To understand the principle of positioning, first popularize what GPS is. GPS (Global Positioning System) is a new generation satellite navigation and positioning system developed by the United States in the 1970s and fully completed in 1994. It has the function of real-time three-dimensional navigation and positioning in all directions at sea, land, and air. The system consists of three parts: the space part (GPS satellite), the ground monitoring part, and the user part. There are a total of 24 satellites in the space section, of which 21 are working satellites and 3 are backup satellites, distributed in 6 orbits at an angle of 60 ° to each other. The distribution of satellites allows for the observation of more than 4 satellites anywhere and at any time in the world, and maintains a geometric shape with good positioning accuracy. This provides continuous global navigation capabilities over time.

　　Our mobile end belongs to the user part of the GPS system, and its built-in GPS module only has the receiving function, not the transmitting function. GPS satellites are equipped with high-precision cesium atomic clocks to ensure the accuracy of time synchronization. The satellite continuously transmits carrier signals to the Earth, which modulate broadcast ephemeris and timestamps representing the satellite's position, as well as C/A and P codes used for ranging. The C/A code is coarse and is generally used for civilian use, while the P code is fine and not easily interfered with. It is used for military use.

　　After the GPS module on the mobile phone receives the GPS signal, the time difference △ t can be obtained by comparing the timestamp in the navigation message with the current time in the device. The speed of electromagnetic waves v is the same as the speed of light propagation, and v multiplied by △ t to obtain the distance between the mobile phone and the GPS satellite (the receiver satellite clock has certain errors and atmospheric propagation errors, and the measured distance is also known as pseudorange), Each GPS terminal can simultaneously obtain navigation messages from at least 4 satellites. By using the method of spatial distance resection, we can calculate the three-dimensional position and other information of the mobile terminal, so that we can understand our current position on the mobile terminal.

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