International GPS development

　　The users of the GPS system are very invisible. It is a one-way system, and users only receive signals without transmitting them. Therefore, the number of users is unlimited. Although the GPS system was built for military purposes at the beginning, it has been rapidly developed in civilian areas, and various GPS receivers and processing software have emerged. At present, the receivers appearing in the Chinese market mainly include NovAtel, ASHTECH, TRIMBLE, CMC, etc. The receiver that can observe two frequencies is called dual frequency receiver. The receiver that can only observe one frequency becomes single frequency receiver. They are quite different in accuracy and price.

　　For users in the surveying and mapping field, GPS has caused revolutionary changes in the field of surveying and mapping. At present, the control network or deformation monitoring network covering several kilometers to thousands of kilometers, with positioning accuracy ranging from hundreds of meters to millimeters, generally use GPS as the preferred means. With the maturity of RTK technology, GPS has begun to penetrate into decimeter and even centimeter level lofting, high-precision dynamic positioning and other fields.

　　Since 1992, IGS, an international GPS geodetic and geodynamic service, has established multiple data storage and processing centers and more than 100 perennial observation stations around the world. China has also set up a number of perennial observation stations in Shanghai Yushan, Wuhan, Xi'an, Lhasa, Taiwan, etc. The observation data of these stations are transmitted to the data storage center in the United States every day through the Internet. IGS also integrates the results of each data processing center almost in real time, And participate in the global coordinate reference system maintenance of the international earth rotation service IERS and the release of the earth rotation parameters. Users can also obtain observation data, precise ephemeris and other products from 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 satellite positions in broadcast ephemeris, to reduce the accuracy of clock correction numbers, to add high-frequency jitter to the satellite reference frequency (reducing the measurement accuracy of pseudo range and phase), and then to implement the A-S policy, that is, to change the P code to Y code, which further restricts the precision pseudo range measurement, The US military and licensed users are not affected by these policies, but in order to obtain greater commercial benefits, these policies will eventually be cancelled.

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