Research On Precision Orbit Determination Theory And Method Of Low Earth Orbiter Based On GPS Technique

In the paper, the basic principle and methods of precision orbit determination (POD) of low earth orbiter (LEO) based on GPS technique are studied. The software of reduced dynamic technique for precise orbit determination of LEO using GPS is developed. To resolve the problems of reduced dynamic technique and extended Kalman filtering (EKF) technique in the LEO satellite orbit determination using GPS, the adaptive Kalman filtering (AKF) technique for LEO post-facto and real-time POD using GPS is researched. The technique for precise relative positioning of formation flying satellites using GPS is researched in the end of thesis. The main works and contributions are summarized as follows.1. The basic theory and method for this paper are presented. The advantages and disadvantages of batch and sequential estimation in the satellite orbit determination are compared and analyzed. The similarities and differences of LEO post-facto and real-time POD using GPS are discussed. The status of research on precise relative positioning of formation flying satellites using GPS is described. The paper points out that reduced dynamic EKF method which is easily realized bu software can meet the demand of post-facto and real-time POD research. Researching orbit determination about GRACE satellites can not only satisfy the needs of single satellite POD, but also satisfy the needs of relative positioning of formation flying satellite.2. Some influence factors on point positioning are analyzed. The parameter choice in the dynamic smoothing method is researched. The results show that the solar radiation pressure and atmospheric drag may not be taken into account in the dynamic smoothing. The dynamic smoothing can obtain high precise result by estimation the parameters of empirical forces. The precision of dynamic smoothing may be reduced when the parameters of empirical forces in radial(R) direction are increased, because of the number of parameter overabundance.3. The method of dynamic orbit determination using point positioning solutions is studied. The results show that point positioning solutions smoothed by the dynamic model can reduce random error effects and improve the point positioning solutions significantly. The accuracy of the dynamic smoothing orbit is better than 10 cm in both radial and normal direction, and 20 cm in the transverse direction compared to the orbit results provided by JPL.4. The application of robust estimation in the dynamic orbit determination using point positioning solutions is studied. The results show that the precision of orbit determination is improved with robust estimation which resists the outlying influences of the observations. But the improvement is limited if the observations are clean in which the measurements with errors larger than three times of root mean square (RMS) values are deleted.5. The method of kinematic orbit determination based on GPS with dynamics information is put forward and realized. The results show that the method of kinematic orbit determination with dynamics information can solve the phenomenon of discontinuity in pure kinematic orbit, but can not solve the steep disturbances. The steep disturbances may be solved by improved dynamics orbit. The precision of the orbit is at a level of better 10 cm in radial, transverse and normal direction respectively.6. The reduced dynamic technique (RDT) is researched in detail from the equations of satellite motion, empirical accelerations and observation equations. The steps of calculation LEO orbit using reduced dynamic technique are presented. The software of reduced dynamic technique (RDT) for precise orbit determination of LEO using GPS is developed. The feasibility of the software is validated by GRACE satellite orbit determination. The results show that the convergence speed is improved when the extended Kalman filtering (EKF) is employed. The accuracy of reduced dynamic orbit is better than 10 cm in radial, transverse and normal direction respectively.7. The influences of priori variance, steady state variance and correlation time on reduced dynamic approach are analyzed. The results show that the steady state variances of RTN empirical accelerations have significant sffects on the reduced dynamic orbit. The precision of orbit will be affected whether increase or decrease the value of the steady state variance of RTN empirical accelerations. An opportune steady state variance value exists which can provide properly weight between the information of observation and dynamic model to get high precision orbit.8. The improvements of Kalman smoothing, bidirectional filtering and robust estimation on reduced dynamic approach are discussed. The results show that POD using Kalman smoothing can improve the accuracy of reduced dynamic result at the beginning of the data arc. The result of bidirectional filtering can not only improve the accuracy of reduced dynamic result at the beginning of the data arc evidently, but also improve the accuracy of orbits in the all arc. The result of bidirectional filtering is about 4-6 cm in radial(R), transverse(T) and normal(N) direction respectively. The orbit of RDT is influenced when outlying measurements appear. While the influences of outlying measurements are resisted by using the equivalent covariance matrices of the observations and the satellite predicted states when introduce robust estimation in RDT.9. The method of extended Kalman filtering(EKF) POD which don't compute the parameter of random processes is put forward. The results show that the orbit is more smoothed than the orbit resolved by reduced dynamic method. Systematic error appears in normal direction in the orbit of the EKF method, but this systematic error can be removed by using bidirectional filtering. The orbit results of two GRACE satellites which computed using seven days data show that the result of bidirectional filtering can get 10 cm in 3-Dimension position and about 5 cm in radial(R), transverse(T) and normal(N) direction respectively. The precision of the orbit by EKF bidirectional filtering is better than the orbit by using reduced dynamic bidirectional filtering.10. An adaptive factor based on the discrepancy between the predicted state vector and the estimated state vector is constructed. The adaptive Kalman filtering (AKF) technique for LEO POD using GPS is realized. The results show that the precision of the orbit computed by EKF, AKF and reduced dynamic technique (RDT) is equivalent approximately, but the orbit corresponding to the first two methods is more smoothed than the orbit provided by the last method. When satellite orbit determined by using AKF, EKF and RDT bidirectional filtering, the orbit precision of GRACE-A and GRACE-B is better than 9 cm and 7 cm (3-dimensional) respectively.The precision of the orbit from the first two methods is better than that from the last method. The project of LEO real time POD using AKF technique, which using point positioning solutions as observation, is put forward and realized. The results show that the AKF technique can avoid setting the state noise compensation matrix, and the solution of AKF POD is more stable than the solution of EKF POD.11. The technique for post-factor precise relative positioning of formation flying satellites using onboard GPS is researched. The software of precise relative positioning of formation flying satellites is realized. The orbits of two GRACE satellites which computed using data with ten days show that the precision of relative positioning can get 5.0mm after the ambiguities fixed to integer values.