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The GPS Navigation System provides specially coded satellite signals that can be processed in a GPS receiver, enabling the receiver to compute position, velocity and time. There are four GPS satellite signals used to compute positions in three dimensions and the time offset in the receiver clock. This explains the reliability of the information retrieved from this device.
The GPS Navigation System was designed by and is controlled by the United States Department of Defense. This can be used by anybody for free. It is divided into three segments: namely, the space, the control and the user. The space segment consists of the GPS satellite constellation of at least 24 satellites in an intermediate circular orbit (ICO). The control segment comprises ground stations around the world that are responsible for monitoring the flight paths of the GPS satellites, synchronizing the satellites' onboard atomic clocks, and uploading data for transmission by the satellites. The user segment consists of the GPS receivers used by both military and civilian.
The system is used by numerous civilians, who can use the GPS's Standard Positioning Service worldwide free of charge. A set is often combined in a bundle with a Personal Digital Assistants (PDA) likened to a palm pilot, a car computer or vehicle tracking system. The system is used as a navigation aid in airplanes, ships, cars, and even by pedestrians.
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