Research On Real-time Kinematic Precise Orbit Determination Of Low-earth-orbit Satellites

Real-Time(RT)Precise Orbit Determination(POD)of Low-Earth-Orbit(LEO)satellites is of great significance for environment monitoring,maneuver control and satellite autonomous navigation applications.On-board Global Navigation Satellite System(GNSS)technology is a reliable way to realize RT POD.In recent years,thanks to the development of the technology of real-time Precise Point Positioning(PPP)and the construction of multi-GNSS,the accuracy of RT POD is hopefully to be greatly improved.However,compared with precise positioning on the ground,the POD of LEO has its particularity.The accuracy and convergence time of on-board real-time PPP remain unclear currently.Furthermore,the orbit of LEO is predictable,which means that the orbit determination performance can be enhanced with the rational use of priori orbits.This research focuses on how to improve the real-time capability,accuracy,convergence speed and stability of on-board GNSS orbit determination,and elaborates the Real-Time Kinematic Precise Orbit Determination(RTKPOD)method from three aspects,that is to say,the basic implementation of RTKPOD,the priori orbit constraint method,and the feasibility analysis of on-board BDS/GPS combined real-time orbit determination.The main research results and innovations are as follows:(1)The broadcast ephemeris,predicted part of ultra-rapid product and real-time precise product are adopted respectively,and different clock intervals and observation intervals are considered to propose various RT POD solutions.The results of case studies show that the accuracies of broadcast ephemeris solutions are at 1-m level,the accuracies of IGU solutions are at decimeter level,and the accuracies of IGC and CNT solutions are at centimeter level.CNT solutions are slightly better than IGC solutions.There are no obvious differences between the solutions with a clock interval of 5 s and 30 s,5-s solutions perform a little better.Shortening the observation interval helps to improve the accuracy and stability.Compared with the solutions with an observation interval of 10 s,solutions with a 1-s observation interval can enhance the accuracy by 9%,9% and 4% in the radial,along and cross component,respectively.The relevant test results can provide references for ephemeris selection and sampling interval setting in actual application scenarios.(2)In view of the fact that LEO may have the advantage of a priori orbit in the context of realtime applications,we present a scheme that the LEO priori orbit constraint(POC)is used in the process of RT POD so that the accuracy,convergence speed and stability of orbit determination are improved.The test results demonstrate that using the priori orbit with a 1-m standard deviation improves the accuracy by 30.6%,36.5% and 43.3% in the radial,along and cross component,respectively,while the convergence time is decreased from 31 min to 4 min.It is revealed that an ideal priori orbit should be of less noise and avoid long-term systematic biases.(3)A priori orbit constraint method based on Sage adaptive filtering is proposed.Using the priori orbit from three-order Chebyshev polynomial extrapolation,an adaptive orbit determination algorithm considering polynomial extrapolation orbit constraint is synthesized.This algorithm can enhance the stability of orbit determination and slightly improve the accuracy in the cross component.The re-convergence time after the loss of lock is also shortened by about 50%.(4)The quality of on-board BDS/GPS observation data from FY-3C is analyzed.The results show that the on-board observation condition of BDS is inferior to GPS,only about 33.3% epochs of the whole day can observe more than 4 BDS satellites.Then,the quality of BDS real-time orbit and clock products provided by CNES is assessed and validated,which indicates that the products are not stable enough currently and suffer from the issues of unavailability and discontinuity,along with the limited product accuracy.Owing to the limited on-board BDS channels,the limited coverage area of BDS-2 and the unstable quality of BDS real-time precise products,the number of usable BDS satellites of FY-3C is few in real-time scenarios.Hence,full-arc real-time kinematic POD with onboard BDS standalone observations cannot be implemented currently,and the performance of BDS/GPS combined real-time orbit determination relies mainly on the GPS observations.(5)BDS real-time PPP and BDS/GPS real-time PPP are realized using the observation from ground stations.The results show that after a sufficient convergence,the accuracy of BDS real-time PPP in simulated kinematic mode reaches 3.02 cm,3.24 cm and 5.41 cm in the north,east and up components,respectively,and the average convergence time is 95.6 min.As for BDS/GPS real-time PPP in simulated kinematic mode,an accuracy of 1.56 cm,2.39 cm and 4.26 cm can be achieved,which is improved by 48.3%,26.2% and 21.3% compared with BDS standalone.The average convergence time of BDS/GPS is 37.1 min,reduced by 61.2%.Afterwards,on-board GPS and onboard BDS/GPS kinematic POD are implemented based on the on-board observations from FY-3C.The stability of POD using on-board BDS/GPS is improved compared with on-board GPS.The accuracies in the radial component and along component are enhanced by 8.0% and 8.3%,while the cross component suffers a slight accuracy loss,which is owing to the inhomogeneous distribution of BDS-2 constellation and the inferior orbit accuracy of GEO satellites.These indicate that using onboard BDS standalone observations to realize real-time kinematic POD with high accuracy is feasible when the number of visible satellites is sufficient and real-time orbit and clock products are accurate enough.Besides,On-board BDS/GPS combined observations help to reach a much more stable and superior accuracy and to decrease the convergence time.With the construction of BDS-3 global system,on-board BDS standalone real-time POD is hopefully expected to be realized in the future.