Research On Key Technologies Of GNSS Time Interoperability

With the development of the global satellite navigation system,it is possible to improve the positioning accuracy by using multiple GNSS satellites and improve the geometric dilution precision.Compared to single satellite navigation independent positioning,multi-GNSS composite services have the potential to provide higher quality of service,especially in challenging environments such as urban canyons and leaf coverage.Multi-GNSS services will have great advantages.In order to achieve multi-combined services and improve the performance of positioning and timing services,each GNSS should have a feature called compatibility and interoperability.Time interoperability is an important aspect of interoperability between satellite navigation systems and a major focus of multi-GNSS converged navigation.The implementation of time interoperability at the system level involves two key elements.One is to accurately monitor the time difference between the navigation systems and to forecast;the other is to determine a parameter to broadcast the time difference forecast information between the systems to the user,and to minimize the user.The paper analyzes the key technical details and implementation methods in the GNSS time interoperability related content from the system level,and the actual data is used to test and verify.The paper mainly analyzes the current GNSS system time generation method,GNSS system time difference monitoring method and time delay monitoring time delay determination method.The atomic clock noise theory is used to analyze the noise characteristics of GNSS time difference sequence,and the GNSS time difference is given.The method of reducing the sequence noise is finally studied.The method of determining the content of GNSS time interoperability parameters is studied.The real data is used for verification.Finally,the method of time interoperability parameter is compared from different aspect.The main research contents and innovations are as follows.(1)The time difference monitoring technology of GNSS system in time interoperation is studied.The different time difference monitoring methods are analyzed in detail,and compared with actual data.Accurately monitoring the time difference of each GNSS system and broadcasting it to users is necessary to improve the positioning performance of multi-mode users.The commonly used GNSS time difference monitoring methods are analyzed,and the technical implementation details of single station time difference monitoring,multi-station time difference monitoring and GNSS system time difference monitoring method based on time comparison link are studied.The error sources and correction methods of different time difference monitoring methods were analyzed in detail.The GNSS time difference monitoring experiment was carried out using the link resources of the National Time Service Center and the multimode receiver.(2)A method for total delay calibration of GNSS time difference monitoring receiver based on link bridging is proposed.The experiment results show that the uncertainty of calibration based on this method is within 5 ns.Compared to the absolute calibration of the time difference monitoring device,this method simplifies the calibration process,reduces the need for expensive equipment and calibration experience for absolute calibration,and ensures accurate calibration results.(3)Applying the noise theory to the noise analysis of the GNSS time-distance sequence,the dominant noise type of the GNSS time-series sequence is clarified.Different GNSS time difference monitoring methods introduce different types of noise.The essence of the GNSS system time difference is the clock difference between the two atomic clocks.It contains the noise of the atomic clock operation,the noise introduced by the driving method in the GNSS system time generation process,and the noise introduced by the GNSS time difference monitoring method.Accurately clear the noise type of the time difference sequence and select the appropriate noise attenuation method to improve the accuracy of the GNSS time difference monitoring results.(4)The traditional noise elimination method of time difference sequence is improved.The Kalman filtering algorithm of forward and backward combination is used to improve the "lag" and "leading" effects of the traditional filtering algorithm.Since the basic equation of the traditional Kalman filter is recursive in the time domain,the process is a process of constant “prediction-correction”.As the epoch increases,the filtered result will have a significant “lag” deviation..Aiming at the above problems,the traditional Kalman smoothing method is improved,and the Kalman filter of the forward and backward combination is used to eliminate the noise of the time difference sequence of the GNSS system.At the same time,the Vondrak cross-certification method is introduced into the noise reduction of the GNSS time difference monitoring sequence to improve the influence of noise on the time difference monitoring sequence.(5)A GNSS combined time scale generation algorithm for noise and stability weighting is proposed.In terms of GNSS time interoperability parameter broadcast,considering the limited factors of navigation message bytes,ESA has proposed the concept of multi-GNSS system integrated paper,but the specific implementation is not described in the proposal.Based on the results of GNSS time difference monitoring,this paper proposes a composite time scale algorithm based on noise weighting and a combined time scale algorithm based on stability and weight.The actual calculation is performed using measured data,and the above algorithms are compared.