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About GPS

GPS is a space-based global radionavigation system which is operated by the U.S. Air Force. Two GPS services are provided. The Precise Positioning Service (PPS) provides full system accuracy primarily to U.S. and allied military users. The Standard Positioning Service (SPS) provides an accurate positioning capability for civil users throughout the world. The GPS has three major segments: space, control, and user.

The GPS Space Segment is composed of 24 satellites in six orbital planes. The satellites operate in circular 20,200 km (10,900 nm) orbits at an inclination angle of 55 degrees and with approximately a 12-hour period. A minimum of four satellites must be visible to a user in order to compute a three-dimensional position solution.

The GPS Control Segment has five monitor stations and three ground antennas with uplink capabilities. The monitor stations track all satellites in view. The information from the monitor stations is processed at the Master Control Station (MCS) to determine satellite clock and orbit states and to update the navigation message of each satellite. This updated information is transmitted to the satellites via the ground antennas.

The GPS User Segment consists of a variety of receiver types to provide positioning, velocity, and precise timing to the user. For more information on GPS see Links.