Acoustic positioning system for KM3NeT

[EN] KM3NeT is the next generation neutrino telescope in the Mediterranean Sea employing the technique of Cherenkov photon detection. The Acoustic Positioning System (APS) is a mandatory sub-system of KM3NeT that must provide the position of the telescope¿s mechanical structures, in a geo-referenced coordinate system. The APS is important for a safe and accurate deployment of the mechanical structures and, for the sake of science, for precise reconstruction of neutrinoinduced events. The KM3NeT APS is composed of three main sub-systems: 1) an array of acoustic receivers rigidly connected to the telescope mechanical structures; 2) a Long Base-Line (LBL) of acoustic transmitters (beacons) and receivers, anchored on the seabed at known positions; 3) a farm of PCs for the acoustic data analysis, on-shore. On shore, the positions of the acoustic receivers are calculated by measuring the ToF (Time Of Flight) of the LBL beacons¿ signals on the acoustic receivers, thus determining, via multi-lateration, the position of the acoustic receivers with respect to the geo-referenced LBL. The synchronized and syntonized electronics and the data transmission/acquisition allows for calculating the latencies of the whole data acquisition chain with an accuracy of better than 100 ns. The APS, in combination with compass and tilt, pressure, current and sound velocity data, is expected to measure the positions of the digital optical modules in the deep sea with an accuracy of about 10 cm. Since data are continuously transmitted to shore and distributed to the local data acquisition network at the shore station, acoustic data are available also for Earth and Sea science users. The KM3NeT APS is also an excellent tool to study the feasibility of an acoustic neutrino detector and a possible correlation between acoustic and optical signals.