A key technology in the movement tracking of marine animals is localization using acoustic transmitters. These are attached to marine animals and are detected by an array of receivers. Then, offline localization is performed by multilateration. However, due to the transmitter's low power and environmental conditions, emissions may be detected by only a limited number of receivers, causing localization ambiguities to arise. This work proposes a solution for such localization ambiguities. The proposed method assumes that the position of acoustically-tagged marine animals follows a hidden Markov model, such that localization ambiguities can probabilistically be resolved using a Forward-Backward algorithm. Our method is able to extrapolate the positions in a data series, as long as one sample in that series is picked up by three receivers, or if the identity of the receivers changes during tracking. Performance analysis shows that the localization accuracy of our method approaches the Cramér-Rao lower bound. Furthermore, to demonstrate the suitability of our method in a real sea environment, we have established a testbed that operated for three months, demonstrating localization of 20 acoustically-tagged sandbar sharks. Compared to the available solutions, roughly 20 times more location estimates were made; thereby, significantly increasing the impact of the test-site.

We present a non-linear navigation solution, referred to as the Reverse Bearing Only Target Motion Analysis (Reverse BO-TMA). Reverse BO-TMA is a passive method for the self-localization of an Autonomous Underwater Vehicle (AUV). Our method relies solely on bearing measurements based on the radiated noise of a passing vessel sailing along a known route. Compared to traditional range-based underwater localization methods, Reverse BO-TMA allows the AUV to remain farther from the reference vessel, and does not require collaboration or message exchange. We formalize the Reverse BO-TMA as an optimization problem, and offer both a least squares solution and an unscented Kalman filter solution. Numerical results show that Reverse BO-TMA provides accurate performance, in terms of both positioning and speed, which are close to the posterior Cramer-Rao lower bound. To demonstrate the effectiveness of our approach, we have implemented a prototype for Reverse BO-TMA and successfully tested it in three sea experiments. We show that the Reverse BO-TMA is suitable for the long-term deployment of an AUV and in cases where energy is scarce and cooperating anchors are not available.