The previous generation of GPS is the "Meridian" navigation satellite developed by the US Navy in 1964, which is a low-orbit satellite. The main purpose is for the navigation of nuclear submarines and surface ships, and also for geodetic surveying. Its most important function is to provide precise positioning for the Polaris nuclear missile to launch a nuclear strike quickly. From April 1960 to the early 1980s, more than 30 satellites were launched. The first is Meridian 1B, which is used to test and identify the navigation satellite scheme and its key technologies, and to verify the principle of dual-frequency Doppler speed measurement, positioning and navigation, and the results prove that satellite navigation is feasible.

　　The first practical navigation satellite Meridian 5B-2 was launched in December 1963; the first finalized navigation satellite Meridian 5C-1 was launched in June 1964 and delivered to the Navy; July 1967 Meridian navigation satellite The networking is practical and allows civilian use. In 1972, the Meridian Instrument Improvement Program (TIP) was implemented. A total of 3 satellites were launched. The main test was the disturbance compensation system, which compensated for the orbital perturbation caused by atmospheric drag and solar radiation pressure in real time, which greatly improved the accuracy of orbit prediction. Drag satellites. In May 1981, the improved and practical Meridian satellite was launched, renamed NOVA.

　　In order to solve the many problems of the "Meridian", the US Department of Defense invested 10 billion US dollars to develop a new satellite navigation system in the 1970s, that is, the GPS that we are familiar with. GPS was highly valued at that time as a part of "Star Wars". After the United States abandoned the Star Wars program in the 1980s, GPS still survived and developed.

　　went through several stages

　　The first stage is the program demonstration and preliminary design stage. From 1973 to 1979, a total of 4 test satellites were launched. Developed ground receiver and established ground tracking network

　　The second stage is the comprehensive development and testing stage. From 1979 to 1984, seven test satellites were launched successively, and various receivers were developed. Experiments show that GPS positioning accuracy far exceeds design standards

　　The third stage is the practical networking stage. On February 4, 1989, the first GPS working satellite was successfully launched, indicating that the GPS system has entered the engineering construction stage. At the end of 1993, a practical GPS network (21+3) GPS constellation had been built, and the failed satellites will be replaced according to the plan in the future.

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