roughly 20,000 kilometers up, carries an atomic clock and constantly broadcasts the exact time. Your phone catches these signals, which travel at the speed of light. By comparing the time stamped in the signal with the moment it arrived, your phone works out how far each satellite is. One distance places you on a huge sphere. Overlap spheres from at least four satellites and they cross at one point: you. This is trilateration, not triangulation: it uses distances, not angles. The fourth satellite fixes your phone's cheap clock.

Here is the strange part: this only works because clocks up there tick in a tiny warp. Gravity is weaker in orbit and the satellites move fast, so their clocks drift. Ignore Einstein and your map would drift kilometers within a day.

Follow The Explainer Vault — because everything has an explanation.