Origin of 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 promote 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.

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