GPS Fundamentals

　　The basic principle of the GPS navigation system is to measure the distance between the satellite with known position and the receiver of the user, and then the specific position of the receiver can be known by integrating the data of multiple satellites. 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 it takes for the satellite signal to propagate 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 the GPS satellite is working normally, it will continuously transmit the navigation message with a pseudo-random code (pseudo-code for short) composed of binary symbols of 1 and 0. There are two kinds of pseudo-codes used by the GPS system, namely Civil C/A code and military P(Y) code. C/A code frequency is 1.023MHz, repetition period is 1 millisecond, code spacing is 1 microsecond, equivalent to 300m; P code frequency is 10.23MHz, repetition period is 266.4 days, code 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 confidentiality performance is better. The navigation message includes satellite ephemeris, working status, 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 each with a length of 6s. Each of the first three frames has 10 characters; each Repeated once every 30 seconds and updated every hour. The last two frames are 15000b in total. The contents of the navigation message mainly include telemetry code, conversion code, 1st, 2nd, and 3rd data blocks, the most important of which is ephemeris data. When the user receives the navigation message, he extracts the satellite time and compares it with his own clock to know the distance between the satellite and the user, and then uses the satellite ephemeris data in the navigation message to calculate the location of the satellite when the message was transmitted. The user's position and velocity 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, because the clock used by the user's receiver and the satellite onboard clock are impossible It is always synchronized, so 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 introduced as an unknown, and then 4 equations are used to solve these 4 unknowns. So if you want to To know the position of the receiver, at least 4 satellites must be able to receive signals. The GPS receiver can receive time information accurate to nanoseconds that can be used for timing; it is used to predict the approximate location of satellites in the next few months. The forecast ephemeris of the location; the broadcast ephemeris used to calculate the satellite coordinates required for positioning, with an accuracy of several meters to several tens of meters (different for each satellite and changes at any time); and GPS system information, such as satellite status, etc. GPS receiver pair The distance from the satellite to the receiver can be obtained by measuring the code, which is called pseudorange because it contains the error of the receiver satellite clock and atmospheric propagation error. The pseudorange measured for the 0A code is called the UA code pseudorange, and the accuracy is about It is about 20 meters, and the pseudorange measured by 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 technologies to modulate the signal on the carrier. After the information is removed, the carrier can be recovered.Strictly speaking, the carrier phase should be called the carrier beat phase, which is the received satellite signal affected by the Doppler shift The difference between the carrier phase and the phase of the signal generated by the receiver's local oscillation. Generally, the measurement is made at the epoch time determined by the receiver clock and the tracking of the satellite signal is maintained, and the phase change value can be recorded, but the initial phase value 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 integer ambiguity, which can only be solved as a parameter in data processing. The accuracy of the phase observations is as high as millimeters, but the premise is to solve the ambiguity of the whole circle, so the phase observations can only be used when there is a relative positioning and there is a period of continuous observations, and the positioning accuracy better than the meter level can only be achieved. 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 method of determining the position of a receiver based on the observation data of a receiver. It can only use pseudo-range observations and can be used for rough navigation and positioning of vehicles and ships. Relative positioning (differential positioning) is a method of determining the relative position between observation points based on the observation data of two or more receivers. Relative positioning using phase observations. GPS observations include errors such as satellite and receiver clock errors, atmospheric propagation delays, and multipath effects, and are also affected by satellite broadcast ephemeris errors during positioning calculations. Cancellation or weakening, so the positioning accuracy will be greatly improved. The dual-frequency receiver can offset the main part of the ionospheric error in the atmosphere according to the observations of the two frequencies. ), a dual-frequency receiver should be used.

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