The basic principle of GPS navigation system is to measure the distance between the satellite with known position and the user's receiver, and then integrate the data of multiple satellites to know the specific position of the receiver.

　　To achieve this goal, the position of the satellite can be found in the satellite ephemeris according to the time recorded by the on-board clock. The distance from the user to the satellite is obtained by recording the time that the satellite signal propagates to the user, and then multiplying it by the speed of light (due to the interference of the atmospheric ionosphere, this distance is not the real distance between the user and the satellite, but the pseudo range (PR): when the GPS satellite is working normally, it will continuously transmit navigation messages with pseudo-random code (PN code for short) composed of 1 and 0 binary symbols.

　　There are two kinds of pseudo codes used in GPS system, namely C/A code for civilian use and P (Y) code for military use. The C/A code frequency is 1.023MHz, the repetition period is 1ms, and the code spacing is 1ms, which is 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. Y code is formed on the basis of P code, which has better security performance. Navigation messages include satellite ephemeris, working conditions, clock correction, ionospheric delay correction, atmospheric refraction correction and other information. It is de modulated from satellite signal and transmitted on carrier frequency with 50b/s modulation. Each main frame of the navigation message contains 5 subframes, each of which is 6s long. 10 character codes for each of the first three frames; Repeat every 30 seconds and update every hour. The last two frames are 15000b in total. The content of navigation message mainly includes telemetry code, conversion code, the first, second and third data blocks, 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 transmitting the message. The user's position and speed in the WGS-84 geodetic coordinate system can be known.

　　It can be seen that the role of the satellite part of the GPS navigation system is to continuously transmit navigation messages. However, since the clock used by the user receiver cannot always be synchronized with the satellite on-board clock, in addition to the user's three-dimensional coordinates x, y, z, a Δ T is the time difference between the satellite and the receiver as an unknown number, and then use four equations to solve the four unknowns. So if you want to know the position of the receiver, you must be able to receive at least four satellite signals.

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