The horizontal position of the remotely operated vehicle (ROV) during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition between November 2019 and September 2020 was measured using an acoustic Long Base Line (LBL) positioning system (LinkQuest Pinpoint). The position was recorded in the SPOT.ON survey systems software (OceanModulesTM). The track was smoothed from initial acoustic fixes and cleaned for most obvious outliers. The position is in a floe-fixed, relative coordinate system (X, Y) with the origin (X=0 m, Y=0 m) at the ROV hole. A quality flag for the position is introduced based on the time to the closest fix with “1” indicating good positon (fix reached 3s & 5s). Depending on the scientific aim, a position with quality flag “3” can still be useful. Vehicle depth was measured by the integrated pressure sensor and calibrated to 0 during pre-survey procedures, when the top side of the vehicle was at the same level as the water surface. Vehicle attitude (roll, pitch, heading) was measured with an onboard inertial measuring unit (IMU, Microstrain) with three axis accelerometer, magnetometer and gyroscope. Depth was measured by a pressure sensor (Keller A-21Y, Keller AG) included in the main electronics housing of the ROV.

Horizontal position (X,Y):• Leg 1: Scale factors and offsets to successfully improve positions for surveys PS122_1_5_62_20191102_1 and PS122_1_6_16_20191105_1 were obtained by comparing positions of markers from the ROV and a terrestrial laser scanner (RIEGL). Scale factors and offsets for surveys PS122_1_6_31_20191106_1, PS122_1_6_118_20191110_1, PS122_1_7_18_20191112_1, PS122_1_7_55_20191113_1 are based on estimates but will be improved also using markers.• Leg 4: Positions for surveys PS122_4_48_213_20200726_1 and PS122_4_49_105_20200728_1 are partly distorted probably due to usage of drifting transponder(s) or erroneous transponder position configuration. Retrieving suitable scale factors and offsets was not successful so far.• Leg 5: probably due to drifting transponder(s). Retrieving suitable scale factors and offsets was not successful so far.