How do GPS satellites know where you are?

GPS works by using at least four satellites that constantly transmit precise time and orbit information. Your receiver compares the travel time of the radio signals from each satellite to estimate its distance from them. With distances from multiple satellites, it uses trilateration to work out latitude, longitude, altitude, and clock error. The system was developed by the U.S. military and became fully operational in 1995. Modern phones can also combine GPS with other navigation systems such as Galileo, GLONASS, and BeiDou for better accuracy.