| The GNSS technique has been developing for a few decades, which provides high accuracy and reliable navigation and positioning service in real time and post modes for wide area applications. The GNSS real time precise positioning system enhances the fundamental GNSS systems by providing real time precise orbit, clock and ionosphere products, enabling precise positioning anytime and anywhere. With the developing and improving of the real-time products, the GNSS real-time service system will expand its application area further, such as LEO precise orbit determination, space weather monitoring, earthquake monitoring, tsunami alarming, plate motion and Earth dynamic research etc. Consequently, the GNSS real-time precise service and application techniques becomes a hot area of research.The positions of navigation satellites service as the known dynamic reference, which is directly related to the users'positioning accuracy. Continuous and reliable precise orbit product provides the precondition and guarantee for the real-time precise service. The PPP-RTK, which is a new technique realizing real-time cm-accuracy positioning in wide area, raises even higher demand on instantaneity and accuracy of orbit products. Traditionally, the real time orbit is predicted based on the legacy observations, which is highly dependent on the accuracy of dynamic model. However, for the Chinese BeiDou regional navigation system, many constraints, i.e. the inaccurate dynamic force model, the switching of different attitude mode and orbit maneuver, reduce the accuracy and reliability of the predicted orbit. As a result, it is of significant meaning to develop the new real time precise orbit determination theory, algorithm and software system to promote the precise real time service capability of BDS.This dissertation systematically studied the key issues of GNSS real time precise orbit determination, such as the BDS satellite attitude control modeling, real time precise orbit determination during the eclipse period and orbit maneuver management. And on mis basis, this paper proposed the real time orbit determination strategy that employs the adaptive square root information filtering. This strategy has been implemented to develop a software system for multi-GNSS precise orbit determination by filtering the real time observations. The strategy and software system have been tested on real data, of which the algorithm and model validity, and product performance have been verified. Specifically, the work includes the following tasks and contribution.1) This paper systematically studies the navigation satellite precise orbit determination theory from the three respects of satellite dynamic model, observation model and parameter estimation. Specifically, the issues that satellite motion equation and perturbation force model, observation errors and observation equation linearization, least squares method and square root information filtering have been investigated. Of all the aspects, more emphasis has been placed on the SRIF method to summarize the processing flow, deduce the nonlinear errors and process noise in the orbit filtering.2) The work summarizes and analyzes the current satellite yaw attitude models, the inverse PPP method for the attitude angle estimation, and the yaw attitude switching mechanism of BDS IGSO and MEO satellites. However, since the recent Precise Orbit Determination (POD) processing for GPS satellites could not provide suitable products for estimating BDS attitude using the reverse PPP because of the special attitude control switching between the nominal and the orbit-normal mode, a modified processing schema was proposed for studying the attitude behavior of the BDS satellites. In the modified processing, only data before the attitude switch is employed in the POD processing. This avoids the effect of the inaccurate attitude model on those parameters of the satellite orbit and ambiguities which are fixed in the phase center offset estimation later on. After this modification, yaw-angles of BeiDou satellites are estimated with the reverse PPP and a preliminary attitude switch model is established and validated. In general, the attitude-mode-switch occurs when the Sun elevation is about 4° and the actual orientation is very close to its target one (the threshold is empirically set to 5°based on lots of computation and analysis). In a special case that the Sun elevation is about 4°, while the yaw angle is empirically larger than 5 but smaller than 20° and the absolute value of yaw angle is increasing, the attitude-switch is executed too. Applying this preliminary attitude switch model to the BeiDou satellite orbit determination observables gives an improved performance compared to the yaw-steering model3) This paper refines the observation model the quality control method, and realizes ambiguity fixing in real time precise orbit determination. Based on the real observations, experimental result shows that the GPS and BDS 3-D orbit accuracy of the ambiguity-fixed solution is improved by 2 cm compared to that of the ambiguity-float solution.4) For the precise orbit determination of maneuver satellite, this paper investigates the aspects of maneuver detection, precise orbit determination during the maneuver period, and quick orbit recovery after maneuver. A new maneuver detection method is developed based on the predicted residuals, and the adaptive square root filtering method is used to determine the real time precise orbit during the maneuver. The proposed method is verified by the real maneuver data of BDS C05 and C08 satellites. Results show that, the proposed method can detect orbit maneuver effectively, and avoid filter divergence caused by orbit maneuver, and the maximum residual is less than 0.3 m for the maneuver satellites. The adaptive filtering can guarantee the parameter continuity, which cuts down the recovery time dramatically to 3-6 h to restore the normal orbit accuracy after maneuver.5) The work applies the real time precise orbit filtering algorithm to the GPS satellites in eclipse, and the efficiency is verified. Experiment results shows that, the orbit of satellites in eclipse determined by the real time precise orbit filtering method is significantly better than the IGU orbit product. If slightly increase the process noise of satellites in eclipse orbit filtering, the 3-D orbit accuracy of the GPS BLOCK IIA satellite can be further improved.6) Based on the PANDA software of Wuhan University, the multi-GNSS real time precise orbit determination system which employs the square root information filter has been developed. The software system has been tested by one-month data. The results show that, compared to the IGS final precise orbit products, the 1-D RMS of GPS and GLONASS real time orbit is 3.9 cm and 5.4 cm. The SLR residuals of the real time orbit of BDS C08, C10 and C11 is-5.7 cm,-3.6 cm and-1.4 cm respectively, which is slightly larger than that of the post precise orbit. The SLR residual of C01 real time orbit is -22 cm, of which the system bias is reduced by 18 cm compared to the -40 cm SLR residual of post orbit. The real time filtered orbit and the traditional predicted ultra-fast orbit were employed to determine the real time clock specifically. The real time clock results show that the real time orbit filtering method can avoid the clock jump caused by the discontinuity of predicted orbit. At last, the real time products were used to for kinematic PPP. The positioning results show that, compared to the predicted real time product, the real time product generated by the proposed real time filtering method improve the BDS-only kinematic positioning accuracy significantly and the accuracy of 3 cm in the horizon and 5 cm in height can be reached for the multi-GNSS positioning. |
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