Echo ranging apparatus and method

Abstract

Echo ranging apparatus to locate fish, vegetation and structure in the sea modifies signals presented to a recorder in the apparatus so as to enhance the capability of the user to distinguish fish in the vicinity of vegetation and structure, including the sea bed. A transducer receives the reflected echo signals. A receiver stage and amplifier stage present signals which, apart from the modification, would be conventionally useful in aiding the user to locate fish. However, a signal modifier connected at the output of the amplifier stage, between the amplifier stage and the input to the recorder, substantially enhances the capability of the apparatus. It does so by shunting signal reducer circuitry in parallel at the output of the amplifier when the level at the output surpasses a predetermined level, to reduce the signal levels at such output while retaining variations in such signal levels. Such retention, as opposed to, e.g., blanking out the output or setting the output to a fixed level, preserves information as to the source of the echoes which is present in such variations. By setting the triggering of the shunt connection at a high level corresponding to the echo from the sea bed, the sea bed will be indicated by a signal at a high level followed, after a time delay, by a steep reduction in level. Recordation of the information may be accomplished by providing the amplifier output signal, as modified, to the stylus of an electrochemical recorder, the stylus burning through a cover layer to a degree which depends on the level of the signal. With a graphite underlayer, and successive, straight, parallel traces along the paper, one per explorer pulse, the fish, vegetation and structure (including the sea bed) are shown on the recorder paper.