Twenty-four GPS satellites orbit the Earth in a 12-hour cycle at an altitude of 12,000 kilometers above the ground. At any time, more than four satellites can be observed at any point on the ground at the same time.
Because of the precise position of the satellites, in GPS observations, the distance from the satellite to the receiver, using the distance formula in three-dimensional coordinates, using three satellites, can form three equations 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 four unknowns, X, Y, Z, and the clock difference. Therefore, the fourth satellite needs to be introduced to form four equations to solve the problem and obtain the observation point. Latitude and longitude and elevation.
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 the signals, the accuracy of civilian GPS positioning is only 10 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 issuing it to the public. . 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.
There are many ways to use GPS for positioning.
If the position of the reference point is different, the positioning method can be divided into:
(1) Absolute positioning. That is, in the protocol earth coordinate system, a receiver is used to determine the position of the point with respect to the center of mass of the protocol, which is also called single point positioning. Here, it can be considered that the reference point coincides with the protocol earth mass center. The protocol used for GPS positioning is the WGS-84 coordinate system. Therefore, the absolute result of the coordinates of the absolute positioning is the WGS-84 coordinate.
(2) Relative positioning. That is, in the protocol earth coordinate system, two or more receivers are used to measure the relative position between the observation point and a ground reference point (known point). That is, the increment of the coordinate from the ground reference point to the unknown point is determined. Since the ephemeris error and the atmospheric refraction error are related, the error can be eliminated by observing the difference between observations. Therefore, the relative positioning accuracy is much higher than the absolute positioning accuracy.
According to different motion states of the user receiver in the job, the positioning method can be divided into:
(1) Static positioning. That is, during the positioning process, the receiver is placed on the survey site and fixed. Strictly speaking, this static state is only relative, which usually means that there is no change in the position of the receiver relative to its surroundings.
(2) Dynamic positioning. That is, the receiver is in motion during positioning.
GPS absolute positioning and relative positioning also include static and dynamic. That is, dynamic absolute positioning, static absolute positioning, dynamic relative positioning, and static relative positioning. According to the principle of ranging, it can be divided into pseudo-range pseudorange method, pseudo-range pseudo-range method, differential localization.