Origin of GPS

　　In the late 1950s, the former Soviet Union launched the first man-made earth satellite. American scientists discovered the Doppler frequency shift phenomenon in their tracking research, and used this principle to promote the Doppler satellite navigation and positioning system. The completion of TRANsIT has achieved great success in military and civilian applications, and is a leap in the history of navigation and positioning. my country has also introduced a number of Doppler receivers, which are used in the fields of island joint testing and earth exploration. However, due to the low orbital height and low signal carrier frequency of Doppler satellites, it is difficult to improve the orbital accuracy, so that the positioning accuracy is low, so as to meet the requirements of geodetic survey or engineering survey, and it is even less likely to be used for astronomical geodynamics research. In order to improve the accuracy of satellite positioning, the United States began to build the Global Positioning System GPS (Global Positioning System) in 1973. After entering the stage of program demonstration and system testing, the official working satellite was launched in 1989, and it was all completed and put into use in 1994. The space part of the GPS system consists of 21 satellites, which are evenly distributed on 6 orbital planes. The ground height is more than 20,000 kilometers, the orbit inclination is 55 degrees, the flat heart rate is about 0, and the period is about 12 hours. The carrier signal of two bands, the carrier signal frequencies are 1575.442 MHz (L1 band) and 1227.6 MHz (L2 band), and a high-precision atomic clock is installed on the satellite to ensure the stability of the frequency. Broadcast ephemeris of satellite position, C/A code and P code for ranging, and other system information, can provide high-precision, all-weather, continuous, real-time three-dimensional speed measurement to any multi-user on a global scale , 3D positioning and timing.

　　The control part of the GPS system consists of 5 monitoring stations located in the continental United States. These stations continuously observe the GPS satellites, and update the satellite information with the information of calculation and forecast by the injection station.

　　The user of the GPS system is very hidden, it is a one-way system, the user only receives and does not have to transmit the signal, so the number of users is also unlimited. Although the GPS system was originally established for military purposes, it soon developed greatly in civilian use, and various GPS receivers and processing software emerged one after another. At present, the receivers that appear in the Chinese market mainly include NovAtel, ASHTECH, TRIMBLE, CMC and so on. A receiver that can observe two frequencies is called a dual-frequency receiver, and a receiver that can only observe one frequency is a single-frequency receiver, and they are quite different in accuracy and price.

　　For users in the field of surveying and mapping, GPS has caused a revolutionary change in the field of surveying and mapping. At present, the control network or deformation monitoring network with a range of several kilometers to several thousand kilometers, with an accuracy of 100 meters to millimeters of positioning, generally GPS is the first choice. With the maturity of RTK technology, GPS has begun to penetrate into the fields of decimeter and even centimeter-level staking and high-precision dynamic positioning.

　　Since 1992, IGS, the international GPS geodetic and geodynamic service, has established a number of data storage and processing centers and more than 100 perennial observation stations around the world. my country has also established Shanghai Yushan, Wuhan, Xi'an, and Lhasa. , Taiwan and other perennial observation 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 various data processing centers in almost real time, and participates in the global earth rotation service IERS. Coordinate reference system maintenance and publication of Earth rotation parameters. Users can also obtain observation data and precise ephemeris products from the Internet for free.

　　The real-time navigation and positioning accuracy of the GPS system is very high. The United States has implemented the so-called SA policy since 1992, that is, to reduce the accuracy of the satellite position in the broadcast ephemeris, reduce the accuracy of the correction number of the star clock, and add high frequency to the satellite reference frequency. Jitter (reduces the measurement accuracy of pseudorange and phase), and then implemented the A-S policy, that is, changing the P code to the Y code, which further restricts precision pseudorange measurement, and the US military and authorized users are not affected by these policies. Influence, but in order to obtain greater commercial interests in the United States, these policies will eventually be cancelled.

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