Time synchronization and localization in underwater environment are challenging due to high propagation delay, time measurement error, and node mobility. Although synchronization and localization depend on each other and have the similar process, they have been usually handled separately. In this paper, we suggest time synchronization and localization based on the semiperiodic property of seawater movement, called SLSMP. Firstly, we analyze error factors in time synchronization and localization and then propose a method to handle those errors. For more accurate synchronization, SLSMP controls the transmission instant by exploiting the pattern of seawater movement and node deployment. Then SLSMP progressively decreases the localization errors by applying the Kalman filter or averaging filter. Finally, INS (inertial navigation system) is adopted to relieve localization error caused by node mobility and error propagation problem. The simulation results show that SLSMP reduces time synchronization error by 2.5？ms and 0.56？ms compared with TSHL and MU-Sync, respectively. Also localization error is lessened by 44.73% compared with the single multilateration. 1. Introduction Ocean infrastructures like offshore plants have been garnering great attention owing to huge potential benefits of marine resources [1, 2]. Also, necessity of real-time monitoring for marine environment is growing to immediately deal with critical accidents that can be caused by unpredicted events like high temperature of sea water, red tide, oil spill, and so on. According to this trend, many researchers from academic and industrial are studying UWSN (underwater sensor networks) recently. UWSN applications can remotely control marine architectures and monitor marine ecosystem. However, UWSN has some challenges due to the nature of underwater communication channel characterized by error-prone and long propagation delay. In addition, constant movement of underwater sensors has to be accounted for network protocol design. So, it is impossible that we adopt well-refined terrestrial communication mechanisms into the underwater environment directly. Although time synchronization is crucial for various applications such as localization and low-power sleep scheduling MAC protocols, existing synchronization schemes did not fully consider practical issues, like channel access delay. The delay can be ignored in terrestrial scenario where propagation speeds are extremely high, but not in the water due to the low speed of acoustic signal. Furthermore, contention based MAC protocols like CSMA may cause
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