How does GPS work?

 

Global Positioning System satellites have a known position in a fixed orbit in space. These satellites then transmit signals to GPS receivers on the ground.

These transmitted signals depend on a very accurate time reference, which is achieved through the use of atomic clocks. Each GPS satellite has an atomic clock on board and transmits data that indicates its location and the current time. All GPS satellites synchronize operations so that these repeating signals are transmitted at the same instant. The signals, moving at the speed of light, arrive at a GPS receiver at slightly different times because some satellites are farther away than others. The distance to the GPS satellites can be determined by estimating the amount of time it takes for their signals to reach the receiver. When the receiver estimates the distance to at least four GPS satellites, it can calculate its position in three dimensions.

A GPS receiver "knows" the location of the satellites, because that information is included in satellite transmissions. By estimating how far away a satellite is, the receiver also "knows" it is located somewhere on the surface of an imaginary sphere centered at the satellite. It then determines the sizes of several spheres, one for each satellite. The receiver is located at the point in space, or in our case on earth, where these spheres intersect.