What is GPS?

　　1. space part

　　The space part of GPS is composed of 24 working satellites [1], which are located 20,200 km above the surface, evenly distributed on 6 orbital planes (4 in each orbital plane), and the orbital inclination is 55°. In addition, there are 4 active backup satellites in orbit. The distribution of satellites makes it possible to observe more than 4 satellites anywhere in the world and at any time, and to maintain a geometric image with good positioning accuracy. This provides continuous global navigation capability in time. GPS satellites generate two sets of codes, one is called C/A code (Coarse/Acquisition Code 11023MHz); the other is called P code (Procise Code 10123MHz), P code is not easy to be interfered with due to its high frequency, and its positioning accuracy is high, so It is controlled by the US military and has a password that cannot be read by ordinary people. It mainly serves the US military. After the C/A code people deliberately reduce the accuracy in order to take measures, they are mainly open to private use.

　　2. ground control section

　　The ground control part consists of a main control station, 5 global monitoring stations and 3 ground control stations. The monitoring stations are equipped with sophisticated cesium clocks and receivers capable of continuously measuring all visible satellites. The satellite observation data obtained by the monitoring station, including ionospheric and meteorological data, will be transmitted to the main control station after preliminary processing. The main control station collects tracking data from each monitoring station, calculates the satellite's orbit and clock parameters, and then sends the results to three ground control stations. The ground control station injects the navigation data and the command of the main control station into the satellite when each satellite runs to the sky. This injection is performed once a day for each GPS satellite, and a final injection is made before the satellite leaves the range of the injection station. If a ground station fails, the navigation information pre-stored in the satellite can still be used for a period of time, but the navigation accuracy will gradually decrease.

　　3. User Equipment Section

　　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 perform 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.

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