1. Global Positioning System (GPS), also known as Global Satellite Positioning System, is a medium range circular orbit satellite navigation system developed and maintained by the US Department of Defense. It began development in the 1970s and was fully completed in 1994. It can provide accurate positioning, velocity measurement, and high-precision time standards for 98% of the Earth's surface. Users only need to have a GPS receiver to use this service without additional payment. Its signals are divided into two categories: civilian standard positioning services and military precision positioning services. Due to the fact that GPS can be used without any authorization, the United States has always been concerned that hostile countries or organizations may use it to launch attacks on the United States. Therefore, selective errors are artificially added to civilian signals to reduce accuracy. Make its final positioning accuracy around 100 meters, and military accuracy below 10 meters. It was not until after 2000 that the Clinton administration abolished interference with civilian signals and improved the accuracy to within a range of about 10 meters.

　　The GPS system can meet the needs of military users located anywhere in the world or in near Earth space to continuously and accurately determine three-dimensional positions, three-dimensional motion, and time. The system includes 24 GPS satellites in space; One main control station, three data injection stations, and five monitoring stations on the ground, as well as a GPS receiver for the user end. At least three satellites are needed to quickly determine the user's location and altitude on Earth; The more satellites can be received and connected, the more accurate the decoded position will be.

　　The GLONASS system was established by the Soviet Union in 1976 and is now operated by the Russian government. The system consists of three parts: satellites, ground measurement and control stations, and user equipment. The current system consists of 21 working satellites and 3 backup satellites, distributed on three orbital planes. Each orbital plane has 8 satellites, with an orbital altitude of nearly 20000 kilometers and an operating period of 11 hours.

　　A satellite navigation system with global coverage was established in 1991. Since December 1982, the navigation satellites of the system have been continuously supplemented. By 1995, the number of satellites in the system had been basically improved. However, as the Russian economy continued to decline, the system also fell on the brink of collapse due to reasons such as disrepair. But from 2001 to 2010, the Russian government had already completed the 24 satellites required for the system, and the theoretical positioning accuracy would reach about 1 meter.

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