Towed array sonar

Abstract: In this study, a vector hydrophone was developed for a towed array sonar system using a shear type accelerometer. In general, inertia type vector hydrophones using accelerometers have higher sensitivity than multimode vector hydrophones. However, the sensitivity still needs to be enhanced as much as possible for practical application. For the purpose, the effect of the structural parameters of a shear type accelerometer on the receiving voltage sensitivity (RVS) of the vector hydrophone was rigorously analyzed, and the structure of the vector hydrophone was optimized to maximize the RVS. The developed vector hydrophone has an external diameter of 23 mm and the minimum RVS over the given frequency range of -201.4 dB. It was also verified that the designed vector hydrophone exhibits the characteristic of a dipole mode beam pattern. Furthermore, a prototype of the vector hydrophone was fabricated with the optimal dimensions, and its acoustic characteristics were measured to verify the validity of the design. The vector hydrophone developed in this work has the highest feasible RVS over the desired frequency range in comparison with other types of vector hydrophone like the multimode hydrophone.

Abstract: This paper presents the development of a high-performance micromachined capacitive accelerometer for detection of sonar waves. The device is intended to replace existing hydrophones in towed array sonar systems, and thus, needs to meet stringent performance requirements on noise, bandwidth, and dynamic range, among others. The in-plane, single-axis accelerometer is designed based on a mode-tuning structural platform. A frame was used instead of a solid plate for the proof-mass of the device, allowing us to push undesired vibration modes beyond the operating bandwidth of the device while enabling us to employ a portion of the area for capacitive sensing elements. The designed accelerometer was fabricated on a silicon-on-insulator wafer with 100- $\mu \text{m}$ device layer with capacitive gaps of $\sim 2.2~\mu \text{m}$ . The sensitivity of the accelerometer is 4 V/g with a noise spectral density of better than ${{350~{\mathrm {ng}}/}}\sqrt {\mathrm {{Hz}}} $ . The fundamental resonant frequency of the device is 4.4 kHz. The open-loop dynamic range of the accelerometer, while operating at atmospheric pressure, is better than 135 dB with a cross-axis sensitivity of less than 30 dB.

Abstract: In this work, a method is proposed for resolving the Left-Right Ambiguity in Passive Sonar Systems with towed arrays. This problem arises in source localization when the array is straight. In practice, the array is not straight and a statistical analysis within the Neyman-Pearson framework is developed for a monochromatic signal in the presence of random noise, assuming that the exact array shape is known. For any given array shape, an expression for the Probability of Correct Resolution (PCR) is derived as a function of two parameters; the signal to noise ratio (SNR) and an array-lateral-displacement parameter. SNR measures the strength of the signal relative to the noise and the second parameter the curvature of the array relative to the acoustic wavelength. The PCR is calculated numerically for a variety of array shapes of practical importance. The model results are found to agree with intuition; the PCR is 0.5 when the array is straight and is increasing as the signal is becoming louder and the array more curved. It is explained why the method is useful in practice and the effects of correlation between beams are discussed. Furthermore, we look at the acoustical Sharpness Methods; these methods have originated from optical imaging, where they were used successfully to correct atmospherically degraded optical images of telescopes. The method entails that an appropriate function, called `Sharpness Function' is supposed to be maximised when the shape of the towed array, used to construct the beamformer image, is the true one. Reviewing carefully previous literature, which indicated that the method has good chances of producing a very good estimate of the array shape, we prove that the proposed Sharpness Function is in fact not maximised and we deduce that the Sharpness Method does not seem appropriate in the sonar context. Therefore, we conclude that, with the proposed Sharpness function, the Sharpness Method does not work, unlike what has been suggested in the literature. We prove however that, most crucially, Optical Imaging is the same as Beamforming. This opens avenues for further exploration of the sharpness analogy between optics and acoustics imaging.

Abstract: A computer-based system that processes hydrophone and beam noise data from a towed horizontal line array sonar has been developed. The system also produces various displays that can be used to help assess the functionality of the sonar and to identify faults that cause degraded performance. The system-and various statistics used for characterizing or quantifying a given aspect of the sonar's performance are discussed. The results are presented in visual formats to aid in rapid assessment and quantification of the sonar's performance. Examples obtained from the system during recent towed array ambient noise measurement exercises are given to illustrate its utility for real-time performance monitoring and its capability for providing clues to aid in fault localization. >