Obtaining absolute water velocity profiles from glider-mounted Acoustic Doppler Current Profilers

ADCP sensors have recently been incorporated into AUV Gliders offering the potential for increased physical and biological oceanographic observations. These vehicles can collect high-density measurements for weeks at a time at a small cost compared to vessel-reliant operations. Glider-based ADCPs can provide absolute water velocity profiles based on the “shear method” developed from LADCP research and offer the ability to perform oceanographic research where stationary platform and vessel-based ADCP observations are unavailable or not suitable. The ability to obtain accurate velocity profiles from gliders will further enable high resolution and cost-effective oceanographic observations. Shear is calculated from water track velocities measurements for every ensemble and averaged within depth intervals to create one shear profile for each glider dive. The vertically averaged shear profile is integrated to establish a velocity profile relative to the bottom of the glider dive. As shear is independent of platform motion, integrated relative velocities will not contain glider motion bias. Absolute velocities are computed by referencing dive-averaged velocity measurements, based on the glider drift. Bottom track velocities, where available, they be referenced to water track measurements and provide absolute velocities. Bottom referenced velocities provide a verification for dive-averaged velocities and are valuable for removing diverging velocity offsets. Effective data processing methods, data filtering, accurate heading information, and instrument settings for high-quality data collection are the tools for obtaining absolute water velocity profiles from gliders. The Oregon State University Glider Research Group is refining these methods by comparing results against concurrent velocity measurements from adjacent ADCP platforms. The findings described in this document result from experiments performed in 2011 by OSU off the Oregon Shelf and during the Off- ce of Naval Research sponsored Lateral Mixing Project in the Sargasso Sea.