Detection and Localization of Multiple Ships Using Acoustic Vector Sensors on Buoyancy Gliders: Practical Design Considerations and Experimental Verifications

Real-time detection of illegal activities in maritime areas, such as drug trafficking or illegal fishing, is a very challenging task. It generally requires important and expensive means (patrol planes, coastguard, or navy ships) to provide an efficient monitoring of large areas. As ships emit noise due to their propulsion (propeller and engines), passive acoustic monitoring can be an effective alternative to accomplish this task, at a reasonable cost. To be efficient, the detection process should be autonomous, run in real-time, and should transmit information within a short period. In this study, we demonstrate the potential of using gliders equipped with a tri-dimensional acoustic vector sensor (AVS) to run a survey mission in total autonomy. Based on the ability of the AVS to measure the direction of arrival of the incoming wave, we develop an embedded and real-time processing chain to detect ships and to cluster detections in a multiple-sources scenario. We also propose a receiver that runs in real time to measure the time difference of arrival (TDOA) if multipath propagation occurs. The sonar contacts are transmitted every 30 s to a command and control (C2) station using an acoustic modem, without interrupting the detection process. The C2 can process the sonar contacts with a tracker for target localization; the article also shows that it is possible to get a good estimate of the source distance by processing the TDOA from multipath propagation. The proposed methods are validated on data recorded during an at-sea experiment in shallow waters.