Vehicle navigation and positioning principle

GPS consists of 24 working satellites, which are located above 20200km from the earth's surface and evenly distributed on six orbital planes (4 satellites per orbital plane), with an orbital inclination of 55. In addition, there are four active backup satellites in orbit. The distribution of satellites makes it possible to observe more than four satellites anywhere in the world at any time, and maintain a geometric image with good positioning accuracy. This provides a global navigation capability that is continuous in time.

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.