The Global Positioning System, run by the United States Space Force, uses about thirty satellites about 20,200 km above the Earth. They are spread over six orbital planes tilted 55°, and each goes around twice a day. Every satellite carries atomic clocks and broadcasts the exact time and its own position. A receiver, such as the one in your phone, picks up the signals of at least four satellites, compares how long each took to arrive and works out where it is, often to within a few metres. Europe’s Galileo, Russia’s GLONASS and China’s BeiDou work the same way, and phones use them together.
Where are the GPS satellites?
Computing…
Questions and answers
How many GPS satellites are needed?
Four in view at once: three for the position and one more to correct the receiver’s clock. The constellation keeps about thirty so that enough are always above every place.
Why do GPS satellites need relativity?
Their clocks tick about 38 microseconds a day faster than on the ground (weaker gravity, high speed); without that correction, positions would drift by kilometres each day.
Where the numbers come from
Values refresh every second in your browser; the published data are checked every 10 minutes and updated several times a day.