Research On BDS Precision Orbit Determination And LEOs-based Augmentation Sytem

BeiDou satellite navigation system(BDS)is in the global deployment stage and it will be developed as a Global Navigation Satellite System(GNSS)by the end of 2020.The determination of satellite orbit and clock,as well as the timely dissemination of information are key factors to maintain the operation and development of GNSS.However,there is still lack of maturely corresponding theory and method to support Precise Orbit Determination(POD)of BDS as its relative short-term of development and operation.In order to further improve the service performance,the relevant models and algorithms for BDS POD must be refined based on the characteristics of the system itself and the current state of development.Meanwhile,it is difficult for the BDS ground control and monitoring stations to achieve global deployment.The stability and accuracy of BDS satellite's orbit and clock are constrained by the limitation of tracking arc.Although China's Inter-Satellite Link technology has been initially implemented,there is still uncertainty,and the design of corresponding backup and enhancement solutions is an urgent problem to be solved.Therefore,it is significantly important to conduct relative researches on the issues of the models,methods and new technologies which are suitable for BDS POD to improve the global competition and overall popularization and application of BDS.This dissertation focuses on the problem of BDS POD and explores LEO-based augmentation technologies from the perspective of new technology.The main contents,results and conclusions of this dissertation are as follows:(1)Aiming at the development trend of compatibility and interoperability between BDS and other GNSSs,the algorithm and processing strategy of the multi-GNSS combined POD are studied and presented.The analytical impact of multi-GNSS combined POD is deduced,and numerical results demonstrated that the GPS/BDS dual system combined POD can significantly improve the accuracy of BDS satellites' orbit and clock.Based on the above theories and methods,the precise orbit and clock products of the GPS,GLONASS,Galileo,and BDS are determined under the unified space-time framework system.The analysis results show that the obtained products can achieve the same accuracy of other similar products.(2)In terms of different types of satellites and various attitude control modes coexisting within BDS,the influences of different attitude modes on POD are compared and analyzed.Two improved POD strategies with pseudo stochastic pulses and empirical force parameters are used to improve the orbit accuracy of IGSO and MEO satellites in the yaw-fixed mode.Based on the measured data,it is verified that BDS IGSO-6 and BDS-3e can provide stable and reliable orbits by conventional strategy during eclipse period.(3)For the modeling of solar radiation pressure(SRP)of BDS GEO satellites,long-term precision ephemeris data is used to invert the time series of ECOM model parameters,and POD experiments are carried out to compare the impact of three optical SRP parameter selection strategies on GEO satellites POD.The results demonstrate that D-direction periodic term parameters of the ECOM model have significant impacts on GEO satellite POD,while the influence of Y-direction periodic term parameter is relatively non-significant.The measured results are in good agreement with the conclusion of the precision ephemeris data analysis.Therefore,it is recommended that 7-parameter or 9-parameter ECOM model is more suitable for GEO satellite POD during the spring or autumn equinox.(4)Ground-based and space-borne observation data is used to analyze the Satellite-induced Code Pseudo range Bias(SCB)effect of BDS-2,BDS-3 and BDS-3e satellites,and a BDS Ambiguity Resolution(AR)method taking account of this error is derived.The experimental results suggest that: 1)The SCB effect of the BDS-2 IGSO and MEO satellite has a certain influence on the ambiguity fixing.Correcting this error can improve the accuracy of the BDS-2 IGSO and MEO satellites.2)The multipath effect of BDS-3 and BDS-3e satellites is distributed as white-noise distribution,and the signal quality of BDS-3 satellites is better than that of BDS-3e satellites.There is no obvious SCB effect can be observed in the Code signal of BDS-3e and BDS-3 satellites.(5)Dealing with the urgent problem that the BDS regional monitoring network cannot cover the full arc of IGSO and MEO satellites,a new scheme that utilizing LEO satellites as space-based monitoring stations is proposed.The necessity and technical characteristics of deploying space-based monitoring stations are elaborated.We provide a mathematical model of joint orbit determination of ground-based and space-based monitoring network and design the corresponding data processing scheme.The experimental results show that: 1)When introduce two GRACE satellites,the three-dimensional accuracy of the GPS satellite POD by a global network and a regional network has been improved by an average of 20.0% and 44.3% respectively,and the regional network's orbit prediction accuracy for the period of 6h and 24 h is better than 10 cm and 13 cm respectively.2)The LEO satellite can significantly enhance the observation geometry of BDS GEOs,and the three-dimensional accuracy of GEO satellite POD by a global network and a regional network is upgraded by m levels.However,due to the incompleteness of the BDS constellations and the unsatisfactory on-board observational data,the POD accuracy of the IGSO and MEO satellites is not significantly improved.6)In order to enhance the service performance of the Beidou Global System,a scheme based on LEO constellation is proposed.The preliminary design of this LEO-based enhancement system is carried out,the system configuration and task assignments are planned,and the work mode and work flow compatible with Beidou Global System are discussed.Based on the self-developed simulation software,the influence of the number of the LEOs on the BDS regional POD is studied.The simulation results show that the addition of 10 LEOs on the basis of 8 regional monitoring stations can effectively improve the orbit determination accuracy of BDS satellite,however,the improvement on the orbit determination by increasing the number of LEO satellites can not be significantly found.Finally,the contribution of LEO navigation constellation to global service performance of BDS is analyzed from the perspective of coverage and PDOP values.The results indicate that the service performance is greatly improved by the LEO navigation constellation in North and South America as well as high latitude regions,it is of great significance to the globalization of the Beidou system.