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24 GPS satellites orbit the earth at an altitude of 12000 kilometers above the ground in a 12 hour cycle, so that more than four satellites can be observed at any time and at any point on the ground at any time.

Because the position of the satellite is accurate, in GPS observation, we can get the interval from the satellite to the receiver. Using the interval formula in three-dimensional coordinates and using three satellites, we can form three equations to solve the position of the observation point (x, y, z). Considering the error between the satellite clock and the receiver clock, there are four unknowns in practice, x, y, Z and clock difference. Therefore, it is necessary to introduce the fourth satellite to form four equations for solution, so as to obtain the longitude, latitude and elevation of the observation point.

In fact, the receiver can often lock more than four satellites. At this time, the receiver can distribute several groups of four satellites in each group according to the constellation of the satellite, and then select the group with the smallest error through the algorithm for positioning, so as to improve the accuracy.

Because of errors in satellite trajectory and satellite clock, the influence of atmospheric troposphere and ionosphere on signals, and man-made SA maintenance policies, the positioning accuracy of civil GPS is only 100 meters. In order to improve positioning accuracy, differential GPS (DGPS) skills are generally used to establish reference stations (differential stations) For GPS observation, use the known accurate coordinates of the reference station to compare with the observed values, so as to obtain a correction number and release it to the public. After receiving the correction number, the receiver compares it with its own observed values, eliminates most errors and obtains a more accurate position. The experiment shows that the positioning accuracy can be improved to 5M by using differential GPS.

The vehicle navigation system is mainly composed of navigation host and navigation display terminal. The built-in GPS antenna will receive the data information transmitted by at least 3 of the 24 GPS satellites circling the earth, so as to determine the current position of the car. The position coordinates determined by the navigation host through the GPS satellite signal can be matched with the electronic map data to determine the position of the car in the electronic map The exact location in the map.

On this basis, various functions such as driving navigation, road introduction, information query and AV / TV broadcasting will be realized. The driver only needs to watch the screen on the display, listen to voice prompts, and operate the remote control in his hand to realize the above functions, so as to drive the car easily and freely.

Article fromJiangmen vehicle navigatorhttp://cnjhl.cn