What is the ADCP?

The hull-mounted ADCP estimates horizontal and vertical velocity as a function of depth by using the Doppler effect to measure the radial relative velocity between the instrument and scatterers in the ocean. Three acoustic beams in different directions are the minimal requirement for measuring the three velocity components. A fourth beam adds redundancy and an error estimate. The ADCP transmits a ping from each transducer element roughly once per second. The echo arrives back at the instrument over an extended period, with echos from shallow depths arriving sooner than ones from greater ranges. Profiles are produced by range-gating the echo signal, which means the echo is broken into successive segments called depth bins which correspond to successively deeper depth ranges. The operator configures the length of each depth bin and the transmit pulse, which determines the degree of averaging in the vertical, depending on whether one is interested more in vertical resolution or profile penetration. The noisy velocity estimates from each ping are vector-averaged into 1- to 10-minute ensembles. The relative velocities are rotated from the transducer's to the earth's reference frame using the ship's gyrocompass. Finally, relative velocities and various ancillary parameters are stored on the ship using a data acquisition system (DAS) which also optionally records navigation information, such as provided by the GPS. Specifics of the instrument capabilities and configuration options are well documented (RDI, 1989).

Routine processing, quality control, and calibration are performed at the host institute. Standard checks include detecting and correcting time errors, applying transducer-level temperatures and salinities to obtain a better estimate of the sound speed for the velocity calculation, editing out bad bins or profiles that have been contaminated by interference with the bottom or some other physical object such as a hydro wire, and verifying the quality of the gyrocompass and the navigation data. The final gyrocompass estimates of ship heading and the navigation information are the primary sources for calibrating the ADCP's relative current velocities. Typically, one is correcting for a "angle" error due to misalignment of the transducer relative to the ship's hull and an "amplitude" component related mostly to minor imperfections of the transducer geometry. Relative current velocity errors caused by these components are orthogonal; the angle errors lead to uncertainties of the athwartships velocity component while the amplitude error introduces uncertainties along the ship track.

The navigation calculation is performed once calibration is complete. Absolute currents over a fixed depth range (reference layer) are obtained by subtracting the average of the ship velocity relative to a reference layer (i.e. ADCP velocities) from the absolute ship velocity over the ground (from navigation, i.e., GPS). The raw absolute current velocities relative to the reference layer are smoothed to reduce the effects of noise in the position fixes and combined with the navigation data to obtain the best estimates of ship positions and velocities, which are stored into the data base. Thus, absolute currents at any depth can be determined from the final ship navigation data and the relative ADCP measurements.

RD Instruments, 1989. Acoustic Doppler Current Profilers Principles of Operation: A Practical Primer. Available from RD Instruments, 9855 Businesspark Av., San Diego, CA 92131

Image picture; scattering of acoustic beam transmitted from ship with the frequency f0.

Picture of Transducer element (RD Instruments)

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Last update : May 12, 1997