Echo sounder is a sonar which measures the depth of the water, by sending sound pulses. The total time that a pulse does to reach the bottom of the sea and return, along with some other parameters (like the speed of sound into water), it is used to determine the depth, with a specific mathimatical formula.
An observer using the speedlog is able to measure the lengths of the sound waves sent by it and their relationship with the moving objects. This measurement is converted into speed using a mathematical formula.
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