The KM3NeT/ARCA: Status and results of the Acoustic Positioning System

The KM3NeT (Cubic Kilometre Neutrino Telescope) Collaboration is currently building two underwater neutrino detectors: ORCA (Oscillation Research with Cosmics in the Abysses) located offshore Toulon, France, at 2500 m water depth, and ARCA (Astronomy Research with Cosmics in the Abysses) offshore Capo Passero, Italy, at 3500 m water depth. The basic element of the detector is the DOM (Digital Optical Module), housing 31 photomultiplier tubes (PMTs), capable of recording single photon arrival time and charge, to be sent to shore for triggering, filtering, and data analysis. A vertical line of 18 DOMs forms a Detection Unit (DU): about 700 m long in ARCA, and about 200 m long in ORCA. A custom Acoustic Positioning System (APS) was designed and developed by the KM3NeT collaboration to obtain an accuracy of a few centimeters in the determination of the position of each DOM. In the case of KM3NeT-ARCA, the subject of this paper, this is required in order to be able to reconstruct neutrino directions with the desired pointing resolution. Building upon the White Rabbit time synchronization, the APS operates as a phased array of several thousand digital acoustic receivers, hosted in all DOMs and the anchors of the DUs. Each receiver records acoustic signals emitted by a long baseline of georeferenced acoustic emitters anchored on the seafloor. The APS allows constant monitoring of the position and movements of the detector elements, subject to underwater currents. Acoustic data are continuously sampled at 195.3 kHz and streamed to shore for analysis. The time of arrival of acoustic signals is used to measure the distance between APS elements. This contribution presents the results and performance of a position and orientation reconstruction system based on multilateration, along with results for ARCA.