GPS positioning technology

　　According to the geometric positioning principle of measuring navigation parameters, satellite navigation can be divided into angle measurement, time ranging, Doppler velocity measurement and combination method. Among them, the angle measurement method and combination method have no practical application due to low accuracy.

　　①Doppler speed measurement positioning: "Meridian" satellite navigation system adopts this method. The user positioning device measures the Doppler frequency shift curve according to the Doppler frequency shift between the frequency of the signal received from the navigation satellite and the frequency of the signal sent from the satellite, and the user's position can be calculated according to this curve and the satellite orbit parameters.

　　② Time ranging navigation and positioning: "Navigation Star" global positioning system adopts this system. The user receiving equipment accurately measures the propagation time of the signals sent by the four satellites in the system that are not in the same plane (to ensure unique results, the four satellites cannot be in the same plane), and then completes a set of model mathematical operations including four equations, The three-dimensional coordinates of the user's position and the error between the user's clock and the system time can be calculated.

　　The difference between the user's own geographic location coordinates measured by the navigation satellite and its real geographic location coordinates is called the positioning error, which is the most important performance index of the satellite navigation system. The positioning accuracy mainly depends on the orbit prediction accuracy, the navigation parameter measurement accuracy and its geometric amplification factor and the user dynamic characteristic measurement accuracy. The accuracy of orbit prediction is mainly affected by the Earth's gravitational field model and other orbital perturbations; the measurement accuracy of navigation parameters is mainly affected by the performance of satellites and user equipment, signal refraction in the ionosphere, troposphere and multipath and other error factors. Its geometric amplification factor is given by It is determined by the geometric relationship between the satellite and the user's position during positioning; the user's dynamic characteristic measurement accuracy refers to the user's heading, speed and antenna height measurement accuracy during positioning

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