First of all, the GPS system is a passive system for our users, that is, the user's receiver is only responsible for receiving, decoding and computing, and will not return data. So as long as the signal covers the place, there can be countless users using the GPS system, which is similar to the principle of our radio station.

　　The standard configuration of GPS satellites is 24 satellites, but at present, the number of satellites in overtime service plus new satellites has actually exceeded the configuration of 24 satellites, and their operation orbits have been adjusted accordingly. The operation orbits of GPS satellites are distributed in 6 orbits, and the standard number of satellites in each orbit is 4. The time of orbit operation is approximately 12 hours (half a star day), which ensures that effective and stable navigation services can be provided in most places.

　　Navigation satellites are equipped with L1 and L2 band signals, and Block IIF satellite has started to add L5 band signals. We will give a brief introduction to the development of its satellites.

　　Block I satellite is the earliest satellite of the GPS system. The purpose is to verify the feasibility of the system design. Therefore, only 11 satellites were manufactured and launched between 1978 and 1985. However, because the satellite can only store 3.5 days of ephemeris, it needs to frequently contact the ground to upload new data. If there is no ground management, the satellite will change its attitude in a short time, which will affect the actual navigation and positioning requirements. Two rubidium/cesium atomic clocks are carried on the satellite. Although the design life is about 5 years, many satellites have worked more than twice as hard, leaving precious data for future satellite design and fault analysis.

　　The Block II satellite is a second generation satellite after learning from the previous generation of satellites and making improvements. A total of 9 satellites were launched in 1989. The built-in storage of the satellite has been increased to 14 days of ephemeris, and independent momentum control has been achieved. This can reduce the frequent contact with ground control, and multiple rubidium/cesium atomic clocks are carried inside. The average design life is 6 years, but the actual working time of the satellite is about 12 years on average. This is also the initial satellite product of GPS system.

　　Later, Block IIA satellite was added, which was very similar to the previous generation satellite, and enhanced some functions of the system. For example, the autonomous operation time was expanded to 180 days, and the satellite could work continuously for 6 months without contact with the ground. A total of 19 were produced.

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