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, 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 geo-referenced beacons
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 193.5 kHz and streamed to shore for analysis. Time of arrival of acoustic signals is used to
measure the distance between APS elements. In this contribution we will present results and
performances of a position and orientation reconstruction system based on multi-lateration, and
provide results for ARCA