The basic principle of GPS navigation system is to measure the distance between the known position of the satellite and the user's receiver, and then integrate the data of multiple satellites to know the specific position of the receiver. To achieve this, the position of the satellite can be found in the satellite ephemeris according to the time recorded by the onboard clock. The distance between the user and the satellite is obtained by recording the time the satellite signal travels to the user and multiplying it by the speed of light (due to the interference of the ionosphere of the atmosphere, this distance is not the real distance between the user and the satellite, but the pseudoran distance (PR) : When the GPS satellite is working properly, it will continuously send navigation messages using pseudo-random codes (pseudo-codes) composed of 1 and 0 binary symbols. There are two types of pseudocodes used in GPS systems, namely C/A code for civilian use and P(Y) code for military use. C/A code frequency 1.023MHz, repetition period of one millisecond, code spacing of 1 microsecond, equivalent to 300m; The P code frequency is 10.23MHz, the repetition period is 266.4 days, and the code spacing is 0.1 microseconds, which is equivalent to 30m. And Y code is formed on the basis of P code, the secrecy performance is better. Navigation messages include satellite ephemeris, working conditions, clock correction, ionospheric delay correction, atmospheric refraction correction and other information. It is modulated from the satellite signal and transmitted at 50b/s modulated on the carrier frequency. Each main frame of a navigation message contains 5 sub-frames, each 6s in length. The first three frames are 10 characters each; Repeat every 30 seconds and update every hour. The last two frames total 15000b. The content of navigation message mainly includes telemetry code, conversion code, data block 1, 2 and 3, among which the most important is ephemeris data. When the user receives the navigation message, the distance between the satellite and the user can be obtained by extracting the satellite time and comparing it with his own clock. Then the satellite ephemeris data in the navigation message can be used to calculate the position of the satellite when the message is sent, and the position and velocity of the user in the WGS-84 geodetic coordinate system can be obtained.

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