Underwater sounding apparatus

摘要

An underwater sounding apparatus transmits acoustic waves into a body of water, receives echoes reflected by underwater targets and displays the target echoes. A first embodiment of the invention includes a filtering processor (6) which performs filtering operations on received echo signals by using filter coefficients suited to a specified target speed range. A band limits generator produces a set of desired filter coefficients by shifting a frequency range suited to a specific target speed range according to the moving speed of the underwater sounding apparatus. The set of filter coefficients are also associated with the direction of motion of the underwater sounding apparatus. A second embodiment of the invention underwater sounding apparatus includes a filtering processor (6) which performs filtering operation on received echo signals by using bandpass filters, each having a stopband formed in part of a passband, wherein the stopband is a frequency band covering frequencies of echo signals received from targets at zero speed. The underwater sounding apparatus thus configured can display echoes received from targets like fish schools traveling at speeds within a specified speed range only by erasing echoes from the sea bottom and substantially zero-speed targets like a plankton layer. When displaying the received echoes, the amount of Doppler shifting can be shown in different colours.