Research On Methodology For BDS Time Service Performance Improvement And Evaluation

Accurate and stable time is the basis of modern electronic systems.The 2019 World Metrology Conference has redefined the units of basic physical quantities,the importance of time is further highlighted and the other six basic physical quantities are derived directly or indirectly from time,which becomes the basis of modern metrology,more and more fields require high-precision time.The real-time one-way time service accuracy is several tens nanoseconds for the current Global Navigation Satellite System(GNSS),which can not meet the requriements of 5G communications,space target detection,synthetic aperture radar and other occasions.Using the precise correction data provided by International GNSS Service(IGS)and the international GNSS Monitoring and Assessment System(i GMAS)can achieve nanosecond and sub-nanosecond time service.However,it can only be implemented by the post-correction,which cannot meet the application requirements of real-time highprecision time service,and the users must pay the additional communicaton or data usage costs.Based on the accurate evaluation of the performance and characteristic of the Bei Dou satellite navigation system(BDS),the method for improving the performance of BDS time service is researched and verified by experiments.The main research contents are as follows:The UTC time service performance evaluation method based on user equipment measurement is proposed.Aiming at the current phenomenon that the time service performance characterization is inconsistent,on the basis of analyzing GNSS one-way time service principle,the distribution of the time service error source and it’s characteristics are analyzed,and the satellite time service performance evaluation criteria and performance characterization parameter set are sorted out.A method to test the GNSS time service performance fromthe user equipment is proposed,and a GNSS time service performance test and evaluation system is established and applied in engineering.The test results have become an important reference for the technical status evaluation of the BDS-3 experimental satellites and BDS-3 satellites constellation access.Combining common-view(CV)time comparison and pseudo-range differential method,a differential time service method that is more suitable for BDS features is proposed.The bias of BDS broadcast time and UTC is measured in the time keeping laboratory,which includes not only the bias between UTC and BDS time(BDT),but also the time service bias of each satellite.The parameters of the model are predicted and broadcasted in real time.The users can use the model to correct the result of oneway time service,which is equivalent to achieving a CV time comparison with the time keeping laboratory.According to the characteristics of the parameter order of the system time traceability model in BDS navigation message,navigation message coding and differential time service model,a broadcast strategy of differential time service model based on BDS navigation message is proposed.Without changing the user’s configuration,BDS time service accuracy can be improved from several tens nanoseconds to 6 ns.A time service performance evaluation and verification scheme based on bidirectional time division multiplexing transmission over a single fiber with the same wavelength time transfer is designed.The experimental verification system is established over the direct-connected optical fiber of 60 kilometers between Xi’an and Lintong.In the laboratory and actual working environment,the accuracy of time transfer is better than 1 ns,which can provide a verification environment for highprecision time service performance evaluation.According to the technical characteristics of differential time service,three basic performances verification methods consisted of a short-baseline direct comparison,a long-baseline direct comparison,and a long-baseline cross-comparison are designed.A long-term verification and analysis of differential time service performance in Xi’an,Sanya,and Kashgar is conducted.The test results depict that the accuracy of differential time service is from 2 ns to 6 ns(RMS).The method significantly improves the accuracy of one-way time service of BDS(several tens of nanoseconds)relying on the UTC provided by National Time Service Center(NTSC),Chinese Academy of Sciences in Xi’an,the central location of the Chinese mainland;When the differential time service information is applied to the single point positioning(SPP),the positioning accuracy can be improved by more than 30%.The research results of BDS time service performance evaluation and differential time service method have been adopted by BDS project,which promote the constriuction and development of Chinese navigation system.It provides a technical means to research the fine variation law of time service error in nanosecond scale,and is expected to drive the development of time frequency,navigation and other related disciplines,which has important scientific significance and practical application value.