GPS positioning principle

　　The GPS receiver can receive time information accurate to nanoseconds that can be used for time service; The forecast ephemeris used to forecast the approximate position of the satellite in the coming months; The broadcast ephemeris used to calculate the satellite coordinates required for positioning, with an accuracy of several meters to tens of meters (different satellites, changing at any time); And GPS system information, such as satellite status.

　　The distance from the satellite to the receiver can be obtained by measuring the code of the GPS receiver. Because of the error of the receiver's satellite clock and atmospheric propagation error

　　Poor, so it is called pseudo distance. The pseudo range measured for 0A code is called UA code pseudo range with an accuracy of about 20 meters, and the pseudo range measured for P code is called P code pseudo range with an accuracy of about 2 meters.

　　The GPS receiver decodes the received satellite signals or uses other technologies to remove the information modulated on the carrier wave

　　Recover the carrier. Strictly speaking, the carrier phase should be called the carrier beat phase, which is the difference between the carrier phase of the received satellite signal affected by Doppler frequency shift and the signal phase generated by the local oscillation of the receiver. Generally, the phase change value can be recorded by measuring at the epoch time determined by the receiver clock and keeping track of the satellite signal. However, the initial phase value of the receiver and satellite oscillator at the beginning of the observation is unknown, and the phase integer of the initial epoch is also unknown, that is, the integer ambiguity can only be calculated as a parameter in data processing. The accuracy of the phase observation value is as high as mm, but the premise is to solve the ambiguity of the whole cycle. Therefore, the phase observation value can only be used when there is a relative positioning and a continuous observation value, and the phase observation value can only be used to achieve the positioning accuracy better than the meter level.

　　According to the positioning mode, GPS positioning is divided into single point positioning and relative positioning (differential positioning). Single point positioning is a way to determine the position of a receiver based on the observation data of a receiver. It can only use pseudo range observation measurement, which can be used for rough navigation and positioning of vehicles and ships. Relative positioning (differential positioning) is a method to determine the relative position between observation points based on the observation data of more than two receivers. It can use either pseudo range observation or phase observation. Geodetic or engineering measurements should use phase observation values for relative positioning.

　　GPS observation includes errors such as clock error of satellite and receiver, atmospheric propagation delay, multi-path effect, etc. During positioning calculation, it is also affected by satellite broadcast ephemeris error. Most of the common errors are offset or weakened during relative positioning, so the positioning accuracy will be greatly improved. Dual frequency receiver can offset the main part of ionospheric error in the atmosphere according to the observation of two frequencies, with high accuracy requirements, When the distance between receivers is far (the atmosphere is obviously different), the dual frequency receiver shall be selected.

　　During positioning observation, if the receiver moves relative to the earth surface, it is called dynamic positioning, such as pseudo range single point positioning with an accuracy of 30-100 meters for rough navigation and positioning of vehicles and ships, pseudo range differential positioning with a precision of meters for urban vehicle navigation and positioning, or centimeter phase differential positioning (RTK) for surveying and setting out, Real time differential positioning requires the data link to transmit the observation data of two or more stations in real time for calculation. During positioning observation, if the receiver is stationary relative to the earth surface, it is called static positioning. During control network observation, it is generally observed by several receivers at the same time in this way, which can maximize the GPS positioning accuracy. The receiver dedicated to this purpose is called geodetic receiver, which is the best type of receiver. At present, GPS has been able to meet the accuracy requirements of crustal deformation observation, and IGS perennial observation stations have been able to form a millimeter scale global coordinate frame.

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