# PTA-Sync: Packet-Train-Aided Time Synchronization for Underwater Acoustic Applications
###### Abstract
Such UATSPs are two-way travel time (TWTT)-based synchronization schemes wherein a packet for synchronization is sent from a source node to a receiver node, which sends back a response packet to the source node; subsequently, the two-way travel time is measured. This scheme has the advantage of the delay time being calculated using only the reference clock information of the source node, without using the clock information of the receiver node. However, the RTT of the packet is considerably high in an environment with propagation delays, e.g., an underwater environment, thus increasing the uncertainty of the propagation delay due to the movements of nodes, ultimately leading to errors in the propagation delay estimation. In contrast, in one-way travel time (OWTT)-based synchronization schemes, the propagation delay is measured based on the time required to send a packet from the source node to the receiver node; these schemes have been frequently applied in studies related to navigation [28; 29]. The OWTT schemes calculate propagation delay by using the clock information of the receiver node and the reference clock of the source node; therefore, propagation delay measurements may vary depending on the synchronization accuracy of the receiver node clocks. However, as multiple nodes simultaneously receive beacon packets to initiate synchronization, the overhead of the entire network can be reduced, thereby lowering the packet reception time by more than 50% compared to that of the TWTT schemes. Because low frequency acoustic communication in bandwidths of several kHz to some tens of kHz is primarily used in underwater environments, message transmission is limited owing to large propagation and transmission delays. Therefore, the OWTT-based synchronization is more advantageous than TWTT-based synchronization in underwater acoustic applications, wherein minimization of message overhead and delay is crucial. In [29], TS and localization were achieved by conducting a sea experiment on an unmanned underwater vehicle (UUV) using the one-way ranging estimation technique. [PERSON] et al. [30] also proposed an energy-efficient OVTT scheme based on nonlinear clock skew tracking in the clock skew and non-Gaussian noises by applying the Gaussian mixture model. In E2 DTS [31], the influence of changes in the propagation delay was minimized using the transmission and reception time difference of continuous packets. However, there was a drawback: the changes in OVTT with respect to the moving speed of the node were assumed to be consistent. TSMA-IGTS [32] improved the synchronization accuracy by estimating the changes in OVTT based on the estimation of relative speed through the Doppler effect in an underwater acoustic network. In this way, the estimated changes in OVTT were deduced from the relative speed, rather than velocity; therefore, the estimated changes were accurate only when the movement directions of the reference node and ordinary nodes were identical, and they were considered inaccurate if the directions were unidentical. This study proposes a packet-train-aided time synchronization (PTA-Sync) protocol of packet-train communication, which can reduce TS errors by tracking the changes in the propagation delay caused by node movements and slow propagation speeds in an underwater acoustic network environment. PTA-Sync adopts OVTT for tracking propagation delay changes and propagation delay asymmetry caused by mobility using the measured linear speed. In particular, synchronization is performed using the position information of the reference node, the measured three-dimensional speed vector of the node, the time difference in the transmitted packets, and the time difference in the received packets. This enables continuous estimation of changes in the reference node and propagation delay for each packet section during the process of continuously transmitting a beacon as several sub-frames in the form of a packet train.