The basic principle of the GPS navigation system is to measure the distance between a satellite with a 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, the position of the satellite can be found in the satellite ephemeris according to the time recorded by the onboard clock. The distance from the user to the satellite is obtained by recording the time that the satellite signal travels to the user, and then multiplying it by the speed of light (due to the interference of the ionosphere in the atmosphere, this distance is not the real distance between the user and the satellite, but Pseudo-range (PR): When GPS satellites work normally, they will continue to transmit navigation messages with pseudo-random codes (referred to as pseudo codes) composed of 1 and 0 binary symbols. There are two types of pseudo codes used by GPS systems, namely: Civil C/A code and military P(Y) code. The C/A code frequency is 1.023MHz, the repetition period is one millisecond, and the code interval is 1 microsecond, which is equivalent to 300m; the P code frequency is 10.23MHz, and the repetition period is 266.4 days. The interval is 0.1 microseconds, which is equivalent to 30m. The Y code is formed on the basis of the P code, and the security performance is better. The navigation message includes satellite ephemeris, working conditions, clock correction, ionospheric delay correction, atmospheric refraction correction, etc. Information. It is demodulated from the satellite signal and transmitted on the carrier frequency with 50b/s modulation. Each main frame of the navigation message contains 5 subframes with a frame length of 6s. The first three frames each have 10 words; each It repeats every 30 seconds and is updated every hour. The last two frames have a total of 15000b. The contents of the navigation message mainly include telemetry codes, conversion codes, and the first, second, and third data blocks, the most important of which is ephemeris data. When the user receives the navigation message, extract the satellite time and compare it with his own clock to know the distance between the satellite and the user, and then use the satellite ephemeris data in the navigation message 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’s receiver and the satellite’s on-board clock cannot always be synchronized, in addition to the user’s three-dimensional coordinates x, y, and z, a Δt, the time difference between the satellite and the receiver, is also introduced as an unknown number. Then use 4 equations to solve these 4 unknowns. So if you want to know where the receiver is, you must be able to receive at least 4 satellite signals.
The GPS receiver can receive time information accurate to the nanosecond level that can be used for timing; the forecast ephemeris for forecasting the approximate position of the satellite in the next few months; the broadcast ephemeris for calculating the satellite coordinates required for positioning , With an accuracy of a few meters to tens of meters (different from satellite, changing at any time); and GPS system information, such as satellite status.
The GPS receiver can measure the code to get the distance from the satellite to the receiver. Because it contains the error of the receiver's satellite clock and the atmospheric propagation error, it is called a pseudorange. The pseudorange measured for the 0A code is called the UA code pseudorange, and the accuracy is about 20 meters. The pseudorange measured for the P code is called the P code pseudorange, and the accuracy is about 2 meters.
The GPS receiver decodes the received satellite signal or uses other techniques to remove the information modulated on the carrier, and then the carrier can be restored. Strictly speaking, the carrier phase should be called the carrier beat frequency phase, which is the difference between the received satellite signal carrier phase affected by the Doppler shift and the signal phase generated by the receiver's local oscillation. Generally measured at the epoch time determined by the receiver clock and keeping track of the satellite signal, the phase change value can be recorded, but the initial value of the phase of the receiver and the satellite oscillator at the beginning of the observation is unknown. The phase integer of the initial epoch is also unknown, that is, the ambiguity of the whole week can only be solved as a parameter in data processing. The accuracy of the phase observation value is as high as millimeters, but the premise is to solve the ambiguity of the entire circumference. Therefore, the phase observation value can only be used when there is a relative observation and a continuous observation value, and the positioning accuracy that is better than the meter level is only Phase observations can be used.
According to the positioning method, GPS positioning is divided into single-point positioning and relative positioning (differential positioning). Single-point positioning is a way to determine the position of the receiver based on the observation data of a receiver. It can only use pseudorange observations and 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 pseudorange observations or phase observations. Geodetic or engineering measurements should be used. Use phase observations for relative positioning.
GPS observations include satellite and receiver clock differences, atmospheric propagation delay, multi-path effects and other errors. They are also affected by satellite broadcast ephemeris errors during positioning calculations. Most common errors are caused by relative positioning. Cancellation or weakening, so the positioning accuracy will be greatly improved. The dual-frequency receiver can cancel the main part of the ionospheric error in the atmosphere based on the observations of the two frequencies. ), dual-frequency receivers should be used.
