How is GPS positioned?

　　GPS global positioning system is a high-precision radio navigation positioning system based on air satellites. It can provide more accurate geographical location, vehicle speed and accurate time information anywhere in the world and in near-Earth space. In a simplified single-point positioning model, GPS positioning is actually to determine the position of the GPS receiver through four satellites with known positions.

　　As shown in the figure, the position of the four satellites and the distance from the GPS receiver are very easy to obtain. Each GPS satellite operating in the space constantly broadcasts its current position coordinate information and time information to the world through satellite signals. Any GPS receiver can easily receive these information through the antenna.

　　After the GPS receiver receives the data packet, subtract the time on the timestamp from the current time, which is the time used for the data packet to be transmitted in the air. Multiply the transmission speed to get the distance of the data packet to be transmitted in the air, which is the distance between the satellite and the GPS receiver. It is required to solve the position of GPS receiver in three-dimensional space. Mathematically, it only needs the position of three satellites and the distance between them and GPS receiver (three unknowns need to construct three equations). In the GPS positioning system, the position information of four satellites is used for correction to eliminate errors.

　　There are many factors that affect the GPS positioning accuracy. According to the different processes in the above model, there can be satellite ephemeris error, satellite clock error and satellite signal transmission antenna phase center error, etc.

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