Research On Performance Improvement Of Precise Relative Orbit Determination For Formation Flying LEOs Based On The Fusion Of Multi-source Data

Formation flying technique in low Earth orbiters(LEOs)has been widely used in many fields,such as Earth gravity field recovery and atmospheric density detection.Precise relative orbit determination(PROD)is a prerequisite for formation flying LEOs to complete specified mission targets.It is prospective to improve the precision and reliability of PROD of formation flying LEOs for implementing the task and expanding the field of relative applications.This thesis mainly focuses on some key problems in PROD,and research on performance improvement of PROD for formation flying LEOs based on the fusion of multi-source data.The contributions are summarized as follows.(1)Due to the observation errors of the onboard GNSS measurements have a negative impact in the PROD.The relative phase center variations(RPCVs)and single-difference(SD)MW model residuals variations(SD MWVs)estimation were proposed to improve the accuracy of PROD.Firstly,the parameters in SD model are estimated with fixed double-difference(DD)ambiguities constraint,and the RPCVs and SD MWVs are obtained.The methods were tested using the onboard GPS data from GRACE and GRACEFO.The maximum values for RPCVs and SD MWVs were 14 mm and 0.32 cycles,respectively.Then,after RPCVs correction,the the orbit overlap differences and the carrier phase observation minus computation(OC)residuals from DD ionosphere-free(IF)combination both decreased significantly,the KBR measurement consistency of GRACE and GRACE-FO increased by 30.1 % and 37.5 %,respectively.This shows that the RPCVs correction can effectively improve the accuracy of PROD.After SD MWVs correction,the DD wide-lane ambiguities fixing success rate of GRACE improved from 85.1 % to97.9 %,and the KBR measurement consistency increased from 0.65 mm to 0.57 mm,indicating that SD MWVs correction can effectively weakened the effect of multipath error on the success rate of the ambiguities resolution.In addition,the results of the RPCVs and SD MWVs estimations both have good stability.(2)The effect of GPS co-worked with BDS2 on PROD for Tian Hui-2(TH-2)are anaylzed.First,the quality of the onboard GNSS data of TH-2 is analyzed from the aspects of available data,code multipath errors and noise levels of carrier phase observations.Then,the differences of inter-agency comparison exhibit an RMS of 1.48 mm outside maneuver periods.The orbit overlap differences of GPS-based PROD are 0.71 mm in3 D,indicating that the accuracy of relative orbit determined by GPS is mm-level.The relative orbit obtained using GPS+BDS2 and BDS2 measurements is assessed using the GPS-based orbit as a reference.When the weight of GEOs is 1,0.5 and 0.2,the RMS of difference between the GPS-based and BDS2-based PROD is 7.08 mm,8.00 mm and11.40 mm in 3D,respectively.The RMS in the Asia-Pacific region and the non-AsiaPacific region is 2.89 mm and 7.96 mm when the weight of GEOs is 1,respectively.The results show that GEO satellites play important roles when BDS2 are applied to PROD of LEO satellites formation.Finally,after adding BDS2,the average of GPS ambiguity dilution of precision(ADOP)reduced from 0.160 cycle to 0.153 cycle.The RMS of the difference in 3D between the GPS+BDS2-based and GPS-based relative orbit is about1.2mm.Average of daily RMS of the 6h orbit overlapping differences in R,T and N are all within 1mm.Integrated GPS and BDS2 can further improve the accuracy of the float ambiguity,and the accuracy and reliability of PROD.(3)A method of united estimation to the parameters of the multi-satellite formation based on the fusion of GNSS and inter-satellite ranging data is proposed.The traditional method of orbit determination is mostly to determine the orbit of a single satellite one by one,it is inability to make full use of the information of inter-satellite ranging to improve the accuracy of orbit.The method based on undifferenced GPS data and intersatellite ranging,adding DD integer ambiguity constraint.The performance for PROD is demonstrated using in-flight data from GRACE and GRACE-FO.After inter-satellite ranging work with GNSS,the PROD accuracy of GRACE and GRACE-FO improved from0.95 mm and 0.84 mm to 0.30 mm and 0.21 mm,respectively.Then,in-flight data from the GRACE-FO and SWARM-A were selected to demonstrate that the method is efficacious.The accuracy of PROD of GRACE-FO improve from 0.83 mm to 0.19 mm when adding inter-satellite ranging to GNSS.The results of simulation experiment shows that the addition of inter-satellite ranging can effectively improve the accuracy of PROD in T direction of the LEOs flying in the same orbit,and improve the accuracy of PROD in R,T and N direction of the LEOs flying in the helix orbit.Integrating GNSS and inter-satellite ranging can further improve the accuracy of PROD.