Field Verification of the Pinnacle ADCP

Several technological advances embodied in the new Pinnacle Acoustic Doppler Current Profiler (ADCP) were introduced at the Current, Waves, Turbulence Measurements and Applications (CWTMA) Workshop held earlier this year. These new technologies will be briefly reviewed to set the stage for the verification testing at the heart of this paper, but with details left to the CWTMA proceedings. “Bottom Track” data are an ADCP's measurement of its velocity relative to the bottom. For Pinnacle the accuracy of its Bottom Track measurements were verified using GPS fixes from a vessel transiting water over 1000 m deep. The vessel's distance travelled was calculated in two ways: by accumulating the distance between repeated GPS fixes; and by accumulating distance obtained by multiplying the ADCP Bottom Track velocity by the time between measurements. As the two calculations of distance are accomplished independently, comparing their results provides a good assessment of the accuracy of the ADCP. The water profiling (both broadband and narrowband) verification was then accomplished by comparing the measured water profile velocities of the Pinnacle ADCP to an Ocean Surveyor ADCP permanently mounted to the survey vessel. The Pinnacle ADCP provides the ability to calculate and apply a post calibration of the compass to the data gathered during a moored deployment. A typical field calibration involving manually rotating the instrument cannot generally be accomplished at a deep ocean deployment site because of the infrastructure and machinery present on any deploying vessel. Therefore, Pinnacle stores the data needed for a field calibration during the deployment, or more to the point, after the vessel and its associated magnetic interference terms have left the area. The data needed are its magnetometer and accelerometer measurements, and Pinnacle adds these measurements to each ADCP velocity measurement made throughout the deployment. The recovery software then uses these data in a manner analogous to the data taken during a typical “flat” field calibration to create a new compass calibration that can be applied to the raw data in post-processing. This does require that the natural rotation of the instrument within the mooring will provide enough heading coverage to create the new calibration. Field data will be presented to illustrate the difference between a good post-calibration and a bad one, with some further explanation of how to tell the difference in deciding whether to retroactively apply it to the gathered data.