GPS signal.

　　The error of C/A code is 29.3m to 2.93m. The general receiver uses C/A code to calculate the location. For its own safety, the United States added SA (Selective Availability) to the signal in the middle of the 1990s, which increased the error of the receiver to about 100 meters. SA was cancelled on May 2, 2000, so our current GPS accuracy should be within 20 meters.

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

　　In a word, Lao Mei is also very tired. A lot of satellites were sent for military positioning. Then I thought it was not worth it and wanted to make some money, so I developed signals for civilian use. The accuracy could not be too high, but people cursed me when the accuracy was low. Because GPS is in the hands of the United States. Although it is free to use, it is not practical for other countries to use it. The United States has processed the GPS signals in this area two days ago when it hit Afghanistan, and the positioning accuracy has become lower.

　　Russia has its own satellite positioning system, Global Navigation Satellite System. Europe should also develop its own positioning system, NAVSAT. China also has its own satellite positioning system, called Beidou, which is a dual satellite system. It can only locate its own country and nearby areas, and is currently only used for the military.

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