Method for elimination of passive noise interference in sonar

Abstract

A sonar depth sounder device and method for processing echo signals, reflected from objects within a body of water, utilizes a processor, a memory, a display, and a keypad connected to the processor. The receiver receives sonar signals indicative of ambient noise in an underwater environment. The processor receives an electrical signal representative of the ambient noise, and calculates a detection threshold. The detection threshold is calculated by multiplying a scaling factor times the variance of the signal indicative of the ambient noise, and adding that product to the mean of the signal indicative of the ambient noise in the underwater environment. A transmitter then excites a transducer which emits sonar pulses into a body of water, and a receiver receives reflected sonar echo signals. When the intensity of the reflected sonar echo signals is less than the detection threshold, the reflected echo signals are eliminated from processing. When, however, the amplitude of a reflected sonar signal is greater than the detection threshold, the processor causes data indicative of the reflected sonar signal to be displayed on the display. Further, the detection threshold is increased from its starting point over time to compensate for increasing a gain in the receiver of the sonar depth sounder device. Additionally, the detection threshold is periodically updated to compensate for changes in ambient noise in the underwater environment. In an alternate embodiment, after a preliminary value is determined, based upon ambient noise or predicted ambient noise, the sonar depth sounder device generates a continuous time-varying detection threshold by sampling a received echo signal and storing the samples in memory, applying a continuous time averaging technique to the sampled data, wherein sample data comprises a time varying detection threshold. That time varying detection threshold is then compared on a sample-by-sample basis with corresponding samples of the originally received data, such that the originally received data that is greater than its corresponding detection threshold sample is displayed, whereas originally received data is less than its corresponding detection threshold sample is rejected and not displayed. In a preferred embodiment, in order to display data indicative of an originally received data sample that is greater than the detection threshold, it must be part of a sequence of consecutive data samples that are greater than the corresponding detection threshold samples, wherein that sequence has an associated time that is greater than a selected time limit.

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