Processing of velocity observations from Ice-Tethered Profilers

Abstract

Ocean velocity in the Arctic is estimated using a three-axis acoustic travel-time current meter onboard an IceTethered Profiler (ITP). The ITP is moored on an ice floe, with the current meter measuring velocity past the sensor as it profiles vertically through the water column along an inclined wire and translates horizontally with the motion of the overlying sea ice. Accurate estimates of ocean velocity rely on simultaneous estimates of instrument pitch, roll, and heading, the velocity at which the sensor crawls along the wire, and the velocity of the overlying sea ice. An inertial measurement unit is used to derive instrument pitch, roll, and heading from a three-axis flux-gate compass, accelerometer, and angle rate gyro. Instrument speed along the wire is derived from pressure as measured by the CTD. Hourly GPS fixes of the surface package are used to estimate ice velocity. Pre-deployment calibrations are preformed to determine magnetometer, accelerometer, pitch, roll, and heading biases. Post-deployment calibration parameters are determined to account for remaining biases. Post-deployment parameters are chosen so that there is no obvious bias related to wire angle, velocity estimates when the profiler is stationary versus moving, or upward versus downward profiles. These procedures are described as implemented for the second-generation velocity sensor deployed in the Arctic Ocean on five Ice-Tethered Profilers during 2013-2015.