Principle analysis of RTK GPS improving positioning precision
GPS positioning principle:
The most basic principle of using GPS for positioning is that four satellites locate the three-dimensional coordinates of an object on the ground. The three-dimensional space information content of the subject is (X, Y, Z), and the indoor space position information includes three unknown variables. Why do four communication satellites need to be used for positioning? This is because the distance required for positioning is obtained by multiplying the rapid propagation of the data signal by the propagation time. Here, the rapid propagation is a clear value, and the propagation time is difficult to be absolutely unified because of the data signal between the receiver and the communication satellite. Therefore it is uncertain. In other words, in fact, the target object information should be four-dimensional information (X, Y, Z, t), so four satellites are needed for positioning. And because the data transmission process needs to pass through the troposphere and the ionosphere, the power propagation path of the signal does not follow the recent standard of the parallel line between two points, thus causing errors in the ionosphere and troposphere. This makes it impossible to accurately obtain the position information of the target object using only the four positioning communication satellites of GPS (the error is usually on the order of 10 meters).
The 10-meter level error obviously cannot meet the positioning requirements of modern society, so is there a way to reduce this type of positioning error? Obviously there are still some, this is the RTK technology that will be recommended next.
(Beitian full-frequency GNSS high-precision RTK antenna BT-800S)
RTK basic elements
The RTK positioning theory is a dynamic differential signal positioning technology based on high-precision carrier phase estimation. The base station first sends the carrier phase estimation and site coordinates obtained by itself to the surrounding dynamic users in real time through the data communication link. . The control module of the data processing method of the mobile station uses the method of dynamic differential signal positioning to determine the relative coordinates of the mobile station relative to the reference station, and then calculates its own instantaneous coordinates according to the coordinates of the reference station.
RTK composition: Base station: During a single measurement, the base station is fixed at a certain position.
Rover station: move with the point to be measured in real time
RTK transmission difference indication: base station: calculate the transmission error and inform the base station
Rover station: solve the error and calculate the distance from the base station to finally determine its own coordinates
RTK consists of a base station and a rover. Among them, the base station simply means that it will not move after it is established in one place, while the rover station will continue to move along with the point to be measured. The principle of the differential signal dynamic positioning technology is based on the spatial correlation between the base station and the rover.
Why use the differential signaling principle? We all know that the original GPS positioning system software, when the signal receiver receives the communication satellite signal, the positioning error is particularly large due to various reasons (such as distance, troposphere and ionosphere, digital clock, etc.), in order to eliminate this type of error, We first install a reference station on the known part of the ground. After receiving the signal, the reference station will start to make appropriate corrections, and compare the part data signal transmitted by the communication satellite with its own specific position information to find the positioning of the propagation process. Then, the error is notified to the rover through technical means, and then the rover adjusts the error based on the signal received in real time, so as to obtain its own precise position.
Many people think that there is also a long distance between the rover and the base station, so will it not cause errors again? In fact, GPS communication satellites are distributed at an altitude of more than 20,000 kilometers from the ground, and the distance between flight control points is from several kilometers to tens of kilometers. This distance is negligible relative to the distance from the satellite station to the ground.
RTK data link principle: place one receiver on the base station, and another or more receivers on the medium (called the rover), and the base station and the rover receive the same time and the same GPS communication satellite at the same time Signal, the estimated value obtained by the reference station is compared with the known position information, and the GPS differential signal is obtained just in time
Coordinate system conversion
When using RTK to determine the position, another important process is to do a good job of coordinate system conversion. The GPS positioning system software uses WGS-84 plane coordinates (the plane coordinates used by other positioning systems are different), but in reality In the application, different consumers usually create other plane coordinates based on objective reasons such as positioning precision, coordinate information confidentiality, and manipulation deformation, which involves the concept of coordinate system conversion.
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