Velocity Imprecision in Finite-Beamwidth Shipboard Doppler Sonar: A First-Generation Correction Algorithm

Abstract

Abstract The finite angular width of Doppler sonar beams introduces errors into the measurement of ocean velocity from moving ships. These errors are exacerbated as ship speed increases and as the acoustic scatter field becomes more inhomogeneous in space. Zooplanktonic scatters often reside in distinct quasi-isopycnal scattering layers. When measured from a moving ship, such layers appear to have a distinct velocity signature, even if the ocean is quiescent. Here, this effect is explored in a simple analytic model and a least squares algorithm is presented for remediating its signature. The algorithm corrects the measured velocity at any given depth, with the correction being a weighted sum of the depth gradient of log acoustic intensity at surrounding depths. The correction weights are a property of the specific sonar beam pattern. Once determined, they can be considered constant. The algorithm, when applied to a 50-kHz sonar on the Research Vessel (R/V) Roger Revelle, is found to remove roughly 90% of the error in vertical wavenumber spectra of shear. More sophisticated algorithms can be developed as experience with the present approach is gained.