Improving Ocean Current Measurement from Gliders AD2CP Hardware, Post-Processing Software Provide Solutions for Gliders By Eric Siegel • Peter J. Rusello
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nderwater 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 con(Top) Profiles of water velocity (left) and glider velocity through water (right) firm evidence of upwelling or downwelling. from the AD2CP mounted on a Seaglider from Port Susan, Washington. In combination with spectrophotometric measurements, they can provide information on The AD2CP mounted in the aft section of the Scripps Spray glider. (Photo vertical migration of phytoplankton. Variance Credit: Jeff Sherman, Scripps Institution of Oceanography) in velocity shear at different locations could because of the necessary infrastructure and kilometers-long explain formation and dissipation of phytoplankton thin transect lines gliders often fly. Velocities measured from the layers. Offshore oil and gas operators can use information glider can refine dead-reckoned position estimates, therefore about currents, especially at depth, to optimize operations improving glider navigation and location accuracy. Using an by anticipating when drilling operations might be interruptinitial position obtained via GPS at the surface and the meaed, thereby reducing downtime. Velocity measurements can sured velocity, the specific location of the glider at every mobe assimilated into numerical circulation models to improve ment in time during the entire glide path can be determined forecast accuracy. by integrating the velocity record. The benefits of the mobile platform, however, are not Using ADCPs to measure ocean currents from glidwithout complications. During a dive, a fundamental probers provides a free measurement of acoustic backscatter lem with gliders is defining the precise horizontal location throughout the water column. Backscatter readings indicate where measurements are made. Typical acoustic baseline particle concentration in the water. An ADCP operating at positioning systems are not well-suited to glider operations Reprinted from Sea Technology magazine. For more information about the magazine, visit www.sea-technology.com
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 glidermounted ADCPs to track zooplankton location in the water column. The AD2CP. AD2CP 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 dataprocessing 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.
Measurement Challenges Despite many reasons to measure velocity from gliders, operational measurements have been slow to become
mainstream because of three challenges: the instrument size was too large, power consumption was too high and data processing was too onerous. The AD2CP provides solutions to these problems, making measurements of ocean currents easier and more efficient. Size. The AD2CP is small, lightweight and wellsuited for integration on glider platforms. The instrument is a cylinder with a diameter of 13.5 centimeters and a height of 12.2 centimeters. It is rated to 1,000 meters depth, and the weight in water is about 0.9 kilograms. Power Consumption. A glider mission is planned based on power consumption. Taking advantage of modern electronics and stringent power management features, the AD2CP consumes very little power. Depending on the sampling configuration, the glider uses between 0.2 to 0.9 watts. It can sample regularly in time (e.g., every 1 second) or with depth (e.g., every 1 meter) using its onboard clock or pressure sensor. The AD2CP operates over a power supply range of 18 to 26 volts DC. Assuming that most gliders have a descent rate around 10 centimeters per second, an efficient (0.2-watt) AD2CP configuration would include a single acoustic ping that profiles more than 20 cells at 1-meter resolution every 10 seconds. This configuThe AD2CP. ration provides 95 percent overlap in measurement profiles, yielding a well-sampled data set for post-processing. Sampling less frequently will decrease profile overlap and further reduce power consumption. Data Processing. The raw velocity measured is the water motion relative to the glider (using a frame of reference attached to the glider). The Nortek post-processing software separates the raw measurements into Earth-referenced glider velocity over ground (for navigation) and the water velocity over ground (for ocean current profiling). Nortek has implemented two solutions to process the data: a linear least-squares solution and integration of measured shear profiles. Both methods yield an average watervelocity profile, while the least-squares solution also produces estimates of glider velocity over ground. The shear solution can be used to estimate glider velocities from the original measurements. The least-squares solution, originally developed to process lowered ADCP casts, is also the preferred processing method for glider data sets. This method is used to process measurements from a variety of underwater vehicles. AD2CP data are collected continuously on descent and ascent. Measured beam velocities are transformed into east, north and up velocities for further processing, placing them in a stationary reference frame attached to Earth. Ideally, the cell size, sample interval, and descent and ascent rates are established so that as the glider descends or ascends, sampling of the water column occurs at the same depths. The bottom location, if within range, is determined by looking at amplitude returns in all beams, and any measurement cells below the bottom are removed from the data set. The near-bottom cells are used to obtain estimates of the
glider velocity by assuming the bottom is a stationary target, thus providing a valid measurement of the glider velocity over ground. The AD2CP pressure sensor and velocity sample interval are used to calculate glider vertical velocity at each sample location. This can be compared against the least-squares vertical velocity estimate to give insight into where and what magnitude errors occur. Initial comparisons showed a small root-mean-square error between the pressure-based velocity estimate and the least-squares glider vertical velocity of only a few centimeters per second.
“AD2CP data are collected continuously on descent and ascent.” Deployments, Development The AD2CP has been used on the University of Washington/iRobot Seaglider and Scripps Spray glider platforms. The first AD2CP was deployed on a Seaglider for a project in January 2012 focused on ocean circulation and zooplankton biomass in Antarctica. The instrument was deployed on several missions to about 900 meters. Other glider missions were conducted for testing, development and velocity profiling evaluation in Cayuga Lake, New York (deployed by Nortek and iRobot); Puget Sound, Washington, and coastal North Carolina (deployed by iRobot); and San Diego, California (deployed by Scripps). The initial development and integration work with iRobot resulted in numerous improvements to the AD2CP hardware and software. For example, a new interface structure was developed to control the AD2CP from a simple glider microprocessor. The carefully considered hardware and firmware interface simplified integration for new users. A working implementation on the Spray glider took only a month to complete before sea trials. It was deployed offshore San Diego for short testing and evaluation experiments from January to March this year. Results were promising, and no updates to the AD2CP were needed. The AD2CP is platform-agnostic and self-contained, both in a physical sense, as it comes in a robust titanium housing, and in terms of the data needed to successfully process and interpret measurements. It fits into a small glider, and needs only power and a control link to the glider. This self-contained attribute, coupled with its small size and low power requirements, make the AD2CP ideal for integration with all gliders and other data-collection systems, such as vertical profilers, AUVs and ROVs. n 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. ©Copyright 2013 by Compass Publications Inc. Sea Technology (ISSN 0093-3651) is published monthly by Compass Publications Inc., Suite 1010, 1600 Wilson Blvd., Arlington, VA 22209; (703) 524-3136;
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