Research On The Key Techniques Of GPS Precise Point Positioning

To overcome the drawbacks in differential GPS positioning, a new technique called precise point positioning (PPP) has been developed. In this approach, the IGS precise ephemeris and satellite clocks are used together with the un-differenced dual-frequency pseudo-range and carrier phase observations of one single receiver for precise point positioning. It can realize static and kinematics positioning in the global area with the accuracy of decimeter level even centimeter level. It provides a very efficient tool to get coordinates based on international terrestrial reference frame and can be used for LEO'S orbit determination, wide area precise positioning and navigation. The paper introduces the principle and the main error sources of PPP in detail, and the key techniques of PPP are also discussed. This paper discusses the following aspects:1,The model of un-differenced positioning and error sourcesThe paper discusses the model of un-differenced positioning in detail. The advantages and disadvantages of the un-differenced, single- differenced, double- differenced and three differenced observation are discussed. Compared with other positioning models, it has such advantages as retaining all the observational information, directly measureing station coordinates, nocorrelation between the different stations and no distance limitation. But the disadvantage is that more unknown parameters need to be estimated, no difference method can be used to eliminate errors, and more sophisticated model is requred .2,The consistence of IGS products One of the basic purposes of the IGS is to provide a global IGS reference which is easy to achieve and maintain for all GPS users and applications. Broadly speaking, the global IGS reference not only includes reference coordinates, but also includes the methods of data processing, such as the troposphere delay, the ionosphere and other inaccurate factors which may affect GPS positioning. By comparing the final IGS precise ephemeris and JPL precise ephemeris, we can draw a conclusion that we must consider the consistence of the IGS ephemeris while using the un-differenced positioning model.3,The method of the un-differenced precise point positioningThe core part of the article are the theory of the un-differenced precise point positioning. The traditional point positioning uses pseudo-range and broadcasting satellite ephemeris and orbital parameters of satellite and clock error correction to realize the positioning. But the precise point positioning uses global positioning precise satellite ephemeris and satellite clock error derived from all IGS tracking station data around the world. Using the method of un-differenced processing with a single receiver based on phase and pseudo-range observation, the accuracy of decimeter level even centimeter level can be achieved. Through the data analysis, the accuracy of PPP can be better than 15 cm in X, Y and Z component, and most of the positioning accuracy is better than 10 cm.