GPS Isn't a Map. It's a Clock.

GPS was never really a map. It's the most precise clock most of us will ever use, and phone networks, the power grid and stock trades all run on its time. We follow one time signal from orbit to the cell tower down your street: how a clock finds you, why Einstein is built into every satellite, what really happens if GPS goes quiet, and why the most valuable thing it has ever done is tell the time.