A Buoyant Tethered Sphere for Marine Current Estimation

The high cost of acoustic Doppler ocean current meters means few are deployed in marine research studies. To address this problem, we have developed a low-cost robust current velocimeter based on the drag-tilt principle. The instrument tilts in response to current flow, for which the angle and direction of tilt are related to the water velocity. Static analytic approximation shows a sigmoid-type tilt response to increasing current speed. We detail a calibration method that models the relationship using a Gompertz curve. Calibration and field tests conducted near Magnetic Island, Australia, show a speed accuracy of 0.05 m/s for current speeds less than 0.6 m/s, and direction accuracy better than 15 ^° for current speeds greater than 0.15 m/s. This instrument should be especially useful for research projects where numerous or spatially dense measurements of ocean currents are required.