Differential GPS positioning principle

According to the information sent by the differential GPS reference station, the differential GPS positioning can be divided into three categories, namely: position difference, pseudo range difference and phase difference. The working principle of these three types of differential methods is the same, that is, the correction number is sent by the base station, and is received by the subscriber station and corrected by the measurement result to obtain accurate positioning results. The difference is that the specific content of the transmission correction number is different, and the differential positioning accuracy is also different.

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Position difference principle

This is the simplest differential method that can be retrofitted and composed of any kind of GPS receiver.

The GPS receiver installed on the base station can observe three satellites and then perform three-dimensional positioning to calculate the coordinates of the base station. Due to orbital errors, clock errors, SA effects, atmospheric effects, multipath effects, and other errors, the calculated coordinates are not the same as the known coordinates of the base station, and there is an error. The base station uses the data link to transmit the corrected number, receives it by the subscriber station, and corrects the coordinates of the user station that it solves.

The corrected user coordinates obtained in the end have eliminated the common errors of the base station and the subscriber station, such as satellite orbit error, SA influence, atmospheric influence, etc., which improves the positioning accuracy. The above prerequisites are the case where the base station and the subscriber station observe the same set of satellites. The position difference method is suitable for the case where the distance between the user and the base station is within 100 km.

2. Principle of pseudorange difference

Pseudorange difference is currently the most versatile technology. Almost all commercial differential GPS receivers use this technology. This technology is also used by the RTCM SC-104 recommended by the International Maritime Radio Committee.

The receiver at the base station requires its distance to the visible satellite and compares this calculated distance to the measured value containing the error. This difference is filtered using an α-β filter and its deviation is found. The ranging error of all satellites is then transmitted to the user, and the user uses this ranging error to correct the measured pseudorange. Finally, the user can correct the position by using the corrected pseudorange to eliminate the common error and improve the positioning accuracy.

Similar to the position difference, the pseudorange difference can cancel the common error of the two stations, but the system error occurs as the distance from the user to the base station increases. This error cannot be eliminated by any difference method. The distance between the user and the base station has a decisive influence on the accuracy.

3. Carrier phase difference principle

The geodetic receiver achieves high accuracy (10-6 to 10-8) using static baseline measurements of the GPS satellite carrier phase. However, in order to reliably solve the phase ambiguity, static observation is required for one or two hours or more. This limits the application in engineering operations. So the method of searching for rapid measurement came into being. For example, using the full-circumference ambiguity fast approximation technique (FARA) to reduce the baseline observation time to 5 minutes, using quasi-dynamic (stop and go), round-trip re-occupation and dynamic (kinematic) to improve GPS efficiency . The application of these technologies has contributed to the promotion of precision GPS measurements. However, these methods of operation are post-processing data processing, and the results cannot be submitted in real time and the quality of the results can be assessed in real time. It is difficult to avoid rework caused by unqualified inspections afterwards.

With the appearance of differential GPS, the position of the carrier can be given in real time with an accuracy of meters, which satisfies the requirements of piloting, underwater measurement and other engineering. Techniques such as position difference, pseudorange difference, and pseudorange differential phase smoothing have been successfully used in various operations. What followed was a more sophisticated measurement technique, carrier phase differential technology.

The carrier phase difference technique, also known as RTK technology (real time kinematic), is based on the carrier phase of the two stations in real time. It provides three-dimensional coordinates of the observation points in real time and achieves centimeter-level precision.

Similar to the principle of pseudorange difference, the base station transmits its carrier observation and station coordinate information to the subscriber station in real time through the data link. The subscriber station receives the carrier phase of the GPS satellite and the carrier phase from the base station, and forms phase difference observations for real-time processing, which can give centimeter-level positioning results in real time.

There are two types of methods for implementing carrier phase differential GPS: the correction method and the difference method. The former is the same as the pseudorange difference, and the base station transmits the carrier phase correction amount to the subscriber station to correct its carrier phase and then solve the coordinates. The latter sends the carrier phase collected by the base station to the subscriber station for the difference calculation coordinate. The former is a quasi RTK technology and the latter is a real RTK technology.