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What is the principle of GNSS positioning?

Satellite based positioning relies on measuring distance between satellite and receivers. With known satellite position and measured distance between satellite and receiver, the coordinate of receiver can be calculated. However, the electromagnetic signals emitted by satellites are perturbed by atmosphere and clock errors, which degrade GNSS positioning accuracy. There are two types of positioning mode, one is point position and the other is relative position (see Fig 1). Point positioning includes standard point positioning (SPP) and precise point positioning (PPP). Their major difference is whether use the carrier phase observation. Relative positioning also includes two types: DGPS and RTK. Their difference is the latter one uses carrier phase observation as well. Relative positioning cancels most error sources in distance measurement by making difference between two receivers, so it is more simple and reliable, but it only measures relative position. In practice, the coordinate of the reference receiver is fixed as known. 

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