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The GPS (Global Positioning System) is a satellite-based radio navigation system that provides its users with high precision position and time information over the entire globe. The system is owned and operated by the U.S. Department of Defense. GPS uses 24 satellites arranged in 6 orbits of four satellites each at an altitude of 20,200 km. The GPS satellites orbit the earth every 11 Hrs 58 Minutes. The GPS satellites send information of their position with respect to the center of the earth, together with a time signal. A GPS receiver uses the information from 4 satellites to calculate its location and the time.
There are two GPS services: the Standard Positioning Service (SPS) available free for all worldwide civilian users and the Precise Positioning Service (PPS) that is only available for the U.S. Department of Defence and authorized partners for security reasons. The Standard Positioning Service has an accuracy ranging from 10 to 15 meters. More precise accuracies are impossible with standard GPS, due to minor timing errors and satellite orbit errors, plus atmospheric conditions that affect the signals and their arrival time on Earth.
GPS uses the following frequencies:
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