Underwater Array Research Articles

A noise-reduction method for UUV communication using a stack-type convolutional autoencoder model of image features

Underwater acoustic (UWA) communication is the primary interaction between UUV and underwater array. However, noise can seriously affect the quality of communication. The conventional filtering algorithms and multiple kernel learning (MKL) models are unsuitable for abrupt-varying noise environments. To address the problem, this study proposes a deep learning (DL) noise-reduction model based on image features. Firstly, in the preprocessing stage, we transform the abrupt-varying noise into a two-dimensional representation of the image characteristics and construct a time–frequency preprocessing structure based on higher-order DEMON spectral representation. Secondly, a feature extraction method based on EMD feature enhancement is built to improve computing efficiency. Finally, we propose a noise-reduction model based on a stack-type convolutional autoencoder aiming to suppress underwater noise and enhance the system’s communication quality. The simulations and experiments on a lake show that the proposed method is more robust over the changing signal-to-noise ratio (SNR) and has a lower bit error rate (BER) than conventional methods.

Two-Stage Learning Model-Based Angle Diversity Method for Underwater Acoustic Array

The diversity combining technique performs well in the inhibition of multipath, it has been widely used in underwater acoustic (UWA) array signal processing. However, the underwater noise can seriously affect the processing results of the diversity. The conventional filtering algorithms cannot deal with the nonlinear components of underwater radiation noise and have a poor processing effect on complex signals. This study proposes a novel underwater array angle diversity method based on a two-stage model to overcome the problem. A noise-reduction model with a deep convolutional neural network (DCNN) as the backbone network for deep residual learning by preprocessing complex-type data on the received and reference noise signals in the first stage. In the second stage, a modified weighted delay summation beamformer group model is constructed. This model adjusts the weights of each channel by a gradient descent criterion. The desired angle estimates and delay information are then obtained. Finally, the delayed combining of the signals of each path is completed by the combining strategy. Simulation test results show that the proposed algorithm has a lower bit error rate (BER) for diverse received signals. On-lake tests further verify the effectiveness of the algorithm.

FPGA implementation of bearing tracking using passive array for underwater acoustics

Within the scope of this study, a Field Programmable Gate Array (FPGA) based system which calculates bearing angles by analyzing the signals emitted by vessels in underwater environment is proposed. An array consisting of 3 non-directional hydrophones were designed and used in tests. Minimum Variance Distortionless Response (MVDR) algorithm was used in the bearing calculations according to the reference hydrophone. In marine tests, hydrophone array was integrated into the buoy, and then tested. The angle between the reference hydrophone and the magnetic north is calculated in order to correct the errors caused by the immobility of the buoy and underwater array. In the developed system, all operations were carried out on Artix 7 FPGA. Fixed point number format is used and implementation stages are designed as pipeline architecture. In the marine tests performed, it was monitored in real time that the bearing information calculated by the system was compatible with the route of the vessel used in the tests. The signals received by bearing information and the hydrophones were recorded. The records were run offline, and the calculated values were compared. The results obtained showed that the developed FPGA-based system successfully calculates the bearing angle of the vessels by passive listening.

Shock wave and bubble characteristics of underwater array explosion of charges

In this study, we focused on the effect of the underwater explosion parameters of multi-point array explosion. The shock wave and bubble parameters of aggregate charge, two charges, and four charges were measured through an underwater explosion test, and their influence on the explosion power field of charge quantity and array distance was analyzed. Results show that the multi-shock wave collision of array explosion can be approximated to a linear superposition, and the interaction of delayed shock wave can be deemed as the increase of the shock wave baseline. Shock wave focusing and delayed superposition increase the shock wave peak pressure. Compared with the aggregate charge, the greater the number of array explosion points is, the higher the impulse and the gain of the bubble peak pressure are. At the same array distance, the smaller the charge quantity is, the higher the bubble impulse will be. At the same charge quantity, the smaller the array distance is, the higher the bubble impulse will be. The bubble period decreases gradually with the increase of the charge quantity, but the test orientation has little effect on the bubble period.

Whitening and source enumeration for large underwater arrays

Source enumeration for underwater arrays typically requires a whitening preprocessing stage due to ocean colored noise. A whitening transform requires the estimation of the noise-only sample covariance matrix (SCM). Estimation of a SCM is a challenging problem for large arrays in nonstationary environments due to the limited number of snapshots available for training. This talk considers a source enumeration method based on the dominant mode rejection (DMR) whitening transform (Diaz-Santos and Wage, 2015), inspired by Abraham and Owsley's DMR beamformer (1990). The DMR whitening transform reduces the number of snapshots required to estimate the noise-only SCM by replacing the small eigenvalues of the SCM with their average. In reduced-snapshot scenarios, replacing zero amplitude eigenvalues makes the SCM invertible. The new source enumeration method based on DMR whitening uses random matrix theory and high dimension, low sample size asymptotics to define a threshold that guarantees a specified probability of false alarm in scenarios where the number of array elements is larger than the number of noise-only snapshots. This talk analyzes the performance of whitened source enumeration method using noise from underwater experiments. [Work supported by ONR and NSWCDD.]

Structural design of an acoustic planar array transducer by using the equivalent circuit method

Underwater acoustic transducers are used to detect and track targets by using sound waves in underwater environments. To enhance the performance, they can be arranged in various formations to create arrays. However, acoustic interaction between the constituent channels of array transducers is inevitable, and it distorts their characteristics. In this study, the transmitting voltage response (TVR) spectrum of an acoustic planar array transducer is analyzed by considering the crosstalk between channels. The transmitting performance of an array transducer is frequently analyzed using the conventional finite element method (FEM). However, when the size of the array is large, the time and cost of computation using the FEM increase to almost prohibitive levels. To overcome this problem, a new equivalent circuit method (ECM) is developed to analyze the acoustic characteristics of an underwater array transducer. The ECM is validated by comparing its TVR spectrum with that analyzed using the FEM. After validation, the ECM is utilized to design the optimal structure of a planar array transducer having the broadest bandwidth. The sample analysis conducted using the ECM was approximately 1780 times faster than that by the FEM. Therefore the ECM developed in this work can facilitate the design of an underwater planar array transducer with a superior speed and efficiency compared with those achievable with the conventional numerical analysis methods such as the FEM.

Optimization and characterization of a wideband multimode Tonpilz transducer for underwater acoustical arrays

A multimode underwater acoustical transducer is well-known for providing wider bandwidth than a conventional single-mode transducer. However, the design of a low-frequency multimode transducer to achieve the desired wideband characteristics for an underwater array is challenging when considering the array’s size and weight limitations. Therefore, this study focused on the development of a multimode transducer structure for superior wideband characteristics. The effect of various structural parameters on the performance of the multimode Tonpilz transducer was first analyzed with emphasis on its bandwidth using the finite element method (FEM). Then, the structure of the transducer was improved by analyzing the effect of the side acoustic window and incorporating realistic design considerations. Finally, the improved transducer structure was optimized to have the widest possible bandwidth while maintaining its transmitting voltage response (TVR) level over a typical power requirement. The final design was validated by fabricating a prototype transducer and evaluating its acoustical performance.

Tracking of multiple surface vessels based on passive acoustic underwater arrays

This paper introduces an approach to localize and track an unknown number of non-cooperative surface vessels based on passive acoustic sensing of their noise radiated underwater. Time-Difference of Arrival (TDOA) measurements are extracted from pairs of hydrophones, and a recently introduced Bayesian framework for multi-object tracking is employed to detect and track vessels from TDOA measurements. Results based on data from a three-dimensional compact hydrophone array towed by an autonomous vehicle confirm that non-cooperative surface vessels can be detected and tracked.

Structure Design and Finite Element Analysis of the 24m Array Truss

In this paper, one type of 24m underwater array truss structure have been designed.Based on the array structure, analysis the force condition, the structure strength and stiffness were calculated, and the establishment of the array structure finite element model, combined with ANSYS analysis, truss strength and stiffness meet the specification requirements, the structure safe and reliable.

Scaling up benthic microbial fuel cells using flyback converters

Benthic microbial fuel cells (BMFCs) are alternative energy sources that can power sensors underwater. However, their use underwater is limited by low conversion efficiencies of the low-voltage energy to higher voltages required by modern electronics. Additionally, BMFC systems detailed in the literature are incompatible with the deployment difficulties associated with underwater sensing. In this work, we present an optimal underwater scaling strategy combined with an integrated power management system. We successfully demonstrate the modular scale-up of BMFCs using in-line flyback converters that held the BMFC input voltage at an optimal cell potential of 0.35–0.5 V while directly increasing output voltage to 12 V. Two flyback converters could operate successfully on a single shared anode, delivering 16 mW of the BMFC power directly to a 12 V rechargeable battery at 77% efficiency. We show that the internal resistance of the BMFC and effective resistance of the power management system determine the transition from start up to stable BMFC operation for up to seventy days. These combined factors have not been demonstrated previously. Such a system allows for a broad range of BMFC underwater array configurations that are critical to the future integration of BMFCs with seafloor systems and sensors.

DOA estimation for wideband LFM signals with a few snapshots

The wideband linear frequency modulation (LFM) signals are widely used in information systems. The conventional direction-of arrival (DOA) estimation algorithms of LFM signals rely on a large number of snapshots, some of which are not reliable in numerous practical applications such as underwater array processing. To solve the above problem, we present a modified sparse iterative covariance (MSPICE)-based estimation method in fractional Fourier transform (FrFT) domain to estimate the DOA of wideband LFM signals. First, we extend the original SPICE algorithm in FrFT domain with a specific transform order for wideband LFM signals. Then, we utilize the energy centrobaric modification method to make the original SPICE more accurate without adding more computational complexity. The simulation results demonstrate the effectiveness of the proposed method.

Artificial light on water attracts turtle hatchlings during their near shore transit.

We examined the effect of artificial light on the near shore trajectories of turtle hatchlings dispersing from natal beaches. Green turtle (Chelonia mydas) hatchlings were tagged with miniature acoustic transmitters and their movements tracked within an underwater array of 36 acoustic receivers placed in the near shore zone. A total of 40 hatchlings were tracked, 20 of which were subjected to artificial light during their transit of the array. At the same time, we measured current speed and direction, which were highly variable within and between experimental nights and treatments. Artificial lighting affected hatchling behaviour, with 88% of individual trajectories oriented towards the light and spending, on average, 23% more time in the 2.25 ha tracking array (19.5 ± 5 min) than under ambient light conditions (15.8 ± 5 min). Current speed had little to no effect on the bearing (angular direction) of the hatchling tracks when artificial light was present, but under ambient conditions it influenced the bearing of the tracks when current direction was offshore and above speeds of approximately 32.5 cm s−1. This is the first experimental evidence that wild turtle hatchlings are attracted to artificial light after entering the ocean, a behaviour that is likely to subject them to greater risk of predation. The experimental protocol described in this study can be used to assess the effect of anthropogenic (light pollution, noise, etc.) and natural (wave action, current, wind, moonlight) influences on the in-water movements of sea turtle hatchlings during the early phase of dispersal.

Array gain of conventional beamformer affected by structure of acoustic field in continental slope area

Conventional beamforming (CBF) is an important processing step in underwater array signal processing. Previous researches have shown that the sound field structure as manifested by amplitude nonhomogeneity and wave-front corrugation can reduce the array gain of CBF. The acoustic environment of the continental shelf slope area is very complex. For an underwater acoustic array in this area, the amplitude and phase of the received signals will be distortional seriously. Recently, the acoustic field correlation has been the focus of research on the array gain relations with the underwater acoustic filed. However, the attenuation of acoustic field correlation is not the only factor that induces the array gain to decline, and the analyses of the array gain in the shallow water based on normal-mode model are not applicable to the continental slope area. In this paper, the array gain relations with the structure of acoustic field in continental slop area are investigated based on the theory of underwater acoustic signal propagation. The effects of acoustic field on the signal and noise gains are considered respectively. The analytic expressions of the array gain of CBF in an isotropic noise field are derived from the primal definition of array gain, which indicates that acoustic field correlation and transmission loss in continental slope are the intrinsic factors that affect the array gain of CBF. A horizontal uniform linear array (ULA) with a wide aperture receiving signals from a source in the deep water region is considered in the upslope propagation condition. The RAM program is utilized in the numerical simulations to generate the sound field of this specific environment with given parameters. The array gains, the ATLs and the horizontal longitudinal correlation coefficients of the acoustic field corresponding to three different locations are compared. Conclusions can be drawn as follows. 1) The array gain of CBF is determined by acoustic field correlation and the acoustic average transmission loss (ATL), and its maximum is less than 10lg M as the signal waveform distortion. 2) when the ATLs corresponding to hydrophones at two different receiving locations are similar, the correlation of acoustic filed is higher, and the array gain of CBF is larger. 3) When the ATLs corresponding to hydrophones at two different receiving locations are greatly different, the relation between the array gain of CBF and the acoustic filed correlation is no longer positive. The simulation results verify the array gain of CBF relations with the acoustic filed correlation and the transmission loss in the continental slope area.

Robust Bearing Estimation in Shallow Water Using Vector Optimization

Correlated multi-paths may cause a biased bearing estimation in shallow water, so most existing methods make fully use of the multi-path nature and the underwater channel property to provide an unbiased bearing estimation. However, an imprecise knowledge of the underwater channel parameters or the array model mismatches will cause performance degradation, especially for the high-resolution methods. Therefore development of robust bearing estimation methods in shallow water is critically important. In this paper, a new approach to robust bearing estimation is proposed in the presence of underwater channel parameter uncertainties or array model mismatches. The proposed method is based on the convex optimization theory, exactly to say, the vector optimization theory, and can be derived by imposing a certain constraint on the Euclidean norm of the source vector. It is shown that the proposed method can be reformulated as a convex secondorder cone program (SOCP) problem and solved efficiently by the well-established interior point method, such as SEDUMI. Computer simulations and experiment analysis show that the proposed method is highly robust against the underwater channel parameter uncertainties and array model mismatches, moreover, demonstrates its excellent performance as compared with existing methods.

Radiometric and photometric determinations of simulated shallow-water environment

Optical remote sensing is increasingly becoming a preferred economic alternative to the traditional in situ observations and physical sampling for mapping and monitoring habitats. Submersed habitats, such as eelgrass and corals, are especially challenging for field work. Even for remote-sensing work, a priori knowledge of environmental factors is required for highly accurate analysis. Background illumination and water clarity are two key factors that affect the optical remote-sensing imagery, which may vary widely with season, time of year, geographic location, or water depth. This article presents efforts to simulate natural oceanic conditions in a laboratory setting. Solar radiation predicted at different latitudes under varying water clarity conditions and depth were replicated using a 2.5 m deep wave tank at the University of New Hampshire. The goals of the study were: (1) to simulate illumination and water clarity conditions that approximate coastal and oceanic waters, and (2) to quantify the impact of the simulated illumination and water clarity conditions at different depths on the apparent colours that can be observed from an aerial platform. The empirical radiometric measurements included irradiance, radiance, and remote-sensing reflectance from an underwater array of light sources. The results of the study show good correlation (r 2 = 0.89–0.93) between the natural daylight spectrum at the water surface and the irradiance measurements between 350 nm and 590 nm, at 3.5 m from the light array. The colours of the clear and murky water types were photometrically calculated from the radiometric measurements and validated using underwater video imagery. Using this methodology, illumination and water clarity can be replicated under controlled laboratory conditions and used to assist in studying the physical, chemical, and biological processes in habitats, at varied geographic locations and differing environments.

A mutual radiation impendence measurement method for 2-element transducer array considering nonlinear interaction

Mutual radiation impedances represent the acoustical interaction between transducers in an underwater array, which are inherent in the distance between transducers and the geometry of the array. The influence of mutual radiation impedances, which are important to evaluate the performance of the underwater array, especially needs to be intensively studied on condition that the power of transducers is high enough because of nonlinear interaction. A method for determining mutual radiation impedances of transducers in a 2-element array is proposed in this paper, which applies to estimation of mutual radiation impedances for an underwater array with high radiation power. This method derives from V method in which the mutual radiation impedances are considered as the acousto-motive force generated on the surface of each transducer. By using nonlinear acoustic analysis, theoretical foundations of this method based on equivalent circuits are described. Detailed experimental procedures to implement the method are ...

Affects of nearby bubbles on underwater array gain

Combining multiple sensor signals coherently (i.e., beamforming) improves spatial or angular resolution and increases signal to noise ratio (SNR). When the array is steered, signals arriving from the steering direction add in phase, while signals arriving from other directions do not (proper choice of signal frequency assumed). Array gain (AG) is a measure of how much the SNR at the array output is increased relative to array input SNR. The degradation in underwater acoustic array AG by scattering from nearby bubbles was measured at the AB Wood tank located at the Institute of Sound and Vibration Research (ISVR), University of Southampton, in June 2008. AG degradation is separate from the effects of bubbles in water to attenuate acoustic signals. Measured statistics of signal phase at the individual sensors show that as bubble density increases, phase differences between the elements increase and AG is degraded. We present a theory and numerical simulation that attributes the phase shifts to scattering from nearby bubbles and provides a way to predict AG degradation from the bubble density. Work sponsored by ONR Undersea Signal Processing.

Affects of nearby bubbles on underwater array gain.

Combining multiple sensor signals coherently (i.e., beamforming) improves spatial or angular resolution and increases signal to noise ratio (SNR). When the array is steered, signals arriving from the steering direction add in phase, while signals arriving from other directions do not (proper choice of signal frequency assumed). Array gain (AG) is a measure of how much the SNR at the array output is increased relative to array input SNR. The degradation in underwater acoustic array AG by scattering from nearby bubbles was measured at the AB Wood tank located at the Institute of Sound and Vibration Research (ISVR), University of Southampton, in June 2008. AG degradation is separate from the effects of bubbles in water to attenuate acoustic signals. Measured statistics of signal phase at the individual sensors show that as bubble density increases, phase differences between the elements increase and AG is degraded. We present a theory and numerical simulation that attributes the phase shifts to scattering from nearby bubbles and provides a way to predict AG degradation from the bubble density. [Work sponsored by the ONR Undersea Signal Processing.]

Random Sample

Space scientists at the University of Leicester are working to protect the people of Britain from a terrifying scourge: counterfeit whisky. And this week's numbers quantify the cost to ferry U.S. astronauts to the International Space Station aboard Soyuz spacecraft, the percentage of MIT's science and engineering faculty who are women, and the number of homes that could run on an underwater turbine array just authorized by the Scottish government.

Convex hull method of acoustic field interpolation.

Empirical knowledge of the local acoustic field typically comes in the form of time samples at discrete points in space corresponding to the locations of, say, omnidirectional microphones or hydrophones on an underwater array. To determine the acoustic field at an arbitrary point in space requires either propagation modeling, signal interpolation, or some combination of the two. In this paper, a method of field interpolation is presented which estimates the acoustic field at any point within the convex hull of all sample points. This method is compared against more traditional inverse beamforming methods using both simulated and measured data.