How GPS works

The GPS fixes its position on the ground by referring to satellites which orbit the earth in six precise orbits. By sending a signal to, and receiving a return message from those satellites, the GPS is able to fix its position by way of time and distance calculations.

A visual example of the GPS constellation in motion with the Earth rotating. Notice how the number of satellites in view from a given point on the Earth's surface, in this example at 45°N, changes with time.