Space part

　　The space part of GPS is composed of 21 working satellites, which are located 20 200 km above the earth's surface, evenly distributed on 6 orbital planes (4 for each orbital plane), and the orbital inclination is 55 °. In addition, there are three active backup satellites in orbit. The distribution of satellites makes it possible to observe more than 4 satellites anywhere and at any time in the world, and the navigation information can be stored in the satellites. The navigation accuracy of GPS satellites will gradually decrease with the passage of time due to atmospheric friction and other problems [1].

　　Ground control system

　　The ground control system consists of a monitor station, a master monitor station, and a ground antenna. The main control station is located in Colorado Spring, USA. The ground control station is responsible for collecting the information transmitted from the satellite, and calculating the satellite ephemeris, relative distance, atmospheric correction and other data.

　　User equipment

　　The user equipment part is the GPS signal receiver. Its main function is to capture the satellites to be measured selected according to a certain cut-off angle of satellites, and track the operation of these satellites. After the receiver captures the tracked satellite signal, it can measure the pseudo distance from the receiving antenna to the satellite and the change rate of the distance, and demodulate the satellite orbit parameters and other data. Based on these data, the microprocessor in the receiver can calculate the location according to the location solution method, and calculate the longitude and latitude, height, speed, time and other information of the user's geographic location. The receiver hardware, onboard software and post-processing software package of GPS data constitute a complete GPS user equipment. The structure of GPS receiver is divided into antenna unit and receiving unit. Generally, the receiver adopts two kinds of DC power supply: internal and external. The purpose of setting the internal power supply is to replace the external power supply without interrupting continuous observation. The battery will automatically charge when using the external power supply. After shutdown, the internal battery supplies power to RAM memory to prevent data loss. At present, various types of receivers are becoming smaller and lighter, which is convenient for field observation. The second is the user receiver. There are two kinds of single frequency and dual frequency receivers. However, due to the price factor, most users buy single frequency receivers.

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