Analysis Of BDS Satellite Clock In Orbit, Modelling And Its Prediction Research

BeiDou Satellite navigation system (BDS) has been constructed by China from 2004, after eight years of development, the regional navigation satellite constellation is completed. The regional navigation, positioning, and also timing services of BDS were declared on December 27,2012. In accordance with the three-step plan, global navigation satellite system will be built in 2020. GNSS performance is closely related to the satellite clock performance and also the ability to estimate and predict satellite clock. Satellite clock is a key device for GNSS, and is also the premise for obtaining the accurate time and frequency information. Characteristic analysis of BDS satellite clock in orbit, and research of BDS satellite clock prediction is important for understanding the current situation and discussing the optimization or improvement of BDS system performance, and is also a research topic with theoretical and practical value.Different from the characteristics analysis and prediction methods of ground atomic clock, satellite clock has its own feature:firstly, satellite clock data can not be directly acquired, while typically obtained by the time comparison performed between ground station and satellite, which includes not only the impact of the satellite clock, but also the impact of the time comparison algorithms; secondly, satellite clock is affected by the orbital dynamics environment. There are two methods to realize the time comparison between satellite and ground station, one-way method and two-way method. The one-way time comparison technology is commonly referred to ODTS (Orbit Determination and Time Synchronization) technology, and the two-way time comparison technology is commonly referred to TWTT (Two-Way radio Time Transfer) technology.It's innovative and unique for BDS system to adopt TWTT technology for the time synchronization between satellite and ground station. The differences between TWTT and ODTS are mainly reflected in the following aspects:observation data, data processing strategy and reference time, etc., satellite clock time offset data obtained by these two algorithms have their own different characteristics:compared with the satellite clock time offset data obtained by ODTS algorithm, the influences of satellite orbit error are smaller for the satellite clock time offset data obtained TWTT algorithm. Comparison of these two type of satellite clock time offset data is of great significance for identifying the error and improving precision of satellite orbit determination and time synchronization.At present, the characteristic analysis of BDS satellite clock in orbit are mainly concentrated in the systematic and stochastic characteristic. The analyzed data is small, and the analyzed time is short, and the periodic variation in the BDS satellite clock has not yet been analyzed. TWTT is one of the innovations for BDS system, while contrast analysis of BDS satellite clock characteristic analysis results derived from the analysis of TWTT and ODTS satellite clock time offsets is still less. Domestic research of satellite clock time offset prediction focus on functional model combinations and mathematical modeling of random component of satellite clock time offset. But the researches about the effect of satellite clock stochastic characteristic on the estimation error of satellite clock state, and also the prediction performance analysis of satellite clock are relatively small.In this paper, the main objective is the performance evaluation of BDS satellite clocks in-orbit, and the main content is the characteristic analysis, and the main purpose is improvement of the prediction accuracy of BDS satellite clock time offset. ODTS and TWTT satellite clock time offset data are used to analyze the systematic, periodic, and stochastic characteristics, and also the prediction performance the of BDS satellite clocks. Based on the present situation of BDS satellite clock time offset prediction, discussion of methods related to the optimization and improvement of prediction accuracy of satellite clock time offset is of innovation and application value for the optimization and improvement of BDS system performance. In this paper, the main research contents and achievements include:1) The role of ODTS and TWTT satellite clock data in the characteristic analysis and modelling of BDS satellite clock is discussed on the basis of the research status about the estimation, characteristic analysis and the prediction theory of satellite clock data. Then the importance and necessity of analysis, modelling and prediction research of BDS satellite clock in orbit for the optimization and promotion of BDS system performance is emphasized.2) Generation and transfer of precise time information in GNSS system is studied. The impact of estimation algorithm and data processing strategies on the satellite clock time offset estimation is analyzed. From the perspective of analysis, modelling and prediction research of satellite clock in orbit, the analysis scheme and research content related to the satellite clock prediction is determined.3) The influence of orbit determination strategy on the estimation of satellite clock time offset is discussed. The internal consistency of WHU-BDS precise clock and the accordance with the BDS satellite clock data of other analytical center is analyzed. Systematic characteristic of BDS satellite clock is analyzed using the ODTS satellite clock data, and the frequency drift caused by linear variation of semi-major axis of GEO/IGSO satellite orbit is also discussed. Periodic signals have been detected in BDS satellite clock. In order to identify the underlying cause of the BDS clock harmonics, the relationship between the periodic signals and the orbital dynamics is studied. BDS sub-daily clock stabilities and its time variation is discussed, and also the power law noises and their spectral density. Performance indicators of BDS satellite clock is obtained from the perspective of frequency stability.4) The internal consistency of TWTT satellite clock data is analyzed. The conclusion that satellite clock data estimated by ODTS algorithm are affacted by the satellite orbit determination error is confirmed through the correlation analysis between the difference of ODTS and TWTT satellite clock data and the orbital dynamics. Systematic, periodic, and also the stochastic characteristic of BDS satellite clock is analyzed based on the TWTT satellite clock data. Comparison of BDS satellite clock analysis result derived from ODTS and TWTT satellite clock data respectively are also discussed.5) Stochastic differential equation of BDS satellite clock system is defined on the basis of the analysis results of BDS satellite clock in orbit, and the Kalman filter is used to solve the differential equation. This differential equation system takes the impact of a variety of random noises into account, and can be used for satellite clock state estimation that takes different kinds of clock observables as input data; ODTS and TWTT satellite clock data are used for the validation of Kalman filter in this thesis, and good results are achieved. Kalman filter is also used to realize the data fusion of ODTS and TWTT satellite clock data.6) Clock prediction accuracy of the BDS broadcast ephemeris is evaluated. Methods for the improvement of broadcast ephemeris clock prediction accuracy is theoretically analyzed. The impact of parameter estimation error in the polynomial model on the BDS satellite clock prediction accuracy is discussed on the basis of sliding window polynomial fitting and predicting experiment. And the hybrid fitting intervals method for the BDS satellite clock predicting is designed on the basis of the discussion result. The significance of periodic variation for BDS satellite clock fitting model and prediction model is discussed on the basis of analysis result about the influences of periodic variations on the fitting and predicting of satellite clock data.