When it comes to the compass, people are very familiar with it. As one of the the Four Great Inventions of the working people in ancient China, it has made great contributions to the development of world civilization. However, due to the uneven distribution of the geomagnetic field, the use of the compass is greatly limited. In order to adapt to the development of various aspects of society, Satellite Navigation System has emerged!

　　When people talk about navigation now, the three letters "GPS" will come to mind. In fact, GPS (Global Positioning System) is a global positioning system built by the US military. Due to preconceptions, people often use it as a synonym for satellite navigation systems. So what is a satellite navigation system? The satellite navigation system consists of three parts, namely navigation satellites, ground stations, and user positioning equipment. Navigation Satellite is the spatial component of a satellite navigation system, consisting of multiple navigation satellites forming a spatial navigation network; Ground stations are mainly responsible for tracking, measuring, and predicting satellite orbits, as well as controlling and managing equipment on the satellite. They usually include tracking stations, telemetry stations, computing centers, injection stations, and time unified systems; User positioning devices typically consist of receivers, timers, data preprocessors, computers, and displays.

　　The working principle of a satellite navigation system is that the navigation satellite is equipped with dedicated radio navigation equipment. Users receive radio navigation signals from the navigation satellite, and obtain navigation parameters such as the user's distance or distance change rate relative to the satellite through time ranging or Doppler velocity measurement. Based on the time and orbit parameters sent by the satellite, the real-time position coordinates of the satellite at the moment of positioning are calculated, thereby determining the user's geographic position coordinates (two-dimensional or three-dimensional coordinates) and velocity vector components. A navigation satellite network (navigation constellation) composed of multiple navigation satellites has the ability to provide three-dimensional coverage in both global and near Earth space, achieving global radio navigation.

　　The four core satellite navigation systems in the world today are the GPS system in the United States, the Galileo system in Europe, the GLONASS system in Russia, and the Compass navigation system in China. So what are the characteristics of these navigation systems? What is the development trend of future global navigation? The editor will provide you with detailed answers below.

　　● US GPS system

　　The most commonly used global satellite navigation and positioning system in the world is the GPS system in the United States. It is the world's first mature and universal global satellite positioning and navigation system. The system consists of 28 medium to high orbit satellites, of which 4 are backup satellites, evenly distributed in 6 tilted orbits about 20000 kilometers above the ground. These satellites form a network with ground support systems, broadcasting information to global users every 1-3 seconds, allowing users from anywhere on Earth to receive signals from at least 4 satellites simultaneously using receivers at any time. After receiving the signal, the receiver can determine its position through differential positioning principle.

　　The GPS system is also constantly improving, and the United States is currently designing new second-generation GPS satellites with plans to launch 20. After the upgrade, the positioning accuracy will be greatly improved.

　　● European GALILEO system

　　The GALILEO system was launched in collaboration between the European Space Agency and the European Union in 1999, and its civilian signal accuracy can reach up to 1m.

　　The planned GALILEO system consists of 30 satellites. On December 28, 2005, the first experimental satellite Glove-A was successfully launched, and the second experimental satellite Glove-B was launched from the Baikonur base in Kazakhstan by a Russian Soyuz carrier rocket on April 27, 2007. In addition, four in orbit verification (IOV) satellites are currently in production and will be launched into orbit in the near future.

　　● GLONASS system in Russia

　　GLONASS, abbreviated as Global Navigation Satellite System, was a global satellite navigation system built by the Soviet Ministry of Defense in the early 1980s. In a sense, it was a product of the Cold War. The system cost over 3 billion US dollars and was put into use in 1995. It is now managed by the Russian Federation Space Agency. GLONASS is the second dual-use global satellite navigation system after GPS.

　　● China Beidou (COMPASS) system

　　The development speed of China's satellite positioning industry is evident, with the market size increasing from less than 1 billion yuan in 2000 to 12 billion yuan in 2005. In this context, based on economic interests and national security considerations, China has begun to build its own BeiDou global satellite positioning and navigation system.

　　The space segment of the system consists of 5 geostationary orbit satellites and 30 non geostationary orbit satellites, providing two modes of open service and authorized service. Open services provide free positioning, speed measurement, and timing, with a positioning accuracy of up to 10m, timing accuracy of 50ns, and speed measurement accuracy of 0.2m/s. Authorization services provide more secure and accurate positioning, speed measurement, and timing services, as well as system integrity information

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