Deep ocean current profiling with a lowered broadband acoustic Doppler current profiler

An RD Instruments 150 kHz Broadband Acoustic Doppler Current Profiler (ADCP) has been mounted on a CTD frame and used to measure current velocity shear below the instrument during casts as deep as 5500 meters. By using processing techniques similar to those described by Firing and Gordon (1990) for a 300 kHz narrowband ADCP and Fischer and Visbeck (1993) for a 150 kHz narrowband ADCP, as well as some more recently developed algorithms, the raw profiles are merged with CTD data, corrected, edited, integrated, and averaged to produce unreferenced profiles of current velocity. Using GPS navigation data and the measured velocities, the profiles are referenced to produce absolute velocity profiles for both the up and down casts. This paper describes data collection and processing procedures, instrument settings, and modifications made to the standard self-contained instrument to accommodate an external, rechargeable battery park. The absolute velocity profiles obtained compare favorably with simultaneous Pegasus measurements. During five such comparisons made an a cruise in the equatorial Atlantic Ocean in June 1993, depth-averaged eastward and northward velocity components agreed to an average of 0.3 cm s^-1 with an rms difference of 0.7 cm s^-1; absolute velocity comparisons between the instruments compare favorably. The lowered ADCP (LAPCP) provides full water column absolute velocity profiles, coincident with CTD measurements, with the accuracy of a Pegasus profiler. It has the advantage of not requiring any extra shiptime and being independent of auxiliary bottom-mounted hardware