When it comes to positioning accuracy, we have to talk about SA and AS.

　　What is SA, AS?

　　GPS signals have two types of C/A codes and P codes.

　　The error of C/A code is 29.3m to 2.93m. Common receivers use C/A codes to calculate positioning. In the mid-1990s, the United States added SA (Selective Availability) to the signal for its own safety considerations, which increased the error of the receiver to about 100 meters. On May 2, 2000, SA was cancelled, so our current GPS accuracy should be within 20 meters.

　　The error of the P code is 2.93 meters to 0.293 meters, which is one tenth of the C/A code. But the P code can only be used by the US military. AS (Anti-Spoofing) is an interference signal added to the P code.

　　In short, the old beauty is also quite tired. A whole bunch of satellites were sent for military positioning. Then I felt it was not worth it and wanted to make some money, so I developed a signal for civilian use. Because GPS is in the hands of the old and the United States, although it is free to use, it is not practical for other countries to use it. The United States processed the GPS signals in the area two days ago, and the positioning accuracy became lower.

　　Russia has its own satellite positioning system, the Global Navigation Satellite System (GLObal NAvigation Satellite System). Europe will also develop its own positioning system NAVSAT. China also has its own satellite positioning, called Beidou, which is a dual-star system that can only locate its own country and nearby areas, and is currently only used by the military.

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