How GPS works: how your phone knows where you are

Before launch, every GPS satellite has its clock set a little slow, on purpose, because of Einstein. That is one of the surprises between the sky and the blue dot on your map. Follow the signal down: why your phone only listens and never answers, how it turns a time into a distance, why it takes four satellites, who tells each satellite where it is, what relativity changes, why assisted GPS finds you faster in town, and why the dot jumps around between tall buildings.