Optimizing acoustic Doppler current profiler settings for noise reduction

The performance of a 600-kHz broadband Acoustic Doppler Current Profiler (ADCP) developed by RD Instruments (RDI) is evaluated to determine the optimum settings for minimizing random noise associated with the inherent single-ping uncertainty in the instrument. The parameters considered are bin size, maximum range, and the resultant sample rate, which is the maximum rate that the instrument is capable of for a given combination of bin size and range. The instrument´s firmware determines the maximum sample rate as a function of range to allow round-trip travel time for the acoustic pulse between pings. The RDI noise model predicts an rms noise velocity that is inversely proportional to the bin size and to the square root of the sample rate for a fixed averaging time period. Data from a set of measurements recorded with an instrument on the research vessel (R/V) Knorr, as the ship was drifting west of Scotland, are consistent with the RDI model. Analysis of these data suggests that there is an optimum number of bins (N ∼ 10) that minimizes the noise level for a specified range requirement while maximum vertical resolution is retained. Ensemble averaging by the instrument does not appear to have a clear benefit over recording single-ping data, although it does reduce the quantity of raw data.