Acoustic position calibration of the KM3NeT neutrino telescope

KM3NeT is a future neutrino telescope with a volume of several cubic kilometres at the bottom of the Mediterranean Sea. It will be composed of at least 300 vertical structures of several hundred metres length, named detection units (DU), anchored on the seabed and kept vertical with a buoy. The DU will host optical sensors able to detect the faint Cherenkov light emitted in the deep sea by charged particles originating from collisions of the neutrinos with nuclei in the sea water or the rock below. A precise knowledge of the optical sensor positions relative to each other, not worse than 20 cm, is needed for an accurate reconstruction of the charged particle tracks. Since the marine currents cause movements of the DUs and thus a displacement of the optical sensors of up to several tens of metres from their nominal positions, an Acoustic Positioning System (APS) is necessary to monitor their positions in deep sea. The KM3NeT APS will be composed of acoustic transceivers at fixed positions on the sea floor and acoustic receivers (hydrophones) along each DU. The hydrophone positions are determined by triangulation using the travel times of acoustic signals between emitters and receivers. This paper presents the acoustic positioning system designed for KM3NeT and in particular the innovative technologies used for transducers, electronics, and signal processing.