Mechanical Line Fit Model to Monitor the Position of KM3NeT Optical Modules from the Acoustic and Compass/Accelerometer Sensor System Data

Abstract

The KM3NeT deep-sea neutrino telescope will use thousands of Digital Optical Modules (DOMs) forming a 3D array to detect the Cherenkov’s light produced by the particles generated after a neutrino interaction in the medium. The DOMs are arranged in Detection Units (DUs), structures anchored and maintained vertical by buoyancy each one containing 18 DOMs at different height. The DOMs are, thus, subject to movements due to sea currents. For a correct reconstruction of events detected by the telescope, it is necessary to monitor the position of each DOM with 10 cm accuracy. For this, an Acoustic Positioning System (APS) with a piezo-ceramic transducer installed in each DOM and a long baseline of acoustic transmitters and receivers on the seabed is used. Besides, there is a system of compass/accelerometers in the DOMs to determine their orientation. Then, a mechanical model is used to reconstruct the shape of the DU taking as input the information from the positioning sensors and using the sea current velocity as free parameter of the DU Line Fit method. The mechanical equations consider the buoyancy and the drag force of any item in the DU line. This work describes the data process of the different sensors and systems to obtain the fit shape of DUs, the situation for the first DUs installed as an example and to study the viability and define the full process to apply in KM3NeT.

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

“…Assuming that the distance between emitter and receiver is known, and the sound speed gradient ( ) is estimated considering the environmental properties (the along a DU is estimated between 1552.0 and 1563.8 m/s in ARCA, and between 1545.6 and 1549.0 m/s in ORCA [ 17 ]), it is possible to determine the ToE. From the ToA and the ToE values for three different emitters in the same receiver, it is possible to calculate the Time of Flight (ToF), so the distance, and do the triangulation to obtain the x , y , and z coordinates for every receiver [ 13 , 18 ].…”

“…The mechanical model (MM) in the DU Line Fit for KM3NeT is a set of mechanical equations used to determine the position and orientation of every DOM from sea current properties and the position of the DU-base. The MM uses mechanical constants ( ) calculated from the mechanical properties of all items in a DU [ 18 ].…”

“…The sea current velocity is assumed to be the same along the entire line and the detector. By integration of over z , the total displacement between the item in height z can be obtained Equation ( 5 ) [ 18 ]: where represents the vertical displacement in the height z of the line considering the sea current velocity v and the mechanical constants ( ) of DU parts.…”

KM3NeT Detection Unit Line Fit reconstruction using positioning sensors data

“…For the DU Line Fit a Mechanical Model (MM) for KM3NeT has been developed, starting from the line shape model for ANTARES detector [3], that combines APS and AHRS data to improve the DU alignment. The MM is a computational method that determines the coordinates in space [𝑋, 𝑌 , 𝑍] from the velocity (𝜈) and direction (𝜔) of the sea current, thanks to the mechanical equations based on known mechanical properties of the DU [9]. The MM distinguishes two different analyses, depending on the data source to study: tilt and position methods.…”