[Paper Review] A positioning system for Baikal-GVD

Baikal-GVD Collaboration, A. D. Avrorin+51 more|arXiv (Cornell University)|Aug 15, 2019

Astrophysics and Cosmic Phenomena4 citations

TL;DR

This paper presents an acoustic positioning system (APS) for the Baikal-GVD neutrino telescope, using hydroacoustic modems (AMs) on detector strings to track optical module (OM) positions in real time. The system achieves a mean positioning accuracy of 12 ± 6 cm, enabling subnanosecond time calibration despite OM drift of up to 50 meters due to water currents.

ABSTRACT

A cubic kilometer scale neutrino telescope Baikal-GVD is currently under construction in Lake Baikal. Baikal-GVD is designed to detect Cerenkov radiation from products of astrophysical neutrino interactions with Baikal water by a lattice of photodetectors submerged between the depths of 1275 and 730 m. The detector components are mounted on flexible strings and can drift from their initial positions upwards to tens of meters. This introduces positioning uncertainty which translates into a timing error for Cerenkov signal registration. A spatial positioning system has been developed to resolve this issue. In this contribution, we present the status of this system, results of acoustic measurements and an estimate of positioning error for an individual component.

Motivation & Objective

• To address positioning uncertainty in the Baikal-GVD neutrino telescope caused by flexible strings drifting due to lake currents.
• To enable precise event reconstruction by minimizing timing errors from inaccurate OM coordinates.
• To develop a real-time, online positioning system using hydroacoustic trilateration for OMs and auxiliary components.
• To quantify OM positioning error and assess its dependence on depth, season, and beacon mobility.
• To validate the system using in-situ measurements and cross-comparison with interpolated coordinates.

Proposed method

• Deployment of EvoLogics S2C R42/65 acoustic modems (AMs) on detector strings, including downward-pointing beacons and upward-pointing nodes.
• Use of trilateration with known node coordinates and measured acoustic distances to determine beacon positions in real time.
• Implementation of the D-MAC protocol for reliable communication between AMs and shore-based data acquisition systems.
• Online polling of beacon distances every ~1 minute per cluster, followed by coordinate reconstruction and buffering at the shore.
• Interpolation of OM positions using piecewise linear models based on trilaterated beacon coordinates.
• Installation of additional test beacons at 811 and 823 m depths to validate positioning accuracy via comparison with interpolated values.

Experimental results

Research questions

• RQ1 What is the achievable positioning accuracy of the acoustic system for optical modules in the Baikal-GVD detector?
• RQ2 How does OM drift vary with depth and season, and what is the typical speed of lateral movement?
• RQ3 To what extent are OM positions correlated across different strings and clusters?
• RQ4 How does the positioning error scale with distance from the reference beacons and with beacon mobility?
• RQ5 Can the acoustic positioning system maintain subnanosecond timing calibration despite environmental drift?

Key findings

• The acoustic positioning system achieves a mean OM positioning error of 12 ± 6 cm over a 11-month period, with a 95% confidence interval.
• OMs exhibit lateral drift of up to 50 meters from their median positions, with average speeds of 0.5 cm/s and maximum speeds of 3 cm/s.
• Beacon mobility decreases with depth, dropping from ~50 m variation at 736 m to ~5 m at 1274 m depth.
• OM coordinates within a cluster and across clusters show strong temporal correlation, indicating coherent motion patterns.
• The system's precision is comparable to that of other large-scale neutrino telescopes, such as IceCube and KM3NeT.
• The positioning error is primarily limited by beacon mobility and distance from reference points, not by measurement noise.