Research On Monitoring And Assessment Method Of Low-frequency Time-code Signal Quality And Timing Performance

Low-frequency time-code is a timing technology which is encouraged and adopted by the International Telecommunication Organization(ITU),characterized by stable signal propagation,wide coverage,and meeting the needs of medium precision users.With the advancement of modern technology and information technology,the application needs of low-frequency time-code timing users are constantly increasing.It is necessary to improve and optimize existing monitoring methods,and propose signal assessment methods to ensure the reliability and safety of timing.The thesis closely combines the deployment plan of China’s low-frequency time-code timing system and conducts research on the design of low-frequency time-code signal monitoring and assessment.The main research content and innovation points are as follows:(1)Based on the propagation characteristics of low-frequency time-code signals,the relationship between propagation distance and arrival wave is studied.The signal propagation characteristics in the ground wave region,sky wave region,and sky wave interference region are systematically sorted and analyzed.Combined with the measured data in 2018 and 2019,the signal field strength calculation model in the ground-sky wave interference region is improved and optimized,providing a theoretical basis for subsequent signal measurement.(2)Propose a low-frequency time-code signal monitoring method based on signal characteristics,and study the monitoring principles of related parameters from two perspectives: signal quality and timing performance.In the monitoring of signal quality parameters,a signal field strength measurement method based on FFT transformation and a phase measurement method based on orthogonal phase detection are proposed,and experimental verification is conducted.The experimental results indicate that the uncertainty of both the signal field strength and phase measurement have been improved.In the timing performance parameter monitoring,firstly,the envelope detection method based on Hilbert transform is used to demodulate the low-frequency time-code signal.Secondly,the k-means algorithm is used to estimate the signal amplitude,proposing a method and strategy for code width measurement and pulse front extraction.The error rate is used as the standard for verifying low-pass filter parameters,smoothing filter parameters,and measurement threshold parameters,and simulation experiments are conducted.Finally,based on the study of the timing principle of low-frequency time-code signal,the composition of the low-frequency time-code signal delay was analyzed in detail.The theoretical calculation of the lowfrequency time-code signal propagation path delay was carried out,and a receiver delay calibration method based on a simulator was studied and verified.(3)Propose an integrity monitoring method for low-frequency time-code signal.Firstly,the concept of integrity monitoring for low-frequency time-code signal is introduced,and theoretical analysis is conducted on integrity monitoring from monitoring indicators,fault modes,and other aspects.Integrity monitoring schemes based on Shewhart Control Charts and Cumulative Sum Control Charts are designed to predict signal parameter alarm thresholds.The two methods are compared and validated using signal field strength data and timing deviation data from November 2022.The results indicate that when there is an offset in the signal field strength or timing deviation,using the Shewhart control chart can detect the changes in the signal and respond in real-time,meeting the requirements of integrity monitoring.(4)Propose an assessment method for low-frequency time-code signal.Firstly,the assessment indicators of low-frequency time-code signal are studied,and the characteristics and data processing methods of the assessment parameters are analyzed.Secondly,using the idea of index priority decomposition,determining the effective range of data,and assigning weights,a low-frequency time-code signal assessment method and process are proposed,and the assessment results are represented by weight values.Finally,the assessment method for low-frequency time-code signal was experimentally validated using monitoring data from November 2022.The test results indicate that the assessment result on November 12,2022 was 0.2308.The reason for this is that the thunderstorm weather caused the broadcasting failure of the Shangqiu low-frequency time-code station,which is consistent with the monitoring data and assessment results,proving that this method can effectively evaluate and analyze lowfrequency time-code signal.(5)Designed and implemented the first low-frequency time-code signal monitoring and assessment system in China.Firstly,starting from the aspects of signal quality,timing performance,integrity monitoring,environmental monitoring,and automatic control,the top-level design of the engineering implementation plan for the low-frequency time-code monitoring and assessment system is carried out.Secondly,for the core parameters,field strength uncertainty test,phase monitoring uncertainty test,amplitude modulation monitoring uncertainty test,and timing deviation uncertainty test are designed to verify the functionality and performance of the monitoring system.The results indicate that in the case of receiving a real signal,the uncertainty of signal field strength measurement is 1.34 d BμV/m,signal phase measurement uncertainty better than 0.033μs.The uncertainty of amplitude modulation measurement is 0.0014%,and the Class A uncertainty of low-frequency time-code timing deviation monitoring is 48.84μs.The combined uncertainty is 49.29μs.All parameters have significantly improved measurement accuracy and stability.By systematically studying the assessment methods of low-frequency time-code timing performance,a low-frequency time-code monitoring system has been established,which has developed and improved China’s low-frequency time-code system,providing strong support for the construction of a resilient and integrated national timing system.