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Feature Article

Improving Ocean Current Measurement from Gliders

By Eric Siegel • Peter J. Rusello

The AD2CP mounted in the aft section of the Scripps Spray glider. (Photo Credit: Jeff Sherman, Scripps Institution of Oceanography)
Underwater ocean gliders, such as the Teledyne Webb Research (Falmouth, Massachusetts) Slocum glider, University of Washington/iRobot Corp.'s (Bedford, Massachusetts) Seaglider and the Scripps Institution of Oceanography Spray glider, are proven platforms for measuring ocean properties like temperature, density, dissolved oxygen and chlorophyll fluorescence. Hydrocarbon sensors mounted on gliders tracked oil spilled from the Deepwater Horizon oil rig in the Gulf of Mexico. Radiation sensors tracked irradiated water from the Fukushima nuclear power plant off the coast of Japan. All these measurements add vital in-situ data to reports, models and forecasts.

Gliders also offer a great opportunity for accurate measurements of ocean currents. The typical sawtooth flight path, profiling vertically down to 1,000 meters and following a transect line hundreds of kilometers long, provides opportunities for measuring ocean currents with high vertical and horizontal resolution over greater depths and larger areas than moored acoustic Doppler current profilers (ADCPs). Ocean current measurements from gliders provide a dynamic interpretation to individual sensor data.

Velocity profiles can be used to interpret other physical variables measured by gliders. For instance, velocity measurements can confirm evidence of upwelling or downwelling. In combination with spectrophotometric measurements, they can provide information on vertical migration of phytoplankton. Variance in velocity shear at different locations could explain formation and dissipation of phytoplankton thin layers. Offshore oil and gas operators can use information about currents, especially at depth, to optimize operations by anticipating when drilling operations might be interrupted, thereby reducing downtime. Velocity measurements can be assimilated into numerical circulation models to improve forecast accuracy.

The benefits of the mobile platform, however, are not without complications. During a dive, a fundamental problem with gliders is defining the precise horizontal location where measurements are made. Typical acoustic baseline positioning systems are not well-suited to glider operations because of the necessary infrastructure and kilometers-long transect lines gliders often fly. Velocities measured from the glider can refine dead-reckoned position estimates, therefore improving glider navigation and location accuracy. Using an initial position obtained via GPS at the surface and the measured velocity, the specific location of the glider at every moment in time during the entire glide path can be determined by integrating the velocity record.

Using ADCPs to measure ocean currents from gliders provides a free measurement of acoustic backscatter throughout the water column. Backscatter readings indicate particle concentration in the water. An ADCP operating at 1-megahertz acoustic frequency is sensitive to zooplankton and other particles with similar size, such as suspended oil droplets. For instance, to understand whale migration and feeding patterns, Woods Hole Oceanographic Institution (WHOI) researchers utilize backscatter data from glider-mounted ADCPs to track zooplankton location in the water column.

Since 2005, Nortek AS (Oslo, Norway) has collaborated with leading researchers at WHOI, the University of Washington, Rutgers University, Memorial University, University of California at Santa Barbara, Scripps Institution of Oceanography and iRobot to develop specialized ADCPs and data-processing methods to measure current velocity from gliders. In 2012, Nortek released the AD2CP-Glider (acoustic Doppler current profiler for gliders) developed specifically for the challenges of measuring current velocity from gliders, such as small size, low power consumption and precise velocity measurements. The instrument must also be able to tolerate frequent pressure cycling and have a high-quality orientation sensor to resolve the pitch and roll angles during descent and ascent.

The AD2CP uses broadband processing for accurate velocity measurements. It operates at an acoustic frequency of 1 megahertz and provides a profiling range of 15 to 30 meters, depending on scattering conditions. The 1-megahertz transducers allow for a small physical size and good return signal strength over the dive profile of 1,000 meters in typical ocean scattering conditions. The instrument uses a four-beam transducer head that creates different symmetric three-beam arrays: one that can be used on descent and the other on ascent. By measuring during both descent and ascent, the AD2CP provides a more complete data set for post-processing compared to instruments capable of sampling only on ascent or descent.

The AD2CP is equipped with a pressure sensor and a microelectromechanical-systems (MEMS) tilt sensor and compass capable of measuring throughout the large pitch range that gliders experience. The AD2CP is controlled over a standard RS-232 interface from the glider's main computer, allowing easy reconfiguration and download of data subsets that can be transferred to shore using the glider communication systems. The interface allows the glider computer to write GPS position data and other relevant dive parameters to the AD2CP memory. Onboard memory (SD card) provides large storage capacities for long-duration missions. Fast data download via an Ethernet interface is available when the AD2CP is retrieved. To continue this article please click here.

Eric Siegel is a physical oceanographer and Nortek's business development manager. He enjoys collaborating with clients to develop new applications and innovative oceanographic measurement solutions. He has a master's in physical oceanography from University of South Florida and an MBA from Northeastern University.

Peter J. Rusello is a scientist at Nortek, focusing on measurements from moving platforms, turbulence and pulse-coherent signal processing. He holds a Ph.D. from Cornell University in civil and environmental engineering, with a focus on environmental fluid mechanics.

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