A Direct Comparison of Two RDI Shipboard ADCPs: A 75-kHz Ocean Surveyor and a 150-kHz Narrow Band

Abstract

During a recent transit from Florida to Rhode Island, simultaneous single-ping data were recorded from two acoustic Doppler current profilers on the R/V Endeavor: an old 150-kHz narrow bandwidth (NB) model, and a new 75-kHz model [Ocean Surveyor (OS)] with a flat phased-array transducer, operating alternately in narrow bandwidth (OSN) and broad bandwidth (OSB) modes. In calm weather the NB, OSN, and OSB data showed nearly perfect agreement; the range of the OSN (up to 800 m) was about twice that of the NB, and the OSB range was 80%?85% of the OSN range. As weather worsened, the returns from all three degraded, with reduced depth range and with occasional pings returning no valid velocity estimates. Reduction in data return was most severe in the OSB, and least severe in the NB. Performance degradation was associated with a velocity bias toward zero in both the OSB and OSN relative to the NB, and a smaller bias toward zero in the NB. The bias in all three is reduced with a suite of editing algorithms that must be applied before the single-ping profiles are averaged. Beam sidelobes were 12?15 dB higher in the OS than in the NB. Although this did not cause obvious velocity profile errors in the present dataset, it is possible that it will do so in regions such as the eastern equatorial Pacific, where strong scattering layers are common.