Satellite signals in GPS satellite navigation can always be used free of charge. The abbreviation of global positioning system is a new generation of space satellite navigation and positioning system jointly developed by the United States Army, Navy and Air Force in 1970s. Its main purpose is to provide real-time, all-weather and global navigation services for land, sea and air, and it is also used for some military purposes such as intelligence gathering, nuclear explosion monitoring and emergency communication. It is an important part of America's global dominant strategy. After more than 20 years of research and experiments, it cost 30 billion dollars. By March 1994, 24 GPS satellite constellations with global coverage of 98% had been deployed. Principle: Twenty-four GPS satellites run at an altitude of 20,200 kilometers from the ground, so that more than four satellites can be observed at any time and at any point on the ground. Because the position of satellite is accurately known, we can get the distance from satellite to receiver in GPS observation. Using three-dimensional coordinates and the distance formula in three satellites, three equations can be formed to solve the position (x, y, z) of the observation point. Considering the error between satellite clock and receiver clock, there are actually four unknowns, X, Y, Z and clock difference, so it is necessary to introduce a fourth satellite to form four equations to solve, so as to get the latitude, longitude and elevation of the observation point. In fact, receivers can often lock on more than four satellites. At this time, the receiver can be divided into several groups with four satellites in each group according to the constellation distribution of satellites, and then the group with the smallest error can be selected by algorithm to locate, which improves the accuracy. Due to the error of satellite orbit and satellite clock, the influence of atmospheric troposphere and ionosphere on signals, and artificial SA protection policy, the positioning accuracy of civil GPS is only 100 meters. In order to improve the positioning accuracy, differential GPS(DGPS) technology is widely used to establish a reference station (differential substation) for GPS observation. By comparing the precise coordinates of the known reference station with the observed values, a correction number is obtained and released to the public. After receiving the correction number, the receiver compares it with its own observation value to eliminate most errors and get a more accurate position. Experiments show that the positioning accuracy can be improved to 5 meters by using differential GPS.