Motion tracking in an acoustic point-measurement current meter

Measurements of velocity structure in the water column under Arctic ice from an Ice-Tethered Profiler (ITP) employed an acoustic point-measurement current meter, MAVS (Modular Acoustic Velocity Sensor). With the velocity sensor it becomes the Ice-Tethered Profiler with Velocity (ITPV). The profiler, containing a Seabird CTD, MAVS, batteries, an inductive modem, and a wire crawling engine, integrated by McLane Labs, was constrained to be deployed through a 24" diameter hole drilled in the ice. The anchor to the ice via a buoy with a satellite transmitter fixed the top of the mooring to a drifting but GPS tracked location while the profiler descending to a depth of 800 m measured velocity relative to the moving mooring and the climbing profiler. A large current-orienting alignment fin was not possible on the ITP due to the limit of the ice hole diameter yet it was known that vortex shedding by the profiler body in the current would cause the instrument to swing and the current sensor to measure horizontal velocities due to the rotation of the profiler around the center of gyration of the package. To remove this platform motion from the current measurement, an inertial sensor, Analog Devices ADIS 16355, was added to the MAVS current meter and three axes of angular velocity and three axes of linear acceleration were added to each data record of time, velocity, temperature, three-axis magnetic vector components and two axes of tilt. From the rate gyro value of angular velocity around the vertical axis, the platform rotation is determined and using the distance that the velocity sensor is displaced from the axis of rotation the horizontal current sensor velocity can be subtracted from the horizontal velocity measured by the sensor. This ITP was deployed in October, 2009 and data from the first profile indicates expected and unexpected performance.