Research On Carrier Tracking In Optic-Fiber Time Transfer System Under Sub-nanosecond Condition

Time and frequency are the basic physical quantities of all ranging and time-measuring systems,including global satellite positioning and navigation system.It is very important and meaningful for the improvement of measurement accuracy by high-precision of transfer and compare of time and frequency.With the increasing measurement accuracy of system and the sub-nanosecond service which is provided,improving the time synchronization between the ground stations of satellite positioning and navigation system is still a problem.Traditional optic-fiber time transfer is based on pseudo distance measurement of time synchronization which is hard to get sub-nanosecond of precision.Researches on carrier phase measurement of time synchronization are insufficient as well as the conclusion of optimized results for the parameters of carrier tracking phase lock loop(PLL).Also,carrier phase tracking based of Kalman filter only take the influences of thermal noise on tracking performances into consideration,which cannot improve the measurement accuracy of carrier phase further.This thesis pays much attention to the optic-fiber time transfer system under sub-nanosecond condition,builds the model of carrier phase tracking in optic-fiber time transfer system and analyzes the characteristics and model of phase noise.Based on the model of phase noise,the influences of phase noise on the accuracy of carrier tracking PLL is analyzed and the optimal design of PLL parameters in the presence of phase noise and thermal noise is given.In order to reduce the effects of phase noise on measurement accuracy,the Kalman filter is used for carrier phase tracking in which the phase-changing characteristic of reference clock remains comparably.The main contents of this thesis are as follows:(1)To analyze the characteristic of phase noise in optic-fiber time transfer system and to get that phase noise still correspond to the power law model after the optic-fiber transfer which is degraded as a result of inducing much white frequency modulated noise(WFM).The parameter values of phase noise model are estimated by the Allan deviation curves of measured signals and the real phase noise is simulated as a result.(2)To analyze the influences of phase noise on carrier tracking PLL under sub-nanosecond condition and the optimal design of loop noise bandwidth and coherent integration time in carrier tracking PLL is proposed in the presence of phase noise and thermal noise.The approach is further testified by simulation.(3)To improve the performance of phase lock loop in noise rejection,the Kalman filter is used in carrier phase tracking and under the influences of phase noise and thermal noise,the effects of phase noise induced by inferior reference frequency sources can be reduced in order to improve the time synchronize precision of time transfer systems.The research results of this thesis can be applied to the two-way optic-fiber time and frequency transfer systems between the ground monitoring stations of global satellite positioning and navigation system.They can also provide the basis theoretical analysis for the time synchronous optical fiber network.Using the results of optimal carrier tracking PLL design,precision of time synchronization will be increased.