Performance of a Single-Bea

We report on the testing of a single-beam 1.7-MHz coherent Doppler sonar. The system is PC-controlled, using a digital signal processor (DSP) to acquire and extract the velocity and backscatter amplitude data. Results from a series of tow- tank calibration tests demonstrate an accuracy in the order of 5 mm sK1 for data rates of 10 profileshecond over a 1-2- m range with 1.5-cm range bins. An expression for system accuracy is developed which allows generalization to other pulse- to-pulse coherent Doppler systems. We present data showing the systematic decorrelation of backscatter signals due to particle advection: increased decorrelation is seen in the transducer near- field. Example observations of velocity profiles in laboratory- generated waves are presented. I. INTRODUCTION ELOCITY measurements over length scales of order V 1 m with centimeter resolution are required in many areas of both field and laboratory study. The small scale of measurement and the short ranges involved are such that even the smallest point sensor can cause some flow disturbance and degrade data quality. In this situation, acoustic sampling can provide an attractive alternative to point-sensor measurements. These remote profiling techniques are noninvasive, and they can provide a complete profile right up to a boundary. In this paper, we report on a general-purpose single-beam 1.7-MHz sonar unit (the Dopbeam system) that is capable of pulse-to-pulse coherent Doppler measurements over ranges of order 2 m. Some background on the operation of coherent Doppler systems is provided, and the basic components of the Dopbeam system are described. We present results of a calibration of the Dopbeam unit which demonstrates velocity- profile measurements with an accuracy of order 5 mm sK1 at a rate of 10 sK1 in 1.5-cm range bins. Using data from tow-tank tests, we develop expressions that allow an estimation of the system accuracy: these expressions are presented in terms of the system operating parameters and so allow generalization to other pulse-to-pulse coherent Doppler systems. The decorrela- tion of the Doppler signal associated with scatterer advection through the sonar beam is quantified.