1. Space part The space part of GPS is composed of 24 working satellites, which are located 20,200km above the surface, evenly distributed on 6 orbital planes (4 in each orbital plane), and the orbital inclination is 55°. In addition, there are three active backup satellites in orbit. The distribution of satellites enables more than 4 satellites to be observed anywhere in the world and at any time, and navigation information can be pre-stored in the satellites. Due to atmospheric friction and other problems, the navigation accuracy of GPS satellites will gradually decrease over time. 2. Ground control system The ground control system consists of a monitoring station (Monitor Station), a main control station (Master Monitor Station), and a ground antenna (Ground Antenna). The main control station is located in Colorado Spring, Colorado, USA. The ground control station is responsible for collecting the information returned by the satellite, and calculating the satellite ephemeris, relative distance, atmospheric correction and other data. 3. User equipment part The user equipment part is the GPS signal receiver. Its main function is to capture the satellites to be tested selected according to a certain satellite cut-off angle, and to track the operation of these satellites. When the receiver captures the tracked satellite signal, it can measure the pseudo-distance from the receiving antenna to the satellite and the rate of change of the distance, and demodulate the satellite orbit parameters and other data. According to these data, the microprocessor computer in the receiver can carry out the positioning calculation according to the positioning solution method, and calculate the longitude, latitude, altitude, speed, time and other information of the user's geographic location. The receiver hardware and on-board software, as well as the post-processing software package for GPS data, constitute a complete GPS user equipment. The structure of the GPS receiver is divided into two parts: the antenna unit and the receiving unit. The receiver generally uses two kinds of DC power sources, inside and outside the machine. The purpose of setting the internal power supply is to replace the external power supply without interruption of continuous observation. The battery is automatically charged when the external power supply is used. After shutdown, the internal battery powers the RAM memory to prevent data loss. At present, various types of receivers are getting smaller and smaller in size and lighter in weight, which is convenient for field observation. The second is the user receiver. There are two kinds of single frequency and dual frequency. However, due to the price factor, the general users buy mostly single frequency receivers.

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