A biomimetic pulse train signal is proposed for underwater acoustic ranging in autonomous underwater vehicles (AUVs). The pulse train mimics the searching phase of dolphins as they emit short pulses continuously at approximately constant intervals and amplitudes. To enhance biological realism, fluctuations in interval timing and amplitude are introduced into each pulse. These variations also provide advantages for signal processing. Accurate travel time estimation is crucial for acoustic ranging. The proposed 127-pulse train, with a nominal inter-click interval of 29 ms, incorporates temporal fluctuations of up to 10⁻² s and amplitude variations within 25%. It exhibits a sharp autocorrelation main peak and low sidelobes, and its correlation properties are superior to those of M-sequence signals of the same length. By mimicking natural dolphin click trains, the signal reduces the likelihood of introducing conspicuous anthropogenic cues. The signal also has a cumulative sound exposure level of 184.8 dB re 1 µPa²s, below auditory injury thresholds for high-frequency cetaceans. This approach offers a promising option for covert AUV operations and potential applications as a pulse compression signal in underwater acoustic systems.

The horizontal target strength (TS) of fish is necessary to estimate the abundance of fish near the sea surface using a quantitative omnidirectional sonar. In this study, we measured the TS of Pacific chub mackerel by varying the yaw, pitch, and sonar tilt angles. We also estimated the theoretical TS using the prolate spheroidal model (PSM) to understand the horizontal TS of fish. The results revealed that the theoretical TS was very close to the measured TS for variation with respect to angle, indicating that PSM is a suitable model for Pacific chub mackerel. The measurements and theoretical prediction revealed that yaw angle distribution was the most important parameter for horizontal TS of Pacific chub mackerel. The influence of pitch angle and sonar tilt angle on horizontal TS was slight. Therefore, it is necessary to make the yaw angle distribution uniform to estimate average TS accurately during sonar surveys.