Global Positioning System
A GPS unit takes radio signals from satellites in space in orbit around the Earth. There are 31 satellites which are  above the Earth. The orbital period is 11 hours and 58 minutes. Each circle is  radius due to the Earth's radius. Far from the North Pole and South Pole, a GPS unit can receive signals from 6 to 12 satellites at once. Each satellite contains an atomic clock which is carefully set by NORAD several times every day.