1. Principle of position difference

　　This is the simplest differential method, and any GPS receiver can be modified and composed of this differential system. After observing four satellites, the GPS receiver installed on the reference station can perform three-dimensional positioning and calculate the coordinates of the reference station. Due to orbital errors, clock errors, SA effects, atmospheric effects, multipath effects, and other errors, the calculated coordinates are different from the known coordinates of the reference station, and there is a difference. The reference station uses a data link to send out this correction number, which is received by the user station, and corrects the calculated user station coordinates.

　　The final corrected user coordinates have eliminated the common errors between the reference and user stations, such as satellite orbit errors, SA effects, atmospheric effects, etc., improving positioning accuracy. The above prerequisite is for the observation of the same group of satellites by both the reference station and the user station. The position difference method is suitable for situations where the distance between the user and the reference station is within 100km.

　　2. Pseudorange difference principle

　　Pseudorange difference is currently the most widely used technique. Almost all commercial differential GPS receivers use this technology. The RTCM SC-104 recommended by the International Maritime Radio Commission also adopts this technology.

　　The receiver on the reference station requires the distance from it to the visible satellite and compares the calculated distance with the measurement value containing errors. Utilize a α-β The filter filters this difference and calculates its deviation. Then, the ranging errors of all satellites are transmitted to the user, who uses this ranging error to correct the measured pseudo range. Finally, users can use the corrected pseudo distance to solve their own position, which can eliminate common errors and improve positioning accuracy.

　　Similar to position difference, pseudo range difference can cancel out the common error between two stations, but as the distance from the user to the reference station increases, system error occurs, which cannot be eliminated by any difference method. The distance between the user and the reference station has a decisive impact on accuracy.

　　3. Principle of carrier phase difference

　　Carrier phase difference technology, also known as RTK (real time kinetics), is based on real-time processing of the carrier phases of two measurement stations. It can provide real-time three-dimensional coordinates of observation points and achieve high-precision at the centimeter level.

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