The geodesic receiver achieves high accuracy (10-6 ~ 10-8) in static baseline measurements using GPS satellite carrier phase. However, in order to reliably solve the phase ambiguity, stationary observation for one or two hours or more is required. This limits its application in engineering operations. So the search for rapid measurement methods came into being. For example, FARA was used to reduce the baseline observation time to 5 minutes, and stop and go, re-occupation, and kinematic were used to improve the efficiency of GPS operations. The application of these technologies has contributed to the promotion of precise GPS measurements. However, the above operation methods are data processing after the event, which cannot submit the results and evaluate the quality of the results in real time, and it is difficult to avoid the rework phenomenon caused by the unqualified inspection after the event.

　　The emergence of differential GPS can give the position of the carrier in real time, the accuracy is meter level, to meet the requirements of pilotage, underwater survey and other engineering. Some techniques, such as position difference, pseudo-range difference and phase smoothing, have been successfully used in various jobs. This was followed by a more sophisticated measurement technique called carrier phase difference.

　　The carrier phase differential technology, also known as RTK technology (real time kinematic), is based on real-time processing of the carrier phase of two measuring stations. It can provide 3D coordinates of observation points in real time, and achieve centimeter level of precision.

　　The reference station transmits its carrier observation and station coordinate information to the user station in real time through the data link, which is the same as the pseudo-range difference principle. The user station receives the carrier phase from the GPS satellite and the carrier phase from the reference station, and forms the phase difference observation value for real-time processing, which can give real-time positioning results of centimeter level.

　　The methods of implementing carrier phase differential GPS are divided into two categories: correction method and difference method. The former is the same as the pseudoran difference, the reference station sends the carrier phase correction to the user station to correct its carrier phase, and then solves the coordinates. The latter sends the carrier phase collected by the reference station to the user station to find the difference and solve the coordinates. The former is quasi-RTK technology and the latter is true RTK technology.

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