A buoyancy glider equipped with a tri-dimensional acoustic vector sensor for real-time underwater passive acoustic monitoring at low frequency

For the first time at CMRE, an acoustic vector sensor, namely the 3D GeoSpectrum M20-040, has been integrated into one of CMRE Slocum buoyancy gliders. This sensor is able to provide directionality in the band from few Hz to 3 kHz, despite its limited size. The sensor beampattern was first measured in a water tank. The glider has been equipped for the first time also with an acoustic modem, able to communicate with a USBL system deployed from a mother ship or a gateway buoy. Through the USBL, a Command and Control Station can get R/T measurements of the glider position and receive its detection alarms during the underwater missions, without need for the glider to surface. First at-sea tests of navigation, underwater communications and acoustic data collection were conducted during REP18-Med trials (Palmaria Island, Italy, Oct. 2018). Evaluating the smoothness of the glider navigation is fundamental to understand whether a glider is suitable to conduct passive survey with a sensor sensitive to acceleration. The USBL position measurements were compared to the estimate achieved in post-processing by filtering the proprioceptive data collected on-board the glider, corrected with the available GPS fixes. The paper provides a detailed description of the system, along with the discussion of preliminary water-tank and at-sea results.