Definition
Transmits acoustic beams, typically at several hundred kilohertz to a few megahertz, and measures the frequency shift of returns. In bottom track mode, available while the seabed is within acoustic range, often up to a couple of hundred metres, it gives true speed over ground, and dual-axis units also give athwartships speed at bow and stern for berthing. Beyond bottom track range it switches to water track, measuring speed relative to a layer of water.
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