The accuracy of GPS is changing every second: the satellite orbit is changing, the ionosphere is changing (the energy difference between day and night is several times), and the temperature, humidity and pressure of the atmosphere are also changing, so the errors introduced by these factors are also changing every second.

　　Generally, the error range of GPS (I mean various handsets of famous manufacturers) produced in the past two years is about 10 meters. Today, the nominal accuracy of GARMIN and MAGELLAN handsets is 7 meters (in 95% cases). This nominal value is calculated under the best conditions, just like the 100 km fuel consumption index of automobiles.

　　In other words, the position data given to you by GPS is relative position data. You need to draw a circle with a radius of 10 meters around this data point. Your absolute position must be here, but you can only use more advanced methods to measure where it is.

　　If possible, the higher the accuracy, the better. In modern times, the general method to improve the accuracy in real time is the difference method: use two GPS stations to observe satellite signals at the same time. If the position of one GPS station is known, you can calculate the GPS signal correction value in that region at that time, and then find a way to notify the other GPS station. Then the other GPS station can calculate a very accurate position. Because the differential correction of GPS for civilian use in China is rarely used, I will not introduce it here.

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A - GPS positioning