The Global Positioning System (GPS) consists of a constellation of (currently about 32) satellites. Orbiting at an altitude of 20,000 km, GPS satellites have orbital period of 12 hours.
So how does GPS work?
Firstly, every GPS satellite continuously broadcast its current time and position. What a GPS receiver must do, is to monitor the time and position information of the GPS satellites in its sight, and use those information to calculate its position on Earth. At a minimum, four satellites must be in view of the receiver for it to compute four unknown quantities (three position coordinates and clock deviation from satellite time).
So these satellites are really like landmarks (or “skymarks”) planted in the sky. Just that to keep them from falling back to Earth, we have to arrange them to keep circling the Earth. 🙂