Forward looking sonar data collected along OFOBS profiles during POLARSTERN cruise PS101 (ARK-XXX/3)

The Ocean Floor Observation System (OFOS), used during Polarstern research cruise PS101 to the Langseth Ridge in the Central Arctic, was a towed underwater camera system equipped with both a high-resolution photo-camera (iSiTEC, CANON EOS 5D Mark III) and a high-definition video-camera (iSiTEC, Sony FCB-H11). The cameras were mounted on a steel frame (150L x 92W x 105H cm), together with two strobe lights (iSiTEC UW-Blitz 250, TTL driven), three parallel laser pointers spaced with a distance of 50 cm used to estimate the size of seafloor structures, four LED lights, and a USBL positioning system (Posidonia) to track the position of the OFOS during deployments. For PS101 the OFOS was upgraded to additionally mount an EdgeTech 2205 bathymetric side scan and a BlueView M900-130 forward looking sonar systems, to become the Ocean Floor Observation and Bathymetry System (OFOBS). The M900-130 has a total horizontal opening angle of 130°, divided into 512 beams of 0.18° x 20° (horizontal x vertical). It operates at an acoustic frequency of 900 kHz, resulting in a sharp image wedge with ranges up to 100 m. The ping rate of the the OFOBS M900 is triggered by the 2205 sonar, resulting in ping rates of around 3 Hz during PS101. The M900 FLS was mounted in the upper front of the OFOBS subsea unit, about 65 cm above the bottom edge of the steel frame with an angle of 5° downward from the horizontal