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This paper considers the problem of accurately measuring the sediment transport over bedforms where flow evolves continuously both in time and space. For this purpose, we have developed a pulse-to-pulse coherent Doppler sonar system designed in bistatic geometry with two fan-beam transmitters symmetrically positioned on each side of a multielement receive array. The system resolves 2D velocity components within a ±20° (~0.5m by ~0.5m) swath. The software-defined radio data acquisition and control system limited us at present to eight independent receiver channels, and consequently the azimuthal resolution of the systemis 48. As a preliminary test of the sonar system, the system operation was simulated using a model developed to predict coherent sonar performance. The uncertainties with respect to the prescribed values and mean measurements in the model results were confined to 0.35 and 0.23 cm s-1, respectively, in the presence of strong shear (~150 s-1) and 50 cm s-1 horizontal flow. An important thing is that the model allowed us to test and develop the signal processing algorithms necessary to invert the multibeam sonar data. Using sand of 0.4-mm median diameter, the laboratory trials were carried out in active sediment transport conditions over dunes with 2-m wavelength and ~0.90m s-1 unidirectional flow velocities. The results presented here focus mainly on 2D velocity field and indicate an average 4% deviation from the wake law and 8% from independent observations made with the wide-band multifrequency coherent Doppler profiler (MFDop) instrument under similar flow conditions. [ABSTRACT FROM AUTHOR]

Knowledge of the velocity measurement distribution is useful for designing new coherent Doppler systems, for resolving velocity ambiguity, and for optimally combining measurements from multiple receivers and multiple carrier frequencies. In this paper, the effects of pulse-pair averaging and pulse-to-pulse autocorrelation are analyzed for ensembles consisting of up to ten pulse pairs. A formula is presented for the probability distribution of velocity measurements from a single pulse pair. This distribution is nonnormal for all values of the autocorrelation coefficient. In the limit of perfect pulse-to-pulse correlation, single pulse-pair measurements obey a Pearson Type VII distribution which has a higher peak and broader tails than a normal distribution. Second and fourth moments of the multiple pulse-pair measurement distributions are evaluated using a Monte Carlo method. The applicability of perturbation analysis for predicting velocity standard deviation is investigated. In the analysis of Zrnić (1977), it is assumed that perturbations in autocorrelation phase are small compared to the mean phase. Three prediction failure mechanisms are demonstrated through changes in the distribution of the complex autocorrelation coefficient; these occur for both high and low autocorrelation magnitude, and for short ensemble lengths. Measurement distributions from the Monte Carlo method are also used to determine the autocorrelation magnitude necessary to resolve velocity ambiguity using a dual-frequency coherent Doppler sonar. Simulations with a high-fidelity coherent Doppler sonar model show how the distribution of velocity measurements is affected by finite particle concentrations. While velocity standard deviation is insensitive to concentration above a threshold of approximately 1 particle per sample volume, kurtosis exhibits a strong dependence on concentration in the range of 0.1–5 particles per sample volume. [ABSTRACT FROM AUTHOR]

A new formula is derived for the asymptotic form of the magnitude of an autocorrelation coefficient for coherent Doppler sonar. The autocorrelation magnitude is shown to be a biased estimator in the limit of infinite ensemble length. Numerical simulation of a Gaussian random process is used to verify the asymptotic formula and to show that a bias persists for finite pulse-pair averages. Validity of the asymptotic formula is also confirmed using a high-fidelity coherent Doppler sonar simulation, and from sonar measurements in a towing tank. It is shown that the distribution of observed autocorrelation coefficients is well predicted by a Gaussian random process once the autocorrelation bias has been removed. [ABSTRACT FROM AUTHOR]

Coherent Doppler sonar allows noninvasive velocity measurements and is suitable for both laboratory and field applications. The approach is particularly attractive in those environments where optical techniques are not suitable either because of power requirements or more critically, water turbidity. Notably, the technique has been employed successfully in oceanic and river boundary layer studies. However, the occurrence of range and velocity ambiguities limit the more general application of the technique. This paper introduces a method to overcome speed ambiguities by acquiring acoustic backscatter at two (or more) frequencies simultaneously with a broadband transmit pulse. The different frequencies have distinct velocity ambiguities allowing disambiguation of the velocity measurements. The approach is conceptually similar to the use of multiple transmit pulse rates but has the advantage that the data can be acquired simultaneously and so there is no loss in data rate. In addition, system geometry often restricts the allowed pulse repetition rate so that disambiguation using frequency is more flexible and more generally applicable. Theoretically, the effective ambiguity velocity of a dual-frequency system can be extended arbitrarily but phase noise in a practical system restricts the method to about a fivefold increase in ambiguity velocity. [ABSTRACT FROM AUTHOR]