Monitoring and Reconstruction of the Shape of the Detection Units in KM3NeT Using Acoustic and Compass Sensors †

The KM3NeT underwater neutrino telescope comprises thousands of optical modules forming 3D arrays to detect the Cherenkov light produced by particles generated after a neutrino interaction in the medium. The modules are arranged in detection units—vertical structures with 18 modules at different heights, anchored to the seabed and kept vertical by the buoyancy of the optical modules and a top buoy. The optical modules are, thus, subject to movements due to sea currents. For a correct reconstruction of events detected by the telescope, it is necessary to know the relative position and orientation of modules with 10 cm and a few degrees accuracy, respectively. For this, an Acoustic Positioning System with a piezoceramic transducer installed in each module and a long baseline of acoustic transmitters and receivers on the seabed are used. In addition, there is a system of compass and accelerometers inside the optical modules to determine their orientation. A model of mechanical equations is used to reconstruct the shape of the detection unit taking as input the information from the positioning/orientation sensors and using the sea current velocity and direction as free parameters. The mechanical equations take the buoyancy and the drag force of the elements of the detection unit into account. This work describes the full process that is implemented in KM3NeT to monitor the modules and the shape of the detection units from the measured position and orientation data.