Correcting for slow platform movement in deep-water ADCP measurements

Sveinn Ó. Pálmarsson, Todd E. Steissberg, S. Geoffrey Schladow

Limnol. Oceanogr. Methods 3:338-348 (2005) | DOI: 10.4319/lom.2005.3.338

ABSTRACT: When boat-mounted acoustic Doppler current profiler (ADCP) velocity measurements are taken in deep water where bottom-tracking capability no longer exists, navigational data via the Differential Global Positioning System (DGPS) are commonly collected to correct for the boat’s contribution to the recorded ADCP velocity. The boat’s cruise speed and the desired averaging interval for the water velocity are often great enough for navigational errors to become insignificant contributors to the uncertainty of the resulting water velocity. When the boat or other measurement platform is nearly stationary, navigational errors become significant and lead to an incorrect water velocity when navigational corrections are applied. Furthermore, when the water current is on the same order of magnitude as the slowly moving boat, the boat movement may not be easily distinguished from the fluid velocity. Factors that affect the quality of DGPS navigational estimates include the number of available satellites and their geometric properties in respect to one another. Here we present a method to correct the navigational velocity by developing criteria for geometric properties of the satellites, thus providing more accurate estimates of the water velocity. Assessment of the method indicates that less temporal averaging of the water velocity is needed to obtain a given uncertainty than when the cleaning procedure is not applied. This is an important property for many applications that often require relatively short temporal intervals, such as particle dispersion studies and the investigation of high-frequency internal waves.