GPS, Global Positioning System, is a system of navigation satellites circling Earth. they constantly send out signals and a GPS receiver in your phone listens for these signals. Once the receiver calculates its distance from four or more GPS satellites, it can figure out exactly where you are. The receiver uses four satellites to calculate latitude, longitude, altitude, and time. The basic GPS service provides all the users with approximately 7 meter accuracy 95 % of the time, on or near the surface of the earth. These constellations are called Global Navigation Satellite Systems (GNSS). All providers such as China and Russia have offered free use of their respective systems to the international community.