The development of GPS system

　　GPS(also known as global navigation satellite system or global positioning system) the predecessor of the system for the United States military development of a meridian satellite positioning system (Transit), 1958 development, 1964 officially put into use. The system works with a network of five to six satellites that circumnavigate the Earth up to 13 times a day, and it does not give altitude information and is not very accurate in positioning. However, the meridional instrument system makes the R&D department get the preliminary experience of 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 superiority in navigation and meridian system has great defects in submarine and ship navigation. The U.S. Navy, army, air force and civilian sectors all feel the urgent need for a new satellite navigation system. To this end, the United States Naval Research Laboratory (NRL) proposed Tinmation with 12 to 18 satellites composed of 10,000 km high global positioning network plan, and in 67 years, 69 and 74 years each launched a test satellite, in these satellites on the initial test atomic clock timing system, which is the basis of GPS system accurate positioning. The U.S. Air Force proposed 621-B's plan of three to four clusters of four to five satellites per cluster. All but one of these satellites were in synchronous orbit, and all of them were in tilted orbit with a period of 24 hours. The plan was based on pseudo-random code (PRN) to transmit satellite ranging signals. Signal density is also detected when it is less than 1% of the ambient noise. The successful application of pseudorandom code is an important basis for the success of GPS system. The Navy's plan is designed to provide low-dynamic, 2-D positioning for ships, while the Air Force's plan is designed to provide high-dynamic services, but the system is too complex. Because the two systems at the same time development will cause huge cost, and here two plans are designed to provide global positioning, so in 1973 the United States department of defense will close 2 for one, the two joint projects agency (darpa), led by the Ministry of National Defense and satellite navigation and positioning (JPO) leadership, will also air force space agencies set up in Los Angeles. The agency has a large membership, including representatives of the U.S. Army, Navy, Marine Corps, Department of Transportation, Defense Mapping Agency, NATO and Australia.

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