Precise Orbit Determination Of Low Earth Orbit Satellites And Gravity Recovery

Low Earth Orbit(LEO)satellites are one of the hotspots in space technology research.The precise orbit of LEO satellites contain gravity field information,which can be used to recover the Earth gravity field model.The Earth gravity field model is of great significance to study the internal structure and mass distribution of the Earth.In this paper,we focus on Precise Orbit Determination(POD)for LEO satellites such as GRACE-FO and SWARM with cm-level accuracy,and carry out the research on LEO POD scheme and orbit validation.Finally,the time-variable gravity field model SWARM?TV and static gravity field model SWARM?ST were recovered by using the self-determined SWARM kinematic(KIN)orbits.The main contents are as follows:(1)Taking GRACE-FO-A as an example to evaluate the influence of attitude data and antenna phase center correction on LEO POD accuracy.Different calculation schemes are designed to determine the KIN and reduced-dynamic(RD)orbits of GRACE-FO-A for DOY60-74 in 2019.The results show that in the 3D direction,the influence of attitude data on KIN POD and RD POD is 1?2 mm.The pseudo-stochastic parameters used in the RD POD have some absorption effect on the error of antenna phase center.Therefore,after adding the antenna phase center correction,the improvement effect on the RD POD accuracy is relatively not obvious,and the 15 days average RMS improvement in the 3D direction is less than 1 mm.KIN POD does not use any dynamic model.Therefore,after adding the antenna phase center correction,the KIN POD accuracy can be improved obviously.In the 3D direction,the average RMS in 15 days is increased by about 3 mm,and the maximum increase in RMS is up to 7mm in a single day.(2)Two schemes of “LEO POD first,then convert orbits” and “convert ephemeris first,then LEO POD” were designed to determine the KIN and RD orbits of GRACE-A for DOY1-7 in 2012.The difference of orbits accuracy between the schemes are discussed to study the conversion method of LEO satellite precise orbit under different reference frames.The results show that the optimal scheme is “LEO POD first,then convert orbits”,that is,to determine the LEO satellite orbits under the reference frame of the precise ephemeris products corresponding to the LEO satellite flight period,and then to convert the orbits to the target reference frame by using the 7-parameter method.(3)Based on the analysis of the POD schemes,the KIN and RD orbits of SWARM in2017 and GRACE-FO in 2019 are determined.The scientific orbits provided by JPL and ESA were used to validate the orbits of GRACE-FO and SWARM,it is concluded that the one-year average RMS of KIN orbits in 3D directions of GRACE-FO-A,GRACE-FO-B,SWARM-A,SWARM-B and SWARM-C are 2.92 cm,3.08 cm,4.15 cm,3.72 cm and 4.40 cm respectively;the one-year average RMS of RD orbits in 3D directions of GRACE-FO-A,GRACE-FO-B,SWARM-A,SWARM-B and SWARM-C are 2.23 cm,2.39 cm,3.76 cm,3.17 cm and 4.02 cm respectively.Using KBR data to validate the KIN and RD orbits of GRACE-FO,the results show that there is no obvious systematic error in the residual,and the one-year average RMS is 1.48 cm and 0.79 cm respectively.Using SLR data to validate the RD orbits of GRACE-FO and SWARM,it is concluded that the one-year average RMS of GRACE-FO-A and GRACEFO-B are 2.21 cm and 2.03 cm respectively;the one-year average RMS of SWARM-A,SWARM-B and SWARM-C are 2.77 cm,2.56 cm and 2.91 cm respectively.The accuracy of GRACE-FO and SWARM POD reaches cm level.(4)Based on the KIN orbits of SWARM-A/B/C in 2017 determined in this paper,12 sets of monthly time-variable gravity field model SWARM?TV up to 60 d/o(degree and order)and static gravity field model SWARM?ST up to 60 d/o are recovered,and the quality of the models are evaluated.In the Amazon River Basin,the SWARM?TV model is consistent with ITSG-Grace2018 and COST-G SWARM model in large-scale at the same period.Using GNSS leveling,deflection of the vertical and marine gravity anomaly data to evaluate SWARM?ST model,the results show that the SWARM?ST model is reliable up to 40 degree,and the accuracy is closer to AIUB-CHAMP03 S,EGM2008 and ITG-Grace03 model.