Global Positioning System (GPS)
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Global Positioning System (GPS) is a space-based positioning, velocity and time system, developed and operated by the U.S. Department of Defense and composed of space, control and user segments. The space segment is composed of 21 satellites (plus three operational spares) in six orbital planes. The control segment consists of five monitor stations, three ground antennas and a master control station. The user segment consists of antennas and receiver-processors that provide positioning, velocity, and precise timing to the user. The satellites broadcast two forms of clock information, the Coarse/Acquisition code, or C/A is freely available to the public, while the restricted Precise code, or P-code is usually reserved for military applications.
GALILEO is a similar satellite navigation system under development by the European Commission.
Global Orbiting Navigation System (GLONASS) is a similar radio-based satellite navigation system, developed by the former Soviet Union and now operated for the Russian government by the Russian Space Forces. It is the Russian counterpart of GPS.