Precise Orbit Determination Of LEO Based On Spaceborne Multi-GNSS Data And The Enhancement For GNSS Satellites

Low earth orbit(LEO)are spacecrafts that cycling the Earth with low orbit altitude and very fast speed.Therefore,in the past decades,LEO satellites have been widely used for many scientific missions such as the recovery of the earth gravity field,monitoring of the oceanic environment,and radio occultation.Precise orbits are prerequisite for successful implementation of these scientific missions.In recent years,Global Navigation Satellite System(GNSS)has become the mainstream technique to determine the orbit of LEO satellites.The Bei Dou Navigation Satellite System(BDS)is developed independently by China,and research on precise orbit determination(POD)for LEO satellites based on BDS-2/3 and multi-GNSS spaceborne observations is significant for promoting the development of China’s integrated PNT system.This thesis focus on the POD of LEO satellites and the enhancement for navigation satellites based on BDS and multi-GNSS spaceborn observations,and the key issues in the orbit determination process are investigated.The main research contents and results of this thesis are summarized as follows:1.The multipath error model and phase center variation(PCV)correction model for FY-3D and Tianjin University-1(TJU-01)satellites are established.Results demonstrate that the inclusion of the PCV correction model leads to a 13.4%enhancement in the carrier phase residuals of the FY-3D satellite.The spaceborne GPS/BDS data quality including the BDS-2/3 and GPS code multipath errors are comprehensively analyzed.Systematic multipath errors correlated with elevation are found for BDS-2 satellites,meanwhile slight near-field relevant multipath errors are found for GPS and BDS-3 medium earth orbit(MEO)satellites resulting from the intricate space environment.Basend on this,the GPS/BDS-2/3 code multipath errors are estimated as piece-wise models to mitigate the impact of multipath error on the LEO orbiting accuracy.Consequently,an orbit overlapping arc accuracy of 2-5 cm has been achieved.2.The orbit determination performance of multi-GNSS spaceborne data for LEO satellites are analyzed systematically,while the contribution of the BDS-3 to BDSbased POD and GPS/BDS combined POD are also studied.Results indicate that GPSbased orbital accuracy of FY-3D can obtain better than 2 cm in the three-dimensional(3D)direction,while the BDS-based solutions are only decimeter level,and GPS/BDS-2 combined POD can reach the same level as that of GPS-based solutions.The GPSbased,BDS-2/3-based,and GPS/BDS-2/3 combined POD of TJU-01 are achieved,and a good orbit consistency of 3.5 cm,2.5 cm,and 2.2 cm in 3D RMS,respectively,is obtained for the three types of solutions.Furthermore,results from both BDS-based and GPS/BDS-2/3 combined POD indicate that the orbit precision can be degraded by22.4% and 23.8% when BDS Geosynchronous orbit(GEO)satellites are involved into calculation.3.The fixed orbit solutions of TJU-01 and FY-3D satellites are obtained by using the double-difference(DD)and single-difference(SD)integer ambiguity resolution(IAR).Results show that with the addition of LEO satellites,more DD ambiguity fixation numbers are provided,so that the accuracy of DD IAR is improved.The uncalibrated phase delay(UPD)products estimated by ground and spaceborn GNSS observation data simultaneously can be successfully used for LEO SD IAR,which is slightly better than UPD products estimated only by ground stations.Both the DD IAR and SD IAR methods significantly improve the orbit accuracy of LEO satellites,with similar improvement effects of around 12%-16%.4.The satellite-ground combined POD is implemented,and the contribution of LEO satellite integration to the precise orbit determination of navigation satellites is quantitatively assessed.The LEO satellites are considered as mobile monitoring stations and combined with ground stations of different network structures to improve navigation satellite orbit accuracy.The results indicate that after adding two LEO satellites,the orbiting accuracy of GPS satellites in the 3D direction of global network and regional network is improved by 30.2% and 18.2%,respectively.Moreover,the accuracy of BDS GEO satellites in along direction is significantly improved by introducing LEO satellites.Specifically,with the addition of LEO satellites,the orbit accuracy in the 3D direction of BDS-2 IGSO satellites improves by 15.0% and 26.9%for global and regional network conditions,respectively,and 18.1% and 14.4% for MEO satellites.With the advantage of tracking to BDS-3 satellite,the effect of introducing TJU-01 satellite on the orbit accuracy of BDS-3 MEO satellites under the conditions of global network and regional network is analyzed,and the orbit accuracy of BDS-3 MEO satellites is improved by 5.3% and 4.4%,respectively.