| Ocean current measurement plays a crucial role in ocean resource development,natural disaster response,and climate change studies.Distributed acoustic measurement of ocean currents relies on the time difference of sound propagation between nodes to calculate current velocity,requiring high time synchronization among underwater nodes.Due to hardware limitations,normal nodes struggle to maintain synchronization with the reference clock,which can affect time accuracy if no calibration is applied.Existing underwater time synchronization protocols based on message exchange assume the reciprocity of the underwater acoustic channel,disregarding the differential propagation delay caused by current velocity.Therefore,this dissertation focuses on time synchronization techniques for underwater acoustic communication in the context of current field measurement.To address the issue of excessive synchronization message transmission by the reference node in the Time Synchronization for High Latency Networks(TSHL),an improved message transmission method is proposed,reducing energy consumption while maintaining good time synchronization performance.To tackle the problem of jitter errors in received messages by underwater nodes,an enhanced clock offset estimation method is introduced,enhancing the estimation performance of the time synchronization approach.Considering the influence of ocean current velocity on propagation delay,a parameter estimation method for underwater node time synchronization is proposed to accurately estimate clock offsets.Simulation results demonstrate the effectiveness of the improved TSHL method and the time synchronization approach combined with current velocity estimation.Based on the self-developed underwater acoustic modem in the laboratory,a segmented time synchronization message data frame structure is designed and implemented to support joint estimation of current velocity and time synchronization.To meet the reliability requirements of the time synchronization process,an equalization module combining fractional fourier transform and virtual time reversal mirror is implemented.The optimization process of the fractional fourier transform order is solved by introducing the simulated annealing algorithm and designing the annealing process with a dynamic cooling coefficient.Field experiments conducted in the lake and sea have preliminarily verified the effectiveness of the proposed time synchronization methods and the designed implementation.This dissertation focuses on time synchronization techniques for underwater acoustic communication in the context of current field measurement,providing technical support for time synchronization implementation in distributed measurement of ocean current.The research work and achievements have practical implications in the field of ocean current measurement. |
