24 GPS satellites operate at an altitude of 20,200 kilometers above the ground, making it possible to observe more than 4 satellites simultaneously at any point on the ground at any time.

　　Since the position of the satellites is accurate, we can obtain the distance from the satellite to the receiver in GPS observation. Using the distance formula in the three-dimensional coordinates and using three satellites, three equations can be formed to solve the position of the observation point (X , Y, Z). Considering the error between the satellite's clock and the receiver's clock, there are actually 4 unknowns, X, Y, Z and the clock difference, so it is necessary to introduce a fourth satellite to form 4 equations to solve, so as to obtain the observation point's Latitude and longitude and elevation.

　　In fact, the receiver can often lock more than 4 satellites. At this time, the receiver can be divided into several groups according to the constellation distribution of the satellites, each group has 4 satellites, and then the group with the smallest error is selected by the algorithm for positioning, thereby Improve accuracy.

　　Due to errors in satellite orbits and satellite clocks, the influence of the atmospheric troposphere and ionosphere on signals, and the artificial SA protection policy, the positioning accuracy of civilian GPS is only 100 meters. In order to improve the positioning accuracy, differential GPS (DGPS) technology is generally used to establish a base station (differential station) for GPS observation, and use the known precise coordinates of the base station to compare with the observed value to obtain a correction number and publish it to the outside world. . After receiving the correction number, the receiver compares it with its own observation value, eliminates most of the errors, and obtains a relatively accurate position. Experiments show that using differential GPS, the positioning accuracy can be improved to 5 meters

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