24 GPS satellites orbit the Earth at an altitude of 12000 kilometers above the ground in a 12 hour cycle, allowing for simultaneous observation of more than 4 satellites at any point on the ground at any time.

　　Due to the precise positioning of satellites, in GPS observations, we can obtain the distance from the satellite to the receiver. Using the distance formula in three-dimensional coordinates and using three satellites, we can form three equations to solve the positions of observation points (X, Y, Z). Considering the error between the satellite clock and the receiver clock, there are actually four unknowns, X, Y, Z, and clock difference. Therefore, a fourth satellite needs to be introduced to form four equations for solving, in order to obtain the longitude, latitude, and elevation of the observation point.

　　In fact, receivers can often lock in more than 4 satellites. In this case, the receiver can be divided into several groups based on the constellation distribution of the satellites, with 4 satellites in each group. Then, the algorithm selects the group with the smallest error for positioning, thereby improving accuracy.

　　Due to the errors in satellite orbit and satellite clock, the influence of atmospheric troposphere and ionosphere on signals, and the artificial SA protection policy, the positioning accuracy of civil GPS is only 100 meters. In order to improve positioning accuracy, differential GPS (DGPS) technology is commonly used to establish a reference station (differential station) for GPS observation. The known precise coordinates of the reference station are compared with the observed values to obtain a correction number, which is then released to the public. After receiving the correction number, the receiver compares it with its own observation values to eliminate most of the errors and obtain a more accurate position. Experiments have shown that using differential GPS, the positioning accuracy can be improved to 5 meters.

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