Multi-frequency, pulse-to-pulse coherent Doppler sonar profiler

Under ideal conditions, pulse-to-pulse coherent Doppler sonar can measure profiles of water velocity with unparalleled accuracy and resolution. However, this technique is limited in application by the occurrence of range and, more critically, speed ambiguities. A simple way to deal with speed ambiguities is to invert velocities using time history or prior knowledge of the flow structure, but these approaches are not always practical or reliable. Another technique is the use of a dual (or multiple) pulse repetition interval: this approach provides a reliable means of improvement but reduces the profile sample rate, and the pulse repetition interval is not always a free parameter (for example in the presence of a boundary). We present a new approach where multiple acoustic frequencies are used simultaneously, allowing a nearly five-fold increase in ambiguity velocity with no reduction in profile sample rate. Results are presented from a prototype multi-static system operating over the frequency range from 1.2 to 2.4 MHz, enabled in part through use of broad-band piezo-composite transducers. The prototype system generates two-component velocity profiles at a rate of 150 profiles/second over a 30 cm range interval with 3 mm range resolution. System performance is demonstrated under laboratory conditions with observations of flow in a turbulent jet.