Research On Precise Point Positioning Based On Multimode Gnss Fusion

The accuracy of positioning result based on traditional standalone positioning is always maintained at the meter level,which cannot meet the high requirements of real-time positioning.For this reason,some scholars proposed a new positioning method,which call precise point positioning(PPP).In PPP,precise ephemeris and clock offset information are received from various international geodynamic service(IGS)reference stations worldwide.Meanwhile,observation message is received from navigation satellites.The positioning result can achieve centimeter-level by establishing appropriate PPP observation models and error models.Compared with the traditional high-precision real-time positioning method,such as Real Time Kinematic positioning(RTK)technology,there are lots of advantage in PPP.For example,the distance between the observation station and the receiver cannot be limited,which can meet the requirements of high-precision real-time positioning worldwide.At present,the research on PPP is mainly limited in single GNSS,such as PPP based on GPS.The positioning results of single GNSS PPP are easily affected by the number of observable satellites.In some places with bad observation conditions,such as urban canyon and mining areas,fewer available satellites often result in failures of positioning,it also results in very long convergence times and poor positioning accuracy.Nowadays,with the continuous development of Beidou satellite navigation system(BDS).By using PPP based on multimode GNSS fusion,researchers can effectively solve the problem of insufficient number of satellites in navigation.At first,the information of GPS and BDS are introduced in the thesis.The difference between two systems are also listed.Therefore,the foundation for the fusion positioning of the dual systems has been laid.Various errors will be encountered during the PPP process.In addition to some kinds of normal errors,such as ionospheric delay,tropospheric delay,satellite clock error,and so on,some special error sources have been considered in the PPP,including the Sagnac effect,antenna phase,center deviation,solid tide,ocean load,relativistic effect,etc.These errors are not considered in the traditional standalone positioning because of the little impact on the positioning results(mostly from the centimeter level to the decimeter level).While in PPP,because of the high requirements of the accuracy in positioning results,it needs to be classified and eliminated.In addition,two observation models commonly used in PPP based on single GNSS are elaborated and compared,including traditional model and Uof C model.Then,the random models used in PPP are classified and discussed.Because there are certain differences in time systems and space systems between GPS and BDS,it is necessary to establish an error model suitable for PPP based on multimode system fusion according to different characteristics of the system.Refer to PPP based on single GNSS,the thesie derived the observation model and random model of GPS/BDS combined PPP,which is suitable for dual-system fusion positioning.It includes GPS/BDS PPP Uof C model based on the traditional error model and GPS/BDS PPP Uof C model based on Helmet's posterior weighting method.By using the latter model,the positioning effects of GPS and BDS in different regions and at different times can be target automatically.The faster convergence speed and higher positioning accuracy are influenced by it.Using the satellite precise ephemeris provided by IGS combined with the original observation data collected by receivers supporting multi-system signals,the final positioning result of PPP can be calculated out.The static and dynamic experiments were done to verify the difference between single GNSS PPP model and combined GPS/BDS PPP model.The experimental result show that the participation of Helmet's posterior weighting method will effectively solve the random model selection problem of fusion positioning between dual systems.Compared with single GNSS PPP based on single GNSS,PPP based on combined GPS/BDS is better in positioning performance.Under static conditions,within 20 minutes,the plane error can be controlled in 1 cm while the elevation error can be controlled in 3 cm used by the GPS/BDS fusion PPP based on Helmet posterior weighting method proposed in the paper.Under dynamic conditions,used by GPS/BDS fusion PPP based on Helmet's posterior weighting method,the plane errors can be controlled in 8 cm within a short convergence time.