Method And Application Of Real-time Clock Bias Estimation For LEO-based Navigation Augmentation GNSS Receiver

At present,there are still some problems for the Global Navigation Satellite System(GNSS),such as slow convergence of precise point positioning,the transmission rate of navigation message is low,the signal of GNSS is weak and easy to be jammed.The orbit height of LEO(low earth orbit)satellite is low,so the velocity of low-orbit satellite is fast and its constellation geometric change quickly,which can greatly shorten the convergence time of integer ambiguity.And the low orbit satellite has high data transmission rate and strong signal anti-interference ability.Using low orbit satellite to enhance GNSS can solve the problems existing in GNSS positioning.The next generation of BDS(Bei Dou Navigation Satellite System)development plan has clearly proposed the construction of using low-orbit satellite to enhanced GNSS positioning.Calculating and broadcasting real-time precise clock bias and orbit parameters of LEO satellite is the core problem of LEO enhancing GNSS.Due to the strong predictability of orbit parameters,the orbit parameters can be calculated and predicted by the analysis center.However,the prediction accuracy of real-time clock bias parameters decays significantly with the stability of onboard clock,so it can be considered that real-time clock bias estimation,short-term prediction and broadcast can be achieved on the onboard platform.In this process,it involves the estimation and prediction of real-time clock bias of LEO satellite and the transmission of message parameters.The accuracy of each item determines the accuracy of the clock bias received by the user,thus affecting the enhanced GNSS positioning of LEO satellite.Therefore,it is of great significance to study the real-time clock bias estimation,prediction and system parameter design of LEO satellite.For the above key problems,this paper studies the theory and method of real-time clock bias calculation of LEO satellite,and proposes a fast convergence algorithm constrained by first epoch clock bias to solve the problem of slow convergence of clock bias.According to the demand of ground users for real-time clock bias of LEO satellite,this paper discusses several clock bias prediction methods and the requirement of clock stability for clock bias prediction accuracy.Finally,the experimental verification of LEO enhancing GNSS on-board subsystem is carried out.The main work and related conclusions are as follows:1.The basic principle of on-board real-time clock bias calculation is studied.A fast convergence algorithm of using first epoch clock bias to constraint solution is proposed to solve the problem of slow convergence of clock bias at the first epoch and after a large number of satellite cycle slip,which makes it impossible to broadcast precise clock bias parameters.The experiment uses the on-board observation data of SF1 B satellite,and the real-time clock bias accuracy of 0.1ns is achieved by using Kalman filter.2.Different clock bias prediction models are studied.Taking two on-orbit satellites SF1 B and GRACE-C as examples,the accuracy of polynomial model,grey model and Kalman filter model in clock bias prediction of LEO satellites is analyzed.Research shows that grey model is better than quadratic polynomial model while only considering the prediction accuracy.For Grey model,although the short-term prediction accuracy is slightly higher,and the prediction accuracy will not decline with the increase of the prediction time,it is not as convenient as the quadratic polynomial coefficient for the broadcast of real-time clock bias and the acquisition of real-time clock bias by ground users.Therefore,the quadratic polynomial model is determined to be used for the ultra short-term prediction of LEO satellite clock bias.3.The relationship between clock bias prediction accuracy and clock frequency stability is analyzed experimentally.The analysis of satellite data from SF1 B,GRACE-C,HY2 B and FY3 D shows that when the prediction accuracy of 10 s and20s is 0.2ns,the stability of satellite clock for 10 s and 20 s should be better than1.9e-12 and 3.5e-12 respectively at least.4.The real-time broadcast parameters and scheduling protocol for LEO enhancing GNSS are designed.The results show that when the fitting time is 60 s and the prediction time is 10 s for LEO satellite clock bias,the clock bias accuracy can meet the needs of the accuracy of 5cm for LEO satellite orbit and the accuracy of 0.2ns for LEO satellite clock bias.5.The sub-system of clock bias estimation and prediction on satellite is completed and the components of the system are introduced,then the semi physical simulation test shows what accuracy can be achieved,which lays a foundation for the subsequent enhancement of GNSS by LEO satellite constellation.