2D Forward Looking SONAR in Navigation Aiding: Development and Testing of Strategies for Autonomous Underwater Vehicles

This work collects the results of the research activity on marine robotics carried out at the Mechatronics and Dynamic Modeling Laboratory (MDM Lab) of the Department of Industrial Engineering of the University of Florence (UNIFI DIEF) during the years 2014-2017. Reliable navigation systems are fundamental for Autonomous Underwater Vehicles (AUVs) to perform complex tasks and missions. It is well known that the Global Positioning System (GPS) cannot be employed in underwater scenarios; thus, during missions below the sea’s surface the real-time position is usually obtained with expensive sensors, such as the Doppler Velocity Log (DVL), integrated within a navigation filter such as an Extended Kalman Filter (EKF), Unscented Kalman Filter (UKF), or Dead Reckoning (DR) strategies. The main goal of this work is to develop and test a framework able to integrate a Forward-Looking SONAR (FLS), by means of linear speed estimations, into an underwater navigation system. On the one hand, the proposed solution can work together with a standard navigation sensors set (comprising, for example, a DVL), and thus leading to a greater number of linear speed measurements. On the other hand, employing an FLS to aid navigation could potentially outline other advantages. Using an augmented set of devices able to provide navigation information represents an intrinsic boost in redundancy; DVL-denied scenarios, such as very close to the seafloor or other surfaces or when a substantial number of gaseous bubbles is present, could be managed. Indeed, as opposed to the DVL, the FLS possesses much more beams that are spread into a broader area, thus improving reliability. DVL failings in the presence of bubbles are well-documented in the current literature and have been experienced during several tests at sea performed by UNIFI DIEF. Conversely, the presence of bubbles, which can be noticed within FLS images as strong return echoes spots, is usually tolerable and not capable of jeopardizing FLS operations. Moreover, although bigger AUVs enable the use of more sophisticated instrumentation and can carry a heavy payload, smaller AUVs are constrained to limited payload carrying capabilities. Hence, in addition to constituting a valuable research interest, multitasking onboard sensors represent a solution that offers compactness and avoids the use of some instruments. Besides this, to better the dynamic modeling of the AUV, a light-weight online estimator for the longitudinal dynamics and a more realistic propulsion model are developed. Lastly, an Adaptive Unscented Kalman Filter (AUKF)-based navigation solution is proposed. Offline validation, through the use of navigation data obtained during sea trials undertaken in La Spezia (Italy) at the NATO STO Centre for Maritime Research and Experimentation (CMRE), is presented. Afterward the results of real autonomous underwater missions performed in La Spezia (Italy) within the activities of the SEALab, the joint research laboratory between the Naval Experimentation and Support Center (Centro di Supporto e Sperimentazione Navale) (CSSN) of the Italian Navy and the Interuniversity Center of Integrated Systems for the Marine Environment (ISME), and at Vulcano Island, Messina (Italy) are reported.