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Top1. Introduction
Human demand for navigation and positioning is produced with the development of science and technology. In early material production life, there is no need for a clear positioning. In the agricultural era, people gradually produced a mapping positioning requirements in the developing agricultural land, building water conservancy, and other related activities. Until the industrial age, Human began the research of the navigation, aviation, intercontinental cruise, communications, and exploration of mineral resources, which needs the precise positioning. These requirements promoted the development of navigation and positioning technology. At the end of twentieth century, electronic computer technology, semiconductor technology, laser technology, and aerospace science appeared, which brought us to the electronic information age and space exploration era. The Soviet Union launched the first man-made earth satellite in 1957, herald the feasibility of a new sky positioning technology, therefore, human beings have entered the era of satellite positioning and navigation.
At present, the global positioning system mainly has the GPS system of the United States, the GLONASS system of Russia, the European Union's Galileo satellite navigation system, and the Chinese Beidou system.
In the seventy's, the U.S. military began to develop GPS satellite positioning system. GPS was established by the US Defense Department for military purposes, aims to solve the problem of navigation and positioning of vehicles in the sea, air and land. Now, GPS has become the world's most practical, but also the most widely used global precision navigation, command and scheduling system. In order to limit the application of different users to GPS, GPS provides two different positioning services for the military and civilian users by transmitting different radio signals. In the United States Department of transportation, a report of the GPS assessment strongly required to improve the technical performance of GPS, in order to obtain higher accuracy, safety and reliability.
GLONASS system is consisted of 24 satellites, its principle and the program are similar to GPS, position precision can reach 1 m, and velocity error is only 15 cm /s. If needed, the system can also be used to accurately attack weapon guidance. The system is developed and controlled by the military. Although its positioning accuracy is lower than that of GPS and Galileo, its anti-jamming ability is the strongest.
Galileo system is the autonomous and independent civil global satellite navigation system in Europe, providing high accuracy, high reliability of the positioning service. The “Galileo” program is a kind of medium height circle orbit satellite positioning scheme, compared with the United States global positioning system (GPS), Galileo system has at least three advantages, First, the coverage area will be two times the GPS system, Secondly, its ground positioning error is not more than 1 meters, accuracy is five times higher than GPS, Third, the Galileo system uses a variety of frequency bands, in the civilian field than GPS more economical, more transparent, more open.
Beidou satellite navigation system is Chinese autonomous construction, independent operation, and compatible with the world's other satellite navigation system. In the global range, day and night for all types of users, it provides high precision and high reliability of positioning, navigation and timing services and short message communication ability. The satellite navigation system of Chinese Beidou satellite navigation system is a kind of fast positioning system of “double stars.” The space segment is consisted of five geostationary satellites and thirty non geostationary satellites, providing two services, namely, open service and authorization service.