GPS (the full name of the system is: NAVigation System with Timing And Ranging Global Positioning System, NAVSTAR-GPS) is intended for both civilian and military use. The civilian signal SPS (Standard Positioning Service) can be used freely by the general public, while the military signal PPS (Precise Positioning Service) is available only to authorized government agencies. The first satellite was placed in orbit on February 22, 1978, and it is planned to have up to 32 operational satellites orbiting the Earth at an altitude of 20,180 km on 6 different orbital planes. The orbits are inclined at 55° to the equator, ensuring that a least 4 satellites are in radio communication with any point on the planet. Each satellite orbits the Earth in approximately 12 hours and has four atomic clocks onboard.
During the development of the GPS system, particular emphasis was placed on the following three aspects:
1. It had to provide users with the capability of determining position, speed and time, whether in motion or at rest.
2. It had to have a continuous, global, all-weather 3-dimensional positioning capability with a high degree of accuracy.
3. It had to offer potential for civilian use.
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