GPS development

　　At the end of the 1950s, the former Soviet Union launched the first man-made earth satellite of mankind. American scientists discovered the phenomenon of Doppler frequency shift in their tracking and research, and used this principle to facilitate the establishment of the Doppler satellite navigation and positioning system TRANsIT, which has achieved great success in military and civil aspects. It is a leap in the history of navigation and positioning. China has also introduced several Doppler receivers, which are used in island joint survey Earth exploration and other fields. However, due to the low orbit height and signal carrier frequency of the Doppler satellite, it is difficult to improve the orbit accuracy, which makes the positioning accuracy low to meet the requirements of geodesy or engineering survey, and it is impossible to apply it to the research of astrogeodynamics. In order to improve the accuracy of satellite positioning, the United States began to build the Global Positioning System (GPS) in 1973. After entering the phase of scheme demonstration and system test, the official working satellite was launched in 1989, and was completed and put into use in 1994. The space part of the GPS system is composed of 21 satellites, which are evenly distributed on 6 orbital planes. The ground height is more than 20000 km, the orbital inclination is 55 degrees, the flat heart rate is about 0, and the period is about 12 hours. The satellite transmits carrier signals of two bands to the ground. The carrier signal frequencies are 1575.442 MHz (L1 band) and 1227.6 MHz (L2 band) respectively. The satellite is equipped with atomic clocks with high accuracy to ensure the stability of the frequency, The broadcast ephemeris representing satellite position, C/A code and P code used for ranging, and other system information are modulated on the carrier wave, which can provide high-precision, all-weather, continuous, real-time three-dimensional velocity measurement, three-dimensional positioning, and time service to any number of users worldwide.

　　The control part of the GPS system consists of five monitoring stations located in the United States. These stations continuously observe the GPS satellites and update the calculated and predicted information from the injection station to the satellite information.

　　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 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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