Implementation of GPS plan

　　The initial GPS plan was born under the leadership of the Joint Planning Bureau, which placed 24 satellites in three orbits at an angle of 120 degrees to each other. There are 8 satellites in each orbit, and 6 to 9 satellites can be observed at any point on Earth. In this way, the coarse code accuracy can reach 100m and the fine code accuracy can reach 10m. Due to budget compression, the GPS plan had to reduce the number of satellite launches and instead distribute 18 satellites in 6 orbits at 60 degrees apart. However, this plan makes the reliability of the satellite unreliable. In 1988, the last modification was made: 21 working stars and 3 backup stars were operating in 6 orbits at an angle of 30 degrees to each other. This is also the working method used by GPS satellites nowadays.

　　The implementation of the GPS plan is divided into three stages:

　　The first stage is the scheme argumentation and preliminary design stage.

　　From 1978 to 1979, four test satellites were launched using Gemini rockets from Vandenburg Air Force Base in California, with a long half axis of 26560km and an inclination of 64 degrees. The track height is 20000km. In this stage, we mainly developed a ground receiver and established a ground tracking network, with satisfactory results.

　　The second stage is the comprehensive development and testing phase.

　　From 1979 to 1984, seven experimental satellites called BLOCK I were launched and receivers for various purposes were developed. Experiments have shown that GPS positioning accuracy far exceeds design standards, and using coarse code positioning can achieve an accuracy of up to 14 meters.

　　The third stage is the practical networking stage.

　　On February 4, 1989, the first GPS working satellite was successfully launched, and the satellites in this stage were called BLOCK II and BLOCK IIA. This stage declares that the GPS system has entered the state of engineering construction. The GPS network used at the end of 1993, known as the (21+3) GPS constellation, has been completed, and in the future, faulty satellites will be replaced according to the plan.

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