Daniel uses GPS every day. He has never paid for it. And he just realized that someone must be paying for all those satellites.

Someone is. The US government spends around two million dollars a day maintaining the GPS system. And you use it for free. Every time you ask your phone for directions, check your location, or track a delivery -- you are using a network of about thirty satellites orbiting twenty thousand kilometers above Earth, owned and operated by the US military, and broadcasting continuously without ever charging you a cent.

Here is how it actually works.

Each GPS satellite broadcasts two pieces of information -- who it is, and exactly what time it is. Your phone picks up signals from several satellites at once. Because each signal takes a slightly different amount of time to reach your phone depending on distance,  ...