Precise Orbit And Clock Determination Of Bei Dou GEO/IGSO/MEO Mixed Constellation With Double-difference Method

The BeiDou Navigation Satellite System(BDS)is currently the only navigation system including the Geostationary Earth Orbit(GEO),Inclined Geosynchronous Orbit(IGSO)and Medium Earth Orbit(MEO)satellites.Many challenges exist in Precise Orbit Determination(POD)of BeiDou satellites duo to the specific design of the mixed navigation constellation.This paper focuses on improving orbits and clocks quality of BeiDou satellites based on double-difference method.Moreover,the performance of the BeiDou regional navigation satellite system applied to Precise Point Positioning(PPP)and POD of Low Earth Orbit(LEO)satellites is analyzed.The main work and contributions of this paper include the following four aspects:First,preprocessing methods of Global Navigation Satellite System(GNSS)observations are investigated with only raw observations and adding a priori orbit and clock products.A new cycle slip detection method is proposed for triple-frequency observations.The BDS and Global Positioning System(GPS)observations from international GNSS Monitor & Assessment System(iGMAS)and Multi-GNSS EXperiment(MGEX)stations are accessed in terms of the observation availability,the pseudorange multipath and the carrier phase noise.Second,the theory of BeiDou satellites POD with double-difference observations is studied.On the one hand,a new Solar Radiation Pressure(SRP)model specific for BeiDou GEO satellites is proposed to solve the classic models unable to depict their SRP.The BeiDou GEO satellite orbits are significantly stabilized with the new SRP model.In addition,the correlation coefficients between GEO orbit overlap accuracies and the elevation angle of the Sun have been obviously decreased by using the proposed model.On the other hand,based on the post-fit residuals,a novel method of selecting reference satellites is developed to obtain double-difference observations with high quality.Compared with the method based on elevation angles only,the accuracy of IGSO satellites orbit is evidently improved and the RMS of the post-fit residuals is decreased using the new method to choose the reference satellites.Third,double-difference ambiguities are introduced to estimate the navigation satellite and the LEO satellite clocks.A prior constraints including not only the traditional satellite orbits,tropospheric zenith path delays,station coordinates and Earth Orientation Parameters(EOP),but also the double-difference ambiguities are used for the navigation satellite clock solutions.For the LEO satellite clock estimations,the stability of the clock solutions is improved and edge effects are eliminated with the double-difference ambiguity constraint.In addition,the day boundary discontinuities are obviously weakened,and the accuracy of the clocks is improved over 30%.Finally,the performance of BeiDou regional navigation satellite system on determining the station positions and LEO satellite orbits is demonstrated.The static daily PPP results achieve an accuracy better than 1 cm in the horizontal and 5 cm in the vertical with the precise orbit and clock products obtained in this paper.More specifically,the accuracy of the LEO satellite orbits determined by BeiDou regional navigation satellite system over different regions is displayed with simulation data.The results show that the orbit quality of a single LEO satellite over the non-service area is similar to that over the service area.The baseline length accuracy is better than 5 mm over the service area,and it is still better than 1 cm over the non-service area.