Passive Sonar Applications Research Articles

Design optimization of an optical fiber air-backed concentric hybrid mandrel hydrophone by an iterative design and finite element analysis for submarine passive array applications

The optical fiber hydrophones are well suited for the submarine passive sonar applications taking into consideration of their high sensitivity, fully electrically passive characteristics etc. and are definitely an equivalent replacement for Lead Zirconate and Lead Titanate ceramic hydrophones. In this paper, finite element analysis of several sets of cases of concentric hybrid material-based air-backed OF mandrel hydrophones are analysed and tuned to their geometric dimensions and their mechanical properties for design optimization, suitable for submarine sonar applications. A finite element method is adopted for analysis, strain and sensitivity calculations using ANSYS software. An inverse dependency of the sensitivity and the natural frequency were observed and was overcome by an innovative method where a hybrid inner hollow tube, combining both Aluminium and high-density polyethylene materials, were used, improving both the stiffness property and compliance of the mandrel and thereby improving both the natural frequency and pressure sensitivity. The harmonic analysis was carried out at 5 KHz acoustic pressure, by adding hydrostatic pressures of 2 MPa, 3 MPa and 4 MPa and obtained satisfying results. The acoustic directivity of this OF mandrel hydrophone was estimated, plotted and an omni directional pattern was obtained.

A Cramér-Rao Lower Bound Derivation for Passive Sonar Track-Before-Detect Algorithms

Track-before-detect (TkBD) algorithms have been shown to greatly abate measurement-to-track association (MTA) challenges. These simplifications are aptly relevant for reducing operator workload in deployed sonar systems that require a human “in-the-loop.” In a prior manuscript a case study of a passive bearings-only target motion analysis TkBD algorithm was demonstrated in complex sonar scenarios relevant to advanced fielded sonar systems. In this manuscript, a Cramer-Rao Lower Bound (CRLB) is derived for the algorithm previously developed. The approximations used in developing the CRLB are validated with a real data set. The CRLB itself, as a predictor of state estimation error performance, is validated with single- and multi-contact simulated data scenarios. The prior algorithm and CRLB derived herein is applicable to passive sonar, active sonar, radar, and optical applications through a change of point spread functions and Jacobians. The CRLB derived is simple to implement, requires minimal statistical assumptions, and is applicable to similarly implemented TkBD algorithms.

Underwater Acoustic Research Trends with Machine Learning: Passive SONAR Applications

Underwater Acoustic Research Trends with Machine Learning: Passive SONAR Applications

A real-time acoustic drifter for active and passive sonar applications

A low-cost, long-duration acoustic drifter has been developed to measure directional ambient noise and man-made acoustic signals in both shallow and deep oceans. Traditional acoustic and oceanographic measurements are typically performed by deploying fixed moorings that would limit retrieving the data only after the recovery. With the advancements in low-power CPUs and satellite telemetry, real-time oceanographic and acoustic measurements are possible with in-buoy data reduction and satellite transmission of processed data. A Raspberry Pi computer with a 16-channel data acquisition board is used for both active and passive sonar applications, including directional ambient measurements, seabed characterization, and ocean acoustic tomography. A specific version of the acoustic drifter was built for on-ice deployments in the Arctic as an ice-tethered acoustic buoy. Several examples of its applications from recent sea-going experiments are given. Successful deployments and instant data analysis would indicate a practical alternative to more traditional acoustic and oceanographic measurement techniques by providing a real-time monitoring capability. [Work supported by the ONR.]

Passive estimation of the waveguide invariant per pair of modes

In many oceanic waveguides, acoustic propagation is characterized by a parameter called waveguide invariant. This property is used in many passive and active sonar applications where knowledge of the waveguide invariant value is required. The waveguide invariant is classically considered as scalar but several studies show that it is better modeled by a distribution because of its dependence on frequency and mode pairs. This paper presents a new method for estimating the waveguide invariant distribution. Using the noise radiated by a distant ship and a single hydrophone, the proposed methodology allows estimating the waveguide invariant for each pair of modes in shallow water. Performance is evaluated on simulated data.

Intrinsic mode characteristic analysis and extraction in underwater cylindrical shell acoustic radiation

Target dimension is important information in underwater target classification. An intrinsic mode characteristic extraction method in underwater cylindrical shell acoustic radiation was studied in this paper based on the mechanism of shell vibration to gain the information about its dimension instead of accurate inversion processing. The underwater cylindrical shell vibration and acoustic radiation were first analyzed using mode decomposition to solve the wave equation. The characteristic of acoustic radiation was studied with different cylindrical shell lengths, radii, thickness, excitation points and fine structures. Simulation results show that the intrinsic mode in acoustic radiation spectrum correlates closely with the geometry dimensions of cylindrical shells. Through multifaceted analysis, the strongest intrinsic mode characteristic extracted from underwater shell acoustic radiated signal was most likely relevant to the radiated source radius. Then, partial information about unknown source dimension could be gained from intrinsic mode characteristic in passive sonar applications for underwater target classification. Experimental data processing results verified the effectiveness of the method in this paper.

Beamforming array techniques for acoustic emission monitoring of large concrete structures

This paper introduces a novel method of acoustic emission (AE) analysis which is particularly suited for field applications on large plate-like reinforced concrete structures, such as walls and bridge decks. Similar to phased-array signal processing techniques developed for other non-destructive evaluation methods, this technique adapts beamforming tools developed for passive sonar and seismological applications for use in AE source localization and signal discrimination analyses. Instead of relying on the relatively weak P-wave, this method uses the energy-rich Rayleigh wave and requires only a small array of 4–8 sensors. Tests on an in-service reinforced concrete structure demonstrate that the azimuth of an artificial AE source can be determined via this method for sources located up to 3.8 m from the sensor array, even when the P-wave is undetectable. The beamforming array geometry also allows additional signal processing tools to be implemented, such as the VESPA process (VElocity SPectral Analysis), whereby the arrivals of different wave phases are identified by their apparent velocity of propagation. Beamforming AE can reduce sampling rate and time synchronization requirements between spatially distant sensors which in turn facilitates the use of wireless sensor networks for this application.

On the sound of snapping shrimp: The collapse of a cavitation bubble

Snapping shrimp produce a snapping sound by an extremely rapid closure of their snapper claw. They usually occur in large numbers providing a permanent crackling background noise, thereby severely limiting the use of underwater acoustics for active and passive sonar, both in scientific and naval applications. Source levels reported for Alpheus heterochaelis are as high as 220 dB (peak-to-peak) re 1 μPa at 1 m distance. Recent ultra-high-speed imaging of the snapper claw closure [Versluis et al., Science (in press)] revealed that the sound is generated by the collapse of a cavitation bubble formed in a fast flowing jet of water forced out from between the claws during claw closure. In this work, we develop a theoretical model for a bubble under such conditions. The dynamics of the bubble radius and the emitted sound can be described by the Rayleigh–Plesset equation. The calculated results are compared with the experimental data. The model fully reproduces the bubble dynamics and it quantitatively accounts for the time dependence of the bubble radius and for the emitted sound. a)Also at Dept. of Physics, Philipps-Univ. Marburg, Renthof 6, 35032 Marburg, Germany.

Exploiting target motion for active matched-field depth estimation

This paper concerns the use of complex multipath propagation for discriminating the depth of a moving target with a low-frequency active sonar. Although matched-field processing techniques, which exploit full-wave modeling of multipath propagation, have been previously demonstrated in passive sonar applications, their use in active sonar has been precluded by uncertainty in the complex backscattering characteristics of the target. In this paper, a matched-field technique is proposed which exploits changes in the complex moving target return seen between consecutive sonar pings at a horizontal receive array. Because the method exploits only changes in the coherent combination of multipath arrivals, it does not require a priori knowledge of the target backscattering characteristics assuming its aspect is only slowly changing from ping to ping. In this paper, simulation results are presented which are modeled after a realistic Mediterranean scenario taken from the Shallow-Water Active Classification Experiment performed in November 1995. Matched-field depth estimation simulation results indicate that target depth accuracies of better than 20% of the channel depth can be achieved at ranges of 10 to 15 km. [Work supported by ONR.]

Wavelet based approach for joint time delay and Doppler stretch measurements

A new wavelet-based algorithm for joint time delay and Doppler stretch estimation in passive radar, sonar, and GPS applications is described. By taking the cross wavelet transform of the noisy outputs from two spatially separated sensors, the wideband cross ambiguity function (WBCAF) can be computed. This in turn provides a direct measurement of the time difference and Doppler stretch between the two signals. The Cramer-Rao lower bound (CRLB) is derived and the performance of the proposed algorithm is analyzed. It is found that the theoretical variances of the estimates are unbiased and approach the CRLB at high signal-to-noise ratio (SNR). In order to improve the accuracy of the estimation under noisy environment, a simple denoising method is introduced. Simulation results are given to corroborate the analysis.

Multidimensional adaptive beamforming for integrated active-passive sonar and medical imaging applications

The present investigation introduces an adaptive beamforming scheme that separates the computationally intensive multidimensional beamforming into two simple modules, which are line and circular array beamformers. This approach minimizes the number of degrees of freedom associated with the adaptation process and provides near instantaneous convergence, which is essential for integrated active-passive sonar and medical ultrasound imaging applications. The proposed adaptive processing concept has been implemented in an integrated active-passive real-time sonar deploying a cylindrical array. Preliminary efforts include also the application of a line array adaptive beamformer in medical ultrasound imaging systems. Real data results from the adaptive and conventional beamforming outputs of the cylindrical array sonar system and the line array of the ultrasound system demonstrate the superior performance of the adaptive beamformer in suppressing the reverberation and cluttering effects in active sonar and medical ultrasound applications. Moreover, for passive sonar applications, the adaptive processing provides substantially improved angular resolution performance as compared with that of the conventional beamformer. Both these two performance improvements for a line and a cylindrical array sonar are of particular importance for mine hunting operations.

An adaptive processing structure for integrated active-passive sonars deploying cylindrical arrays

The paper details the development and implementation of an adaptive processing structure for integrated active–passive sonars deploying cylindrical arrays. The main concept includes decomposition of the computationally intensive multidimensional beamformer into two simple modules, which are line and circular array beamformers. Thus, the multidimensional beamforming process can now be divided into coherent subprocesses which lead to efficient implementation in real-time sonar systems. Furthermore, the application of spatial shading to reduce the side-lobe structures can now be easily incorporated. Moreover, the new approach makes the implementation of adaptive schemes in multidimensional sensor arrays practically achievable. The proposed adaptive processing concept has been implemented in an integrated active–passive real-time sonar deploying a cylindrical array. Real data results from the adaptive and conventional beamforming outputs of the cylindrical array sonar system demonstrate the superior performance of the adaptive beamformer in suppressing the reverberation and cluttering effects in active sonar applications. Moreover, for passive sonar applications, the adaptive processing provides substantially improved angular resolution performance as compared with that of the conventional beamformer. Both these two performance improvements for a cylindrical array sonar are of particular importance for mine hunting operations.

Implementation of adaptive processing schemes in active and passive sonar systems

This paper details the implementation of adaptive processing schemes with near-instantaneous convergence in integrated active–passive sonar applications. The adaptive schemes of this investigation include a generalized sidelobe canceller (GSC), a coherent broadband adaptive algorithm based on a multiple steered covariance matrix and the MVDR method. The effort is part of a major study that includes the design of a generic signal-processing structure which allows the implementation of adaptive and synthetic aperture signal-processing schemes in real-time sonar systems. Application results from synthetic and real experimental datasets show that the adaptive processing schemes of this study provide continuous beam time series at the input of a matched filter and a spectrum analyzer for active and passive sonar applications, respectively. The same set of results demonstrates that the implementation scheme of this study eliminates undesirable adaptive performance degradation effects such as signal cancellation. In addition, the adaptive beam time series have temporal coherence properties equivalent to those of the conventional beam time series. Moreover, their amplitude of correlation with the reference FM signal of active sonars is the same with that of the conventional ‘‘main stream‘‘ schemes, which indicates that the adaptive algorithms have achieved near-instantaneous convergence.

A neural network harmonic family classifier

A method of automatic target discrimination and classification for passive sonar applications is obtained using an unsupervised, self-organizing, Kohonen neural network. Utilizing detected, smoothed spectral data, the kth-order frequency differences are histogrammed into constant percentage resolution spectral bins prior to presentation to the network. This nonlinear transformation results in good neural network classification when the input data is topologically close to the training set. Simulations of the algorithm show highly robust performance to missing spectral detections, spurious spectral detections and Doppler shifts.

Differential delay-doppler estimation using second and higher-order ambiguity functions

Estimation of differential delay-Doppler parameters in passive sonar and radar applications is addressed. Without assuming Gaussianity it is shown that the conventional ambiguity function is asymptotically chi-squared and yields consistent delay-Doppler estimates for narrowband signals observed in uncorrelated sensor noises. Related asymptotic covariance expressions and their computable forms are also derived. It is further shown that the performance of the ambiguity function degrades severely when the sensor noises are correlated. To deal with such noises a novel third-order ambiguity function is proposed and is shown to be consistent and theoretically immune to Gaussian and symmetrically distributed disturbances. Further, to take into account the errors due to variances of sample statistics, alternative delay-Doppler estimators are defined to minimise a cumulant matching criterion and related issues are discussed. The case of wideband signals is also addressed and, finally, extensions to kth-order ambiguity functions are proposed. Computer simulations are performed to confirm the theory. Throughout the paper the analysis treats both deterministic and stochastic signals on a common framework. Connections with active sonar and radar problems are also shown as a special case.

A balanced, centrally stiffened design for PVDF hydrophone arrays

As PVDF thick films become more readily available, there has been a growing interest in the development of a large-area hydrophone for passive sonar applications. These new PVDF materials offer the advantages of having a lower density, greater mechanical strength and flexibility, and better impedance match to water than conventional piezoelectric ceramics. Furthermore, they are now available in large sheets, from which arrays can be developed for acoustic detection covering large surface areas. Prototype PVDF hydrophone arrays were fabricated and tested to show that in-plane flexural motion greatly affected hydrophone responses due to the in-plane anisotropy of the piezoelectric properties of PVDF. To circumvent this difficulty, a balanced, centrally stiffened configuration was adapted by sandwiching two pieces of PVDF to a common damped plate, while aligning in parallel the stretched “3—1” directions of the PVDF sheets. With this design, constant receiving sensitivity and directivity patterns in agreement with theoretical predictions were obtained. Further refinement of this design was also achieved through mathematical modeling. [Work supported by ONR.]

Use of rectangular vandermonde matrix decomposition to resolve damped sinusoidal oscillation components

Signals that are sums of decaying sinusoids arise frequently in passive and active sonar applications, both as spontaneous acoustic transients and as echoes from elastic objects. When the oscillations are highly damped, identification of component frequencies and damping factors is more amenable to least‐squares Prony analysis than to Fourier analysis, and is particularly suited to a related technique that we call “rectangular Vandermonde matrix decomposition.” Our results indicate that the Vandermonde method is robust in the presence of noise, but not in the presence of distortions created by ideal bandpass filters. On the other hand, analysis of simulated replica‐correlated echoes of FM pulses bounced off an object having a pair of heavily damped resonances give different results. Surprisingly, the Vandermonde method identified the resonant frequencies even without replica correlation, despite the fact that the analyzed waveform looked not at all like a sum of decaying sinusoids. [Work supported by the ONR.]