At present, the positioning accuracy provided by the GPS system is better than 10m. To obtain higher positioning accuracy, we usually use the differential GPS technology: place a GPS receiver on the base station for observation. According to the known precise coordinates of the reference station, the distance correction number from the reference station to the satellite is calculated, and the data is sent out by the reference station in real time. The user receiver also receives the correction number sent by the reference station while making GPS observation, and corrects the positioning result, so as to improve the positioning accuracy. Differential GPS can be divided into two categories: pseudo range difference and carrier phase difference.

　　1. Pseudo range difference principle

　　This is the most widely used difference. On the base station, observe all satellites, and calculate the true distance from each satellite to the base station at each time according to the known coordinates of the base station and the coordinates of each satellite. Compared with the measured pseudo range, the pseudo range correction number is obtained and transmitted to the user receiver to improve the positioning accuracy. This difference can obtain meter level positioning accuracy, such as the "beacon difference" widely used in coastal areas.

　　2. Carrier phase difference principle

　　Carrier phase difference technology, also known as RTK (Real Time Kinematic) technology, is a difference method for real-time processing of carrier phase observation of two stations. That is, the carrier phase collected by the reference station is sent to the user receiver to calculate the coordinates. The carrier phase difference can make the positioning accuracy reach centimeter level. It is widely used in the field where high precision position is required dynamically.

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