1. Space

　　The space part of GPS is composed of 24 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 four 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 maintain a geometric image with good positioning accuracy. This provides continuous global navigation capability in time. GPS satellite generates two groups of code, one is called C/A code (Coarse/Acquisition Code 11023MHz); One group is called Procise Code 10123MHz. Because of its high frequency, low interference and high positioning accuracy, the P code is under the control of the US military and has a password that cannot be interpreted by ordinary people. It mainly serves the US military. The C/A code is mainly open to civilian use after the precision is deliberately reduced by taking measures.

　　2. Ground control part

　　The ground control part consists of one master control station, five global monitoring stations and three ground control stations. The monitoring stations are equipped with precise cesium clocks and receivers capable of continuously measuring all visible satellites. The monitoring station will transmit the satellite observation data, including ionospheric and meteorological data, to the main control station after preliminary processing. The master control station collects tracking data from each monitoring station, calculates satellite orbit and clock parameters, and then sends the results to three ground control stations. The ground control station injects these navigation data and master control station commands into the satellite when each satellite moves overhead. This injection is carried out once a day for each GPS satellite, and the final injection is carried out before the satellite leaves the scope of the injection station. If a ground station fails, the navigation information stored in the satellite will be available for some time, but the navigation accuracy will gradually decrease.

　　3. 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 range from the receiving antenna to the satellite and the rate of change of the range, 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, all types of receivers are becoming smaller and lighter, which is convenient for field observation

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