Global Positioning System (GPS), space-based radio navigation system, consisting of 24 satellites and ground support, that provides accurate, three-dimensional position, velocity, and time, 24 hours a day, everywhere in the world, and in all weather conditions. Because the user does not communicate to the satellite, GPS serves an unlimited number of users. GPS is available in two basic forms: Standard Positioning Service (SPS) and Precise Positioning Service (PPS). SPS provides a horizontal position that is accurate to 100 m (109 yds). PPS horizontal accuracy is 20 m (22 yds). For authorized users, usually the US military and its allies, PPS also provides greater resistance to jamming and immunity to deceptive signals (antispoofing). . HOW GPS WORKS GPS Satellite A total of 24 US Global Positioning System (GPS) satellites orbit overhead and provide accurate positioning and navigation information for both military and civilian use. Solar cells power each satellite and its atomic clocks. Antennas on a satellite continuously transmit timing information from the clocks. The signals can be picked up and processed by a GPS receiver to determine exact location and altitude.The NAVSTAR Global Positioning System Joint Program Office GPS satellites carry atomic clocks that measure time to a high degree of accuracy. The time information is placed in the codes broadcast by the satellite so that a receiver can continuously determine the time the signal was broadcast. The signal contains data that a receiver uses to compute the locations of the satellites and to make other adjustments needed for accurate positioning. The receiver uses the time difference between the time of signal reception and the broadcast time to compute the range to the satellite. The receiver must account for propagation delays caused by the ionosphere and the troposphere. With three ranges to three satellites and knowing the location of the satellite when the signal was sent, the receiver can compute its three-dimensional position.