How do GPS satellites calculate your location?

Each GPS satellite carries an atomic clock and broadcasts precise timing signals from orbit about 20,200 km above Earth. Your receiver compares the arrival times of signals from several satellites and uses the differences to estimate distances to each one. With signals from at least four satellites, it can solve for latitude, longitude, altitude, and clock error. The system was developed by the U.S. military and became fully operational in 1995. Accuracy can be improved to within a few metres for civilian users and even better with augmentation systems.