30+ Low-earth orbit satellites with known trajectories that broadcast really precise, accurate measurements of the time according to their on-board clock.
GPS Satellites orbit the earth 2x a day. They are not geostationary, which would put them too high up to perform their communications task. They are 12,000 miles up. There should be at least 4 satellites within view anywhere on the earth.
Each satellite has an on-board atomic clock. That’s how GPS Receivers are able to determine how far away they are from any given satellite they can detect. Using Trilateration and 3 satellites they can pinpoint your location to within 5m.
