1. Overview
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In the 1950s, the former Soviet Union launched the first artificial satellite of mankind. In the follow-up study of American scientists, the phenomenon of Doppler shift was discovered, and the principle was used to promote the Doppler satellite navigation and positioning system. The completion of TRANsIT has achieved great success in military and civilian use. It is a leap in the history of navigation and positioning. China has also introduced multiple Doppler receivers for use in island joint surveys and earth exploration. However, due to the low orbital height of the Doppler satellite and the low signal carrier frequency, the orbit accuracy is difficult to improve, which makes the positioning accuracy lower, to meet the requirements of geodesy or engineering measurement, and is less likely to be used in astronomical geodynamics research. In order to improve the accuracy of satellite positioning, the United States began to build a Global Positioning System (GPS) in 1973. After entering the program demonstration and system test phase, the official working satellite was launched in 1989 and was completed and put into use in 1994. The space part of the GPS system consists of 21 satellites, evenly distributed on 6 orbital planes. The ground height is more than 20,000 kilometers, the orbital inclination is 55 degrees, the flat heart rate is about 0, the period is about 12 hours, and the satellite launches two to the ground. The carrier signal of the band, the carrier signal frequency is 1575.442 megahertz (L1 band) and 1227.6 megahertz (L2 band) respectively. The satellite has a highly accurate atomic clock to ensure the stability of the frequency. Broadcast ephemeris at satellite position, C/A code and P code for ranging, and other system information, providing high-precision, all-weather, continuous, real-time 3D speed measurement to any user on a global scale. , 3D positioning and timing.
The control part of the GPS system consists of five monitoring stations located in the United States. These stations continuously observe GPS satellites and update the calculation and forecast information from the injection station to the satellite information.
The user of the GPS system is very concealed. It is a one-way system, and the user only receives and does not have to transmit signals, so the number of users is also unlimited. Although the GPS system was originally established for military purposes, it has been greatly developed in civilian use, and various types of GPS receivers and processing software have emerged. At present, the receivers appearing in the Chinese market mainly include NovAtel, ASHTECH, TRIMBLE, CMC, and the like. A receiver that can observe two frequencies is called a dual-frequency receiver. A receiver that can only observe one frequency becomes a single-frequency receiver, and they are quite different in accuracy and price.
For the users of the surveying and mapping community, GPS has caused revolutionary changes in the field of surveying and mapping. At present, the control network or deformation monitoring network ranging from several kilometers to several thousand kilometers in range, the accuracy is from 100 meters to millimeters. GPS is the preferred method. With the maturity of RTK technology, GPS has begun to penetrate into the fields of decimeter and even centimeter level, high-precision dynamic positioning.
International GPS geodetic and geodynamic services IGS has established a number of data storage and processing centers and more than 100 stations for perennial observations since 1992. China has also established Shanghai Yushan, Wuhan, Xi'an and Lhasa. Taiwan, and other perennial observing stations. The observation data of these stations are transmitted to the data storage center in the United States through the Internet every day. IGS also integrates the results of each data processing center in almost real time and participates in the global earth rotation service IERS globally. Coordinate reference system maintenance and release of Earth rotation parameters. Users can also obtain observation data and precision ephemeris products from the Internet for free.
The real-time navigation and positioning accuracy of the GPS system is very high. The United States implemented the so-called SA policy in 1992, which is to reduce the accuracy of the satellite position in the broadcast ephemeris, reduce the accuracy of the star clock correction, and add high frequency to the satellite reference frequency. Jitter (decreases the accuracy of pseudorange and phase measurement), and then implements the AS policy, which changes the P code to the Y code, which further limits the precision pseudorange measurement, while the US military and licensed users are not subject to these policies. Impact, but in order for the United States to gain greater commercial benefits, these policies will eventually be lifted.
2. GPS positioning principle
The GPS receiver can receive time information accurate to the nanosecond level that can be used for timing; a forecast ephemeris used to predict the approximate location of the satellite in the next few months; and a broadcast ephemeris used to calculate the satellite coordinates required for positioning. The accuracy is from a few meters to tens of meters (different satellites, changing at any time); and GPS system information, such as satellite status.
The GPS receiver measures the distance from the satellite to the receiver. It is called pseudorange because it contains the error of the receiver satellite clock and the atmospheric propagation error. The pseudorange measured by the 0A code is called the UA code pseudorange, and the precision is about 20 meters. The pseudorange measured by the P code is called the P code pseudorange, and the precision 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 recover the carrier. Strictly speaking, the carrier phase should be referred to as the carrier beat frequency phase, which is the difference between the received satellite signal carrier phase affected by the Doppler shift and the phase of the receiver local oscillator generated signal. Generally, the epoch time determined by the receiver clock is measured, and the tracking of the satellite signal is kept, and the phase change value can be recorded, but the initial phase values ​​of the receiver and the satellite oscillator at the start of observation are unknown. The phase integer of the 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 is as high as millimeter, but the premise is that the whole-circumference ambiguity is solved. Therefore, the phase observation value can be used only when the relative positioning is performed and there is a continuous observation value, and the positioning accuracy is better than the meter level. 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 be measured by pseudo-range observation 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 observation data of two or more receivers. It can use both pseudo-range observation and phase observation. Geodesy or engineering measurement should be performed. Phase observations are used for relative positioning.
The GPS observations include errors such as the clock difference of the satellite and the receiver, the atmospheric propagation delay, and the multipath effect. They are also affected by the satellite broadcast ephemeris error during the positioning calculation. Most of the common errors are caused by relative positioning. Offset or weaken, 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 observation of two frequencies. When the accuracy is high and the distance between receivers is far away (the atmosphere is obviously different) ), should use dual-frequency receiver.
In the positioning observation, if the receiver moves relative to the surface of the earth, it is called dynamic positioning, such as pseudo-single point positioning with accuracy of 30-100 meters for rough navigation and positioning of vehicles and ships, or for urban vehicle navigation and positioning. Pitch-precision differential positioning with meter-level accuracy, or centimeter-level phase differential positioning (RTK) for measuring stakeouts. Real-time differential positioning requires a data link to transmit observations from two or more stations in real time. In the positioning observation, if the receiver is stationary with respect to the surface of the earth, it is called static positioning. When performing control network observation, it is generally observed by several receivers in this way. It can play GPS to the least extent. Positioning accuracy, a receiver dedicated to this purpose is called a terrestrial receiver and is the best performing class in the receiver. At present, GPS has been able to meet the accuracy requirements of crustal deformation observation, and IGS's perennial observing stations have been able to form a millimeter-scale global coordinate framework.
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