Journal
IEEE JOURNAL OF OCEANIC ENGINEERING
Volume 40, Issue 4, Pages 841-852Publisher
IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC
DOI: 10.1109/JOE.2015.2469955
Keywords
Acoustic communications; clock synchronization; localization and navigation; multiagent systems; optimization; underwater acoustic networks; underwater sensor network
Funding
- NATO Allied Command Transformation (ACT), Collaborative Anti-Submarine Warfare Programme
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Proliferation of deployed sea-going autonomous platforms, such as autonomous underwater vehicles (AUVs), unmanned surface vehicles (USV), and sensor nodes anchored to the seabed, make the deployment of true underwater acoustic networks more and more feasible. An important feature of any network is the ability to synchronize the clocks of the participants, for the purpose of, e.g., time-slotted media access control (MAC) and navigation. Terrestrial clock synchronization protocols, such as the well-established network time protocol (NTP), are not readily applicable to underwater acoustic networks, because of long propagation times, low packet delivery success rates, communication ranges that vary over time in an unpredictable manner, and, in the presence of mobile nodes, the ad hoc nature of the composition of the network. This paper proposes a continuous estimation of internode clock offset and drift, based on the continuous exchange of modem packets, possibly containing transmission and reception timestamps. The proposed solution takes explicitly into account the limitations of the acoustic communication channel and network node mobility. This robust, opportunistic clock synchronization (ROCS) is robust against modem reset, and will work even if packet delivery success rates are not optimal or if no communication is possible for extended periods of time. Experimental results are given from the COLLaborative Asw Behaviours-Next Generation Autonomous Systems (COLLAB-NGAS14) campaign, held October 19-31, 2014, off the west coast of Italy. During the sea trial, the proposed clock synchronization algorithm was deployed and successfully tested within an underwater acoustic network composed of mobile and fixed nodes.
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