Study On BDS Differential Code Biases Estimation And The Performance Analysis Of Real-time Ionospheric Product

Ionospheric delay is one of the main errors of GNSS navigation and positioning,which should be taken into consideration.The ionospheric model can correct the ionospheric delay for single-frequency users,and accelerate the convergence speed for double-frequency/multi-frequency users.Differential code bias is a subsidiary product of ionospheric grid.It not only affects the accuracy of ionospheric model,but also has significant influence on amending clock offset datum of integrated positioning with different observations.To meet the needs of engineering survey and scientific research,international satellite navigation service and analysis centers have been conducting research to improve the accuracy and timeliness of ionospheric products.Since 2018,Beidou Data Analysis Center of China University of Mining and Technology has been estimating global real-time ionospheric products.However,ionospheric products have not been released officially due to the internal testing stage.In this paper,the main contents and conclusions are as follows.(1)We introduced the basic theory of GNSS navigation and the theoretical methods for monitoring and modeling ionosphere in detail.Specifically,the combinational schemes of some commonly used linear observations,and their comparison in terms of application conditions and advantages or disadvantages are discussed;the advantages and disadvantages of two cycle slip detection methods in data processing are summarized and verified by experimental test;and the conventional methods and theories for ionospheric monitoring and modeling are introduced.(2)In this study,we proposed a multi-frequency differential code biases estimation method for BD-2 satellites based on closure difference constraint.The test results show that the differences are trivial between estimated multi-frequency differential code biases and those calculated by CAS.Due to the limitation of station conditions,the stability of DCB estimated by DCB and CAS in this paper is lower than that published by DLR.Besides,we analyzed the influence of DCB correction on the positioning results of different combinations of observations in different positioning modes.(3)This study explored the principle and method of global real-time ionospheric product in Beidou Data Analysis Center of China University of Mining and Technology.We compared the products against the final ionospheric product ‘whug' from Wuhan University Analysis Center.Statistical results show that the accuracy of the real-time ionospheric products is equivalent to that of final products,and the difference is within 5 TECu,which meets the requirements of ionospheric products accuracy.Besides,we also analyzed the impact of real-time ionospheric products on standard single point positioning and undifferenced and uncombined PPP.