# PTA-Sync: Packet-Train-Aided Time Synchronization for Underwater Acoustic Applications
###### Abstract  
In underwater acoustic applications, time synchronization errors accumulate severely with time, owing to the unpredictable propagation delay change induced by mobility and low sound propagation speed. In previously reported schemes, error accumulation during underwater acoustic time synchronization occurred, decreasing their performance and affecting their applicability. To overcome this limitation, we propose packet-train-aided time synchronization (PTA-Sync) for underwater acoustic application. The proposed PTA-Sync adaptively tracks one-way travel time (OWTT) change using three-dimensional linear velocity vector, the transmit time difference and arrival time difference of two adjacent packets in a packet train, and the position information of a mobile reference node. Thus, this scheme enables us to reduce the synchronization error accumulation rate by estimating the propagation delay change more accurately. Simulation results show PTA-Sync achieves higher accuracy than existing OVTT-based synchronization schemes. Thus, PTA-Sync can be effectively used in underwater exploratory activities because it can successfully reduce time  
acoustic time synchronization protocols (UATSPs), which employ acoustic waves, need to consider several unique disadvantages, such as inability to use a global positioning system (GPS), slow propagation speed, narrow bandwidth due to physical limitations of acoustic sensors, rapid changing of channels over time, and high mobility of nodes due to ocean currents [13; 14; 15; 16]. In particular, the propagation time delay caused by the underwater acoustic wave speed, which is approximately 200,000 times lower than that of the radio wave, is the most important constraint in TS design. The propagation time delay varies depending on the acoustic speed and the node's mobility. Acoustic speed is a spatial-temporal variable as it varies depending on the depth of the water owing to the influence of temperature, pressure, and salinity, and it is difficult to predict the node's mobility because of ocean currents that change depending on the surrounding environment. The performance of UATSP is substantially affected by the estimation accuracy of propagation delay; therefore, for improving the estimation accuracy of the propagation delay, the effects of acoustic wave speed and mobility must be considered. TSHL [17], which is the first UATSP that considers the underwater propagation delay, was proposed for stationary nodes. MU-Sync [18] is a cluster-based UATSP that considers the mobility of nodes; it improved the synchronization performance by calculating the half of round-trip time (RTT) as propagation delay. To improve the accuracy and energy efficiency, Mobi-Sync [19] uses a spatial correlation of node speeds for estimating the changes in the propagation delay. D-Sync [20], DA-Sync [21], TSMU [22], and DE-Sync [23] are UATSPs that utilize Doppler shift caused by the relative mobility of nodes to improve the synchronization accuracy and energy efficiency. APE-Sync [24] aims to achieve a high-precision and energy efficiency by combining DE-Sync and the Kalman filter tracking time-varying clock skew. PCDE-Sync [25] combines with partial clustering method built on the artificial fish swarm algorithm and clock correction method by the Doppler shift, and it achieves a trade-off of accuracy and energy consumption. Joint time synchronization and localization (JSL) [26] compensates the stratification effect caused by the inhomogeneity of underwater media, and improves the accuracy by combining the localization and TS. Robust joint localization and synchronization (RJLS) [27] improves the synchronization precision by repeatedly performing calculations until clock skew and offset converge after constant application of the linear least squares method to the method combining synchronization and localization.