Baikal-GVD Component Positioning.pdf

The detector's components are mobile and may drift from their initial coordinates or change their spatial orientation. This introduces a reconstruction error, particularly a timing error for PMT hits. The problem is mitigated by a combination of a hydroacoustic positioning system and per-component acceleration and orientation sensors. Under regular conditions, the average positioning accuracy for a GVD component is estimated to be less than 13 cm.

The hydroacoustic positioning system (APS) is an array of EvoLogics S2C R42/65 acoustic modems mounted along the strings. The modems communicate with each other using the D-MAC protocol. The modems installed along the optical modules are called beacons and are directed downwards. Some strings have modems installed near the anchor (nodes). The node coordinates are determined shortly after string installation and are assumed constant.

During cluster operation, the beacons are regularly polled from the shore for acoustic distances to the lake floor acoustic antenna formed by the nodes. This data is used to trilaterate beacon coordinates with an accuracy of several centimeters. OM coordinates are then interpolated from beacon positions assuming a piece-wise linear model of the string. The coordinates for the calibration light sources like lasers and LED matrices are acquired the same way.

The trilaterated coordinates for beacons on one string are provided on Figures 5 and 6. The drift mainly occurs in the XY plane. Season-long depth variations are within 0.5 m (except in the 'active' period, when they can reach up to several meters). Coordinate variation decreases with depth from about 25 m at the top of the cluster to about 2.5 m below the bottom OM (about 50 m to about 5 m for active period).

The acoustic data acquired in 2017 can be used to observe the dynamics of GVD. The maximum speed of the most shallow and, therefore, the most mobile, beacon is estimated to be below 3 cm/s. The mean speed for the most shallow beacons is about 0.5 cm/s. As with coordinate range, the speed falls with depth. The distance travelled by the AM between polls may reach several meters during the 'active' period.

The beacon coordinates are correlated. As shown in Figure 9, the distance between beacons installed at similar depths within one cluster is consistent even at the most shallow depths. A similar dynamic is present within one string and even between clusters.

The spatial orientation of OMs is determined from accelerometer and compass data provided by intra-OM sensors polled independently from APS. As of 2018, the system is fully deployed.

The error in OM positioning using the above described procedure varies with speed of the OM and its distance to the nearest beacon. To gauge the OM positioning error for the APS, an additional beacon has been installed on the central string of cluster 3, between beacons 3 and 4. Its coordinates have then been reconstructed as if it was an OM and compared to the coordinates acquired via acoustic trilateration.

The analysis used data acquired from April 30th to October 1st 2018. The period between September 20th and September 29th 2018 is characterised by an unusually high drift and speed, and has thus been processed separately. As can be seen in Figure 10, the mean positioning error is 13 ± 3 cm for the regular period, and 21 ± 10 cm for the 'active' period.