A new broad-band acoustic Doppler current profiler (ADCP) is described, with a useful range comparable to that of a commercially available narrow-band (incoherent) system of the same acoustic frequency, but having enhanced performance. The extra performance may be traded off among 1) reduced velocity variance, 2) reduced averaging time, and 3) finer depth resolution. This improvement permits the observation of phenomena with smaller time and space scales than is now possible with available ADCP's. An expression predicting rms velocity error in terms of system parameters and the measured acoustic data is given and is shown to be consistent with the independently measured velocity error among redundant beams. Two major sources of bias error in incoherent ADCP's are shown to be much reduced for the broad-band system. Field data demonstrating the improved performance over the existing incoherent ADCP is shown for cases of both strong and weak shear. Since then he has been Vice President of Engineering at RD Instruments, where he has worked on various advanced product development projects, including long-range coherent, high-resolution coherent, and broad-band Doppler and acoustic scintillation systems.Eugene A. Terray for a photograph and biography, please see page 337 of this issue.
REVIEW OF DOPPLER SONARSThe practical feasibility of developing a Coherent Acoustic Doppler Current Profiler (ADCP) with spatial resolution and range comparable to those of commercially available incoherent systems but with much smaller velocity measurement variance is being studied. This improvement in measurement accuracy will reduce required averaging times, thus permitting the observation of short timescale currents (i.e. internal wave breaking) which are beyond the capabilities of available incoherent ADCP's. Several strategies have been analysed and tested. Among them, transmission of coded acoustic pulses combined with coherent signal processing techniques has been given special attention. This approach may allow the measurement of watermass velocities with high accuracy without the range limitations of conventional pulse-to-pulse coherent systems due to velocity ambiguity and pulse-to-pulse decorrelation. Several schemes based on this approach have been analysed and their relative merits assessed.
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