Research On GNSS Real-time Precise Satellite Clock Fast Estimation

With the continuous development of the Global Navigation Satellite System(GNSS),high precision,high reliability,high compatibility and real-time accessibility have gradually become the focus of attention.Precise Point Positioning(PPP)with the characteristics of simple model,good real-time performance and flexible positioning,has become a research hotspot in the field of navigation.It has been widely adopted,but it relies on the support of real-time high-precision clock products in real-time precision positioning.At this stage,IGS has provided real-time orbits with accuracy comparable to that of post products,but limited by factors such as the stability of the satellite clock and the space environment in which the satellite clocks must be estimated in real time through observations from ground tracking stations.Studying real-time precision clock estimation is of great significance for real-time precision single-point positioning.With the addition of more data such as GLONASS,GALILEO and Beidou navigation system,while improving the performance of navigation services,it also restricts the timeliness of data update and brings new challenges to GNSS data processing.1.The basic theory of GNSS precision data processing is systematically studied.The main error sources and related correction models of GNSS observations are studied.The GNSS data processing methods of this paper are introduced,including basic mathematical models,parameter estimation methods and related quality control strategies.2.The theory and method of GNSS real-time precision clock estimation are studied.Based on the original observation equation of GNSS,the implicit clock and ambiguity reference conditions in the ionospheric-free combination estimation model are given.The correlation between satellite clock error and receiver clock error is analyzed and the time reference is given.Aiming at the problem that the clock calculation efficiency is low in the un-difference mode,a method using a graphics processing unit(GPU)is proposed to accelerate the GPS/GLONASS/GALILEO/BDS four with 90 stations.Taking this satellite clock calculation as an example,the average time consumption of a single epoch is decreased from the initial 78.15 s to 0.86 s,which fully satisfies the requirement of 1Hz high-frequency clock.On the basis of improving the efficiency of GNSS data calculation,real-time precision clock error estimation of GPS/GLO/GAL/BDS is carried out by using the measured data of 061 to 067 days in 2019,and the solution is discussed.Efficiency and accuracy,the results show that the estimated clock agrees well with the final clock difference of WHS,most of which is within 0.1 ns.The GPS/GAL/BDS satellite wide-lane and narrow-lane fractional products are extracted by using the ambiguity of the clock difference estimation,and their respective changes in the time domain are analyzed,and the FCB products are verified through the ambiguity residual.The ambiguity of the clock difference solution is fixed by the solution of the wide and narrow lane products.The results show that the fixed rate of non-difference ambiguity of GPS and GALILEO system is 75% and 76%,but for the clock precision after convergence The fixed solution clock is not much improved compared to the floating point solution clock.3.The theory and method of fixing un-differenced ambiguities at a single station are studied.The real-time satellite clock products and the wide-line and narrow-lane FCB products extracted during the clock estimation are used to verify the PPP application.Firstly,the positioning performance of float solution and fixed solution of GPS/GLO/GAL/BDS is compared.The results show that the float PPP positioning accuracy of each system can reach the level of 2～5 cm in horizontal and 5～10 cm in vertical.After applying the wide-line and narrow-lane fractional products to fix the ambiguity,the positioning accuracy of each system in the East direction is greatly improved,and the positioning accuracy is equivalent to the North direction,but improvement of the Up direction is not obvious.If the correct un-difference ambiguity is quickly fixed,the PPP convergence can be significantly accelerated.Multi-GNSS can also significantly accelerate the convergence of PPP.Especially after adding GLONASS system,the positioning accuracy and convergence speed are obviously improved.After the ambiguity is fixed,the horizontal precision of multi-GNSS float solution is improved from 1 to 2 cm to within 1 cm,the vertical precision is improved to about 2 cm,and multi-GNSS also helps to improve the ambiguity fixed rate,and the fixed rate is mostly above 98%.4.Developed a real-time precision positioning service system,gave the basic framework of the software,and used 61 real-time stations of MGEX network and Hong Kong CORS network to verify the operating efficiency,real-time product integrity rate and the accuracy of the system.Finally,the real-time PPP is used for verification.The results show that the clock precision generated by the system is comparable to the realtime products provided by IGS analysis centers,and the real-time PPP positioning horizontal accuracy is better than 5cm,while the elevation is better than 10 cm.