Research On GNSS Satellite Real-time Precise Orbit Determination Technique

Real-time precise PNT service is one of the basic requirements in human activities.With the advantage of all time,global coverage and high precision,GNSS becomes one of the most popular techniques for real-time PNT service.While real-time PNT service with broadcast ephemeris is the most basic forms of GNSS-based real-time PNT service,it is hard to achieve decimeter or higher-accuracy,due to the orbit/clock error of broadcast ephemeris.The PPP technique is able to achieve real-time precise PNT service with a single receiver through the correction of GNSS satellite orbit and clock et al.error represented in the state space.Real-time precise GNSS satellite orbit and clock products are the prerequisite of real-time precise PPP.Therefore,this study focuses on the method and software for GNSS satellite real-time precise orbit and clock estimation,the major researches and achievements are as follows:· With the current research status reviewed,the trend of GNSS real-time PNT service is analyzed,and a more rigorous and flexible method,in which the real-time orbit and clock are estimated in an integrated way,is proposed.· The time and coordinate system,the orbit model and the measurement model are introduced in detail with emphasis on the handling of EOP and the modeling of satellite attitude in time and coordinate system,the perturbation forces in orbit model and the correction terms in observation model.· The GNSS real-time precise orbit determination model is derived.The high accuracy synchronization of observations is achieved with simplified state transition formulas,which avoids multiple integration in the interpolation algorithm.With the IGS dual-frequency IF-combination clock datum,the IFCB at both station and satellite end are considered,and there are no hardware delays absorbed by the phase residual in the third frequency.The real-time quality control and fast estimation method are studied.The Open MP parallel algorithm is introduced into real-time precise orbit determination and the robust extended Kalman filter with linear algebra library acceleration is given.A suite of algorithms for GNSS real-time precise orbit determination are set up and the corresponding software is developed.· The methods for improving the efficiency of real-time precise orbit determination are compared,include sequential processing of observations in measurement update,the Open MP parallel acceleration,the Eigen library acceleration and the Open BLAS acceleration.The results indicate that,the Open MP improves the efficiency slightly compared to sequential processing,and the linear algebra library acceleration is more efficient than the Open MP parallel acceleration.Therefore,the use of libraries is recommended when available.With about 100 stations(1500parameters),the parameter estimation time for GPS is around 2.0s,which is able to satisfy real-time services within 10.0s.· The accuracies of GPS/Galileo real-time precise orbit determination are evaluated.In consideration of the weak correlation of systems,the estimation is separated for each system.The results indicate that,the RMS of GPS dual-frequency real-time orbit achieve 3.0cm(radial),5.0cm(tangential)and 4.0cm(normal),the STD of realtime clock is about 0.10 ns,and the RMS of Galileo dual-frequency real-time orbit achieve 6.0cm(radial),8.0cm(tangential)and 5.0cm(normal),the STD of real-time clock is about 0.16 ns.Due to the model differences,there exists a systematic bias of1.0-2.0cm in the radial direction and the STD of clock is not affected.In general,the accuracy of real-time orbit and clock reach the international level of similar products.· The process noise of real-time precise orbit determination is analyzed.To avoid the divergence of filter,a relative conservative value is suggested.The influence factors of real-time orbit convergence are compared,include the initial accuracy of satellite position,velocity and SRP coefficients.The results indicate that,the convergence is more sensitive to initial position accuracy than initial velocity accuracy,and the apriori SRP model can reduce the radial error in the initial phase.Due to the constraint of force models,the convergence of radial direction is slower than the other two directions.In general,about 12 hours is needed to achieve a complete convergence of real-time orbit.· The correlation of parameters in real-time precise orbit determination is analyzed,include the ambiguity parameters,the ZWD parameters,the orbit parameters and the clock parameters.The results indicate that,for parameters of same type,the correlation is higher for the same station/satellite than different station/satellite,the situation for clock parameters is different,there is a linear correlation between station clock and satellite clock,with the datum introduced,the correlations of clock parameters between different station/satellite are close to 1.0.;for parameters of different type,due to the various degrees of correlation with ambiguity parameters,the ambiguity resolution is able to improve the accuracy of ZWD parameters,the orbit parameters and the clock parameters.· The IFCB in multi-frequency data processing is studied.The results indicate that,the IFCBs of GPS triple-frequency satellites show apparent time-varying characteristics,compared to that of BLOCK IIF satellites,the IFCB of BLOCK IIIA satellite is largely reduced;the IFCBs of Galileo triple-frequency satellites are nearly constants,when corrected with DCB products for the IFCBs at satellite end,the phase residuals show anomaly at day-boundaries due to the discontinuity of DCBs;since the observation information of dual-frequency is already sufficient,the triple-frequency has no apparent advantages than dual-frequency in real-time precise orbit determination currently,except for very extreme situation.· The UPD in undifferenced ambiguity resolution is studied.The independent ambiguity datum is generated with the minimum spanning tree algorithm and maintained with the UPD.The float ambiguities are corrected with the convergenced satellite UPD to perform quality control.The results indicate that,the undifferenced wide-lane UPDs of GPS are rather stable with daily variations less than 0.20 cycle and STD less than 0.10 cycle except for some satellites,while that of Galileo are very stable with daily variations less than 0.10 cycle and STD less than 0.05cycle;the GPS dual-frequency real-time undifferenced wide-lane UPDs agree well with the post-processed products from CNES with difference less than 0.10 cycle.