Precise Orbit Determination For Mixed-type BeiDou Constellation And Maneuvering Low Earth Orbiters

Precise orbit determination is crutial for global navigation satellite system and high resolution earth observation system.In order to promote the performance of both systems and expand the field of relative applications,it is prospective to improve the precision and reliability of satellite orbit(s).This thesis mainly focuses on precise orbit determination for mixed-type BeiDou costellation and maneuvering low Earth orbiters.The contributions are summarized as follows.(1)Extensive 1-cycle slips have been found in low-elevation BeiDou GEO carrier phase observations,which are collected by the MGEX tracking stations.We proposed an enhanced cycle slip detection method based on the series of dual-frequency phase geometry-free combinations.To deal with the effect of ionospheric variation,the longterm trend is first removed from the L1-L2 series by a robust polynomial fit algorithm.Then,the fit residuals are treated as a nonstationary and heteroskedastic time series.The GARCH model is employed to estimate the time-variant conditional variance,which will be used as an adaptive threshold in cycle slip detection subsequently.Simulated and real data tests reveal that most of the 1-cycle slips can be detected by the proposed method.The proper modeling for ionospheric variation even allows us to detect small cycle slips in the case of ionospheric scintillation.(2)The performance of CODE's new solar radiation pressure(SRP)model EECOM is analyzed on precise orbit determination for mixed-type BeiDou constellation.The validation of Satellite Laser Ranging(SLR)reveals that the EECOM model can improve orbital accuracy of GEO satellites by 17.4% and 35.1%,compared with traditional CODE's ECOM-9 and ECOM-5 models,respectively.As to IGSO and MEO satellites,however,the ECOM-5 model performs better than EECOM as well as ECOM-9 model.It means that CODE's new SRP model is not able to improve orbital accuracy of IGSO and MEO satellites.External orbit validation by IGS data analysis centers of GFZ,WHU and CODE indicates that the accuracy(3D RMS)of our current BeiDou precise orbit is 1-4 m for GEO satellites,25-30 cm for IGSO satellites and 10-20 cm for MEO satellites,respectively.(3)Orbital maneuvers are usually performed as needed for Low Earth Orbiters(LEOs)to maintain a predefined trajectory or formation-flying configuration.To avoid unexpected discontinuities and to connect pre-and post-maneuver arcs with a minimal set of parameters,a maneuver has to be considered in the routine GPS based orbit determinations.We propose a maneuver handling method in a reduced-dynamic scheme.With the proper thrust modeling and numerical integration strategy,the effects caused by orbital maneuver can be largely eliminated.The performance for both single-satellite precise orbit determination(POD)and inter-satellite precise baseline determination(PBD)are demonstrated using selected data sets from the Gravity Recovery and Climate Experiment(GRACE)mission.For the POD results,external orbit validation shows that the GRACE-B orbits obtained from our approach match the DLR reference orbits better than3 cm(3D RMS).For the PBD results,an RMS of the K-Band Ranging system(KBR)residuals of better than 0.7 mm can be achieved.Furthermore,the actual maneuver performance derived from the POD results provides rigorous feedback on the thruster system,which is not only beneficial for current maneuver assessment but also for future maneuver plans.(4)High-precision inter-satellite baseline determination is a fundamental of the distributed InSAR system.However,the Center-of-Mass(CoM)of satellite in orbit is usually different from the point calibrated on ground.First of all,the CoM errors of 1 cm have been added to the satellite-fixed x,y and z direction,respectively.Simulation results reveal that the effects of such 1 cm errors in x and y directions are very tiny and can be neglected safely.While the effect of CoM errors in z direction is significant and results in a systematic variation in the baseline product.We propose two methods in GNSS-based InSAR baseline determination to mitigate the CoM errors: a)by adding constant empirical acceleration in the radial direction;b)by adding a bias parameter in z direction of the satellite-fixed frame.Both methods are validated by simulation and are proved to be effective.More than 98% of the CoM errors can be mitigated.