How does GPS develop

　　The predecessor of the GPS system is a "transit" navigation satellite system developed by the US military. It was developed in 1958 and officially put into use in 64. The system works with a satellite network composed of 5 to 6 satellites, bypassing the Earth 13 times at most every day, and cannot give altitude information, and its positioning accuracy is not satisfactory. However, the transit system enables the R&D department to gain preliminary experience in satellite positioning, and verifies the feasibility of positioning by satellite system, laying the groundwork for the development of GPS system. Because satellite positioning shows great advantages in navigation and the meridian system has great defects in submarine and ship navigation. The United States Navy, Land, Air Force and civilian sectors all feel an urgent need for a new satellite navigation system. To this end, the US Naval Research Laboratory (NRL) proposed a global positioning network plan called Tinmation, which uses 12 to 18 satellites to form a 10000 km height. In 67, 69 and 74, it launched a test satellite respectively. The atomic clock timing system was preliminarily tested on these satellites, which is the basis for the accurate positioning of the GPS system. The US Air Force has put forward the 621-B plan to form three to four star clusters with four to five satellites per star cluster. Except one of these satellites uses synchronous orbit, the rest of these satellites use inclined orbits with a period of 24h. The plan uses pseudo-random code (PRN) as the basis to spread satellite ranging signals. Its powerful function can also detect when the signal density is lower than 1% of the environmental noise. The successful application of pseudorandom code is an important basis for the success of GPS system. The naval plan is mainly used to provide low dynamic 2D positioning for ships, while the air force plan can provide high dynamic services, but the system is too complex. Because the simultaneous development of the two systems will cause huge costs and the two plans here are designed to provide global positioning, the U.S. Department of Defense combined the two into one in 1973, led by the Joint Satellite Navigation and Positioning Program (JPO) led by the Department of Defense, and set up its office in the Air Force and Space Office in Los Angeles. The agency has many members, including representatives of the United States Army, Navy, Marine Corps, Department of Transportation, National Defense Mapping Agency, NATO and Australia.

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Schematic diagram of GPS satellite navigation system