Ocean velocity observations were taken on the ACCE cruise along 6oN, 0 and 6oS using two acoustic Doppler current profiler (ADCP) systems and accurate navigation data. The two systems are the hull-mounted ADCP and a lowered ADCP mounted on the rosette with the CTD. The data were taken aboard the R/V SEWARD JOHNSON to doucment the upper ocean horizontal velocity structure along the cruise track, and to measure vertical profiles of the horizontal velocity components at the individual hydrographic stations. The observations provide absolute velocity estimates including the ageostrophic component of the flow.
The hull-mounted ADCP is part of the ship's equipment aboard the SEWARD JOHNSON. The ADCP is a 150 kHz unit manufactured by RD Instruments. The instrument pings about once per second, and for most of the cruise the data were stored as 1.5-minute averages or ensembles. The user-exit program, ue4, receives and stores the ADCP data along with both the P-code navigation data from AOML's Trimble receiver and the Ashtech gps receiver, which accounts for pitch and roll. The P-code data are used as navigation for the ADCP processing. The ship's gyro-compass provides heading information for vector averaging the ADCP data over the ensembles. The user-exit program calculates and stores the heading offset based on the difference between the heading determination from the Ashtech receiver and from the ship gyro. As setup parameters, we used a blanking interval of 4 meters, a vertical pulse length of 16 meters and a vertical bin size of 8 meters. Data collection during the cruise used four ping ensembles.
Shipboard data processing was limited due to software problems. Final editing and calibration of the ADCP data are not finished. This involves the usual editing of CTD wire interference and the determination of the actual transducer orientation. In addition, the CTD and underway temperature and salinity must be used to correct the speed of sound and check the ADCP thermistor and account for any possible effects. Some problems arose with the Ashtech receiver's failure to swap satellites. This resulted in an occasional lose of pitch and roll measurements. Although the exact cause of this problem was never determined, manually cycling the Ashtech receiver through available satellites restored operation.
The second ADCP system is the lowered ADCP (LADCP), which was mounted to the rosette system with the CTD. The LADCP yields vertical profiles of horizontal velocity components from near the ocean surface to near the bottom. The first unit used is a broadband, self-contained 150 kHz system manufactured by RD Instruments. We used single ping ensembles. We use four ping ensembles. The data from each instrument are transferred to a PC between casts.
The vertical shear of horizontal velocity was obtained from each four ping ensemble. These shear estimates were vertically binned and averaged for each cast. By combining the measured velocity of the ocean with respect to the instrument, the measured vertical shear, and accurate shipboard navigation at the start and end of the station, absolute velocity profiles are obtained. Depth is obtained by integrating the vertical velocity component; a better estimate of the depth coordinate will be available after final processing of the data together with the CTD profile data. The shipboard processing results in vertical profiles of u and v velocity components, from a depth of 35 meters to near the ocean bottom in 5 meter intervals.
At the beginning of the cruise, a data/battery cable had a suspiciously bad connection that occasionally caused the LADCP to restart new casts when being plugged in. Several casts have a barotropic offset, that we believe were caused by the faulty cable.
AOML installed a Trimble P-code receiver for navigation, with data coming in at once per second. We have stored this once per second data for the entire cruise. The Ashtech receiver uses a four antennae array to measure position and attitude. The heading estimate was used with the gyro to provide a heading correction for the ADCP ensembles. The Ashtech data was stored by the ADCP user-exit program along with the ADCP data.