# GPS principle four

- Apr 12, 2018-

GPS principle four

The basic principle of GPS positioning is to determine the position of the point to be measured by using the method of spatial distance resection, based on the satellite's instantaneous position of high-speed motion as known starting data. As shown in the figure, assuming that the GPS receiver is placed on the ground to be measured at time t, the time at which the GPS signal arrives at the receiver can be measured. With the addition of other data such as the satellite ephemeris received by the receiver, the following four can be determined. Equations:

The coordinates x, y, z and Vto of the point to be measured in the above four equations are unknown parameters, where di=cΔti (i=1, 2, 3, 4).

Di (i=1, 2, 3, 4) is the distance between satellite 1, satellite 2, satellite 3, and satellite 4 to receiver, respectively.

Δti (i=1, 2, 3, 4) is the time that the signals of satellite 1, satellite 2, satellite 3, and satellite 4 reach the receiver, respectively.

c is the speed of propagation of the GPS signal (ie, the speed of light).

The meaning of each parameter in the four equations is as follows:

x, y, z are the spatial rectangular coordinates of the coordinates of the point to be measured.

Xi, yi, zi (i=1, 2, 3, 4) are the spatial rectangular coordinates of satellite 1, satellite 2, satellite 3, and satellite 4 at time t.

It can be obtained from satellite navigation messages.

Vt i (i=1, 2, 3, 4) is the clock difference of the satellite clocks of satellite 1, satellite 2, satellite 3, and satellite 4, respectively, and is provided by satellite ephemeris.

Vto is the receiver's clock difference.

From the above four equations, the x, y, and z coordinates of the point to be measured and the clock difference Vto of the receiver can be solved.

In fact, the receiver can often lock more than four satellites. At this time, the receiver can be divided into several groups according to the satellite's constellation distribution, with 4 in each group. Then the algorithm is used to select the group with the smallest error for positioning. Improve accuracy.

Because of the errors in satellite orbits and satellite clocks, the influence of atmospheric troposphere and ionosphere on signals, and the artificial SA protection policy, the accuracy of civilian 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 station) for GPS observation, and the known reference station precise coordinates are used to compare with the observed values, thereby obtaining a correction number and publishing it. . After receiving the correction number, the receiver compares it with its own observation value, eliminating most of the errors and obtaining a more accurate position. Experiments show that using differential GPS, positioning accuracy can be improved to 5 meters.

The vehicle navigation system is mainly composed of two parts: a navigation host and a navigation display terminal. The built-in GPS antenna will receive data from at least 3 of the 24 GPS satellites orbiting the earth, thereby determining where the car is currently located. The position coordinates of the navigation host determined by the GPS satellite signals are matched with the electronic map data, and the exact position of the car in the electronic map can be determined.

Based on this, it will achieve navigation, route recommendations, information query, play AV / TV and other functions. The driver can realize the above functions by simply watching the screen on the monitor, listening to voice prompts, and manipulating the remote control in his hand, thereby allowing him to drive freely.