Research On Algorithms Of Precise Clock Offset And Quality Evaluation Of GNSS Satellite Clock

Time system is one of the core elements of satellite navigation and positioning system. Time is the most basic observation of navigation system. Time-frequency observations of satellite navigation system are provided by the satellite atomic clocks. Satellite clocks which provide time scale of navigation systems directly determine the accuracy of navigation and positioning plus time-frequency transferring. Nowadays, for GNSS, especially China’s BeiDou satellite navigation systems (COMPASS), there are many theoretical and practical problrms in the satellite clocks characteristics and performance maintenance. The theoretical significance of atomic clock’s time-frequency characteristics and various satellite clock errors are for exploring the time-varying regularity of satellite clock system and providing theoretical basis for dynamic characteristics predicting of satellite clock and performance maintenance. The practical significance of the study on precise satellite clock characteristics is for providing accurate time offset and supporting to enhance the accuracies of the satellite orbit determination for user’s navigation and positioning. Therefore, the research on characteristics and performance maintenance of satellite atomic clocks is one of the hot theoretical exploration and practical research issues in the field of satellite navigation..In the research, GNSS clock products are used to study on clock characteristics, error models and prediction methods of the satellite clocks on orbit. The main work and contributions of this thesis are as follows:(1) IGS clock products from2001to2010are used to analyze the GPS satellite clock qualities such as frequency stabilities and clock noise level. We find out that the frequency stabilities and clock noise of the clocks of Block IIA satellites are ten times worse than that of the Block IIR and IIR-M satellites. Moreover, the linear relationships between frequency stabilities and clock residuals have been deduced with an accuracy of better than0.02nanoseconds. Specially, it is noticed that the clock of the PRN27is instable and the relationship between the frequency stability and residuals can be described by a quadratic curve. Therefore, we suggested that GPS satellite clocks should be weighted by their quality levels in application, and the observations of the Block IIA should not be used for real-time positioning in the situation with required precision better than one meter.(2) Through the analysis of GPS satellite clocks noise by Allan variance and Hadamard total variance, as well as the research on estimation and prediction of satellite clock errors by Kalman filter given stochastic model of satellite clock noise, precision abnormity was computed among different style of clock. Meanwhile, combined with satellite clock stability, some conclusions can be obtained:Stability and precision of GPS Block IIR-M Rb clock is better than that of Block IIR Rb clock, Stability and precision of GPS Block IIR Rb clock is better than that of Block IIA Rb clock, compared with Rb clock, Cs clock’s stability and precision is worse by2-3times.(3) In order to estimate the satellite clock offset in a real-time mode, a new algorithm of adaptively robust Kalman filter with classified adaptive factors for clock offset estimation is proposed. Compared with standard Kalman filter clock offset model, the new method can detect and control the influences of outliers and clock jumps automatically in real-time. Moreover, the clock model parameters, which contain the clock offset, clock speed and clock shift, are classified to decide the adaptive factors in the new model. Thus, clock jumps with different characteristics can be distinguished more effectively. Meanwhile, the dynamic noise characteristics of clock offset series are used for stochastic modeling. An actual numerical example is presented, which shows that the proposed filter can give a better performance than other commonly used filters.(4) Based on the properties of clock velocity and shift, a robust quadratic polynomial model to fit single-day clock error and to mitigate the effect of exceptional data is provided firstly. Then we fuse the long-term clock parameters fitted from former clock time series and use the ARIMA model to predict the long-term clock parameters. This novel algorithm is tested to predict clock errors for100days, and the prediction accuracy is better than34ns, which is significantly better than other traditional algorithms.(5) Improved prediction models for GPS and GLONASS satellite clocks are proposed in order to enhance the precision of predicted clock offsets. Firstly, the proposed prediction models are proposed, in which a few cyclic terms is added to absorb the periodic effects and a time adaptive function is used to adjust the weight of the observation in the prediction model. Secondly, initial deviations of the predictions are reduced by using a modified method to a constant term. The simulating results have shown that the proposed prediction model can give a better performance than that from IGU-P clock products. In addition, RTPPP method was chosen to testify the efficiency of the new model for real-time static and kinematic positioning.(6) Because IGS has no useable products of GLONASS real-time clock prediction, real-time forecasting is studied based on the GLONASS clock products which offered by the three major IGS research institutions. The improved model is proposed and the validity and accuracy of the improved model are computed. In addition, the spectral analysis method is used to solve the significant periodic terms of the GLONASS satellite clock.(7) Clock error sequence obtained by the traditional single-day PPP is not continuous, which exists day-boundary. This problem directly influences the popularization and application of PPP technique in the realm of precise time and frequency transfer. In this paper, parameter-based Bayesian estimation with prior information is proposed to solve the problem of day-boundary. Namely, by constraining the prior information with different parameters, the author realized continuous PPP solution or approximate PPP solution, which successfully suppresses the day-slip resulted from single-day solution. The simulation results show that the proposed new algorithm for timing accuracy and stability of the PPP algorithm is significantly superior to the traditional, and it can effectively solve the problems of day-boundary.(8) The characteristic of GPS Common View (GPS CV) and GPS Carrier Phase time transfer (GPS CP) methods are introduced, and a new continuous time and frequency transfer algorithm based on GPS single Different Carrier Phase observations is proposed. This new algorithm uses ionosphere-free combination of single different observations from the same satellite, and its processing steps are also consistent with simple common GPS CP. In addition, the Bayesian estimation of prior information is proposed to smooth the day-boundaries of time transfer results. The simulation results, which compared to that of GPS CP method, show that the proposed new algorithm has a remarkable improvement in the precision and frequency stability of the time and frequency transfer results.(9) The standard GNSS combined PPP algorithm cannot be used satisfactorily in the real-time and high frequency precise positioning because of its low compute efficiency. A new algorithm based on the parameter equivalent reduction principle is proposed. First, the observation equations and the normal equations which belong to the single navigation system can be solved independently. Second, the normal equations of overlapping parameters between the different systems can be obtained by using parameter equivalent reduction principle. At last, the combined PPP resolutions can be computed easily by using the least squares method. The proposed algorithm can improve the calculating efficiency immensely. In addition, an adaptively combined method which can automatically adjusts the contributed weight of different GNSS systems is also proposed.(10) A new method of GNSS time offset monitoring is proposed based on the combined PPP model and the different IGS clock products. The real GNSS time offset, the hardware delay difference between different systems and the user time offset are obtained. Meanwhile, the stability, accuracy and variation characteristics of these products are analyzed. The practical application of these products in the real navigation and positioning is tested. In the end, it was concluded that the monitoring and forecasting of the user time offset has more important practical value for the users of navigation and positioning.