How does GPS know where you are?

GPS knows your location by timing radio messages from satellites. Your phone measures how long each signal took to arrive, turns that time into distance, and compares distances from at least four satellites. The overlap of those distances gives your spot on Earth.

The clever part is the clock check. Satellite clocks are extremely precise, but your phone's clock is not nearly as exact. A fourth satellite helps fix that tiny timing mistake, because even a blink-small error can move a map point far down the road.