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To address the challenges of large‐aperture sonar systems passive localisation, this paper proposes the application of sparse Bayesian learning (SBL) for passive target localisation in the wideband beam domain. The proposed algorithm aims to overcome the issues of massive computational requirements for two‐dimensional SBL scanning and increased localisation errors due to interference energy leakage. The wideband beam domain SBL focusing localisation algorithm is developed by constructing an azimuth‐range two‐dimensional transformation matrix to preprocess array data, which effectively reduces the computational load of SBL processing while suppressing strong interference energy leakage in passive sonar operating environments, thus improving the range resolution and parameter estimation accuracy of focusing localisation. Simulation and sea trial data analyses demonstrate the feasibility of the proposed algorithm, with results indicating its superior performance compared to existing algorithms. [ABSTRACT FROM AUTHOR]

Conventional active SONAR systems often use beamformers and matched filters separately to extract bearing and range information from the received signal and offer a straightforward way of creating a two‐dimensional map of the environment. In SONAR systems the minimum‐variance‐distortionless‐response beamformer (MVDR beamformer) is a commonly used type of beamformer, which will reconstruct the receive signal from a certain direction optimally. In terms of detecting the transmit signal, the most used method is the conventional matched filter. Both algorithms are simple to implement and perform well under various noise scenarios. The proposed method combines the beamformer and matched filter by introducing an extended channel model that allows the derivation of a multichannel Wiener filter to solve for the unknown reflection coefficients of the complete two‐dimensional environment. This results in adaptively calculated filter weights that will drastically improve the performance compared to a separate MVDR beamformer and matched filter. In addition, a parameter is introduced with which one can arbitrarily adjust the focus between angular and temporal resolution depending on the application. After the derivation, the performance is demonstrated with simulations and measurements. [ABSTRACT FROM AUTHOR]

The performance of Doppler velocity logs (DVLs) in terms of velocity estimate error is directly linked to the geometry of the beam and the pulse transmitted. Beyond a specific transmitted bandwidth, the phase‐shift beamformer can introduce significant errors in velocity estimation. To delineate the operating mechanism of phase‐shift errors within a phased array of acoustic DVLs, the correlation between bottom echo and velocity distribution, in conjunction with the power‐weighted function, was initially examined predicated on spectral estimation theory. Subsequently, numerical and analytical models of the Gaussian‐shaped Doppler spectrum were formulated. The models are employed to evaluate the velocity estimation inaccuracies attributed to phase shifts in extant DVLs, and the comparative results with field experiments corroborate the model's efficacy in forecasting errors. The theoretical findings evaluate the performance limitations of the current phased array transducer design and provide insights for developing new designs. Pool experimental results show that this design effectively reduces the velocity estimation error caused by phase shift under static conditions and in the presence of Doppler frequencies to a level of almost complete elimination of the error compared to conventional configurations. [ABSTRACT FROM AUTHOR]

Sonar systems are frequently used to classify objects at a distance by using the structure of the echoes of acoustic waves as a proxy for the object's shape and composition. Traditional synthetic aperture processing is highly effective in solving classification problems when the conditions are favourable but relies on accurate knowledge of the sensor's trajectory relative to the object being measured. This article provides several new theoretical tools that decouple object classification performance from trajectory estimation in synthetic aperture sonar processing. The key insight is that decoupling the trajectory from classification-relevant information involves factoring a function into the composition of two functions. The article presents several new general topological invariants for smooth functions based on their factorisations over function composition. These invariants specialise to the case when a sonar platform trajectory is deformed by a non-small perturbation. The mathematical results exhibited in this article apply well beyond sonar classification problems. This article is written in a way that supports full mathematical generality. [ABSTRACT FROM AUTHOR]

Underwater acoustic target recognition (UATR) technology based on deep learning and automatic encoding has become an important research direction in the underwater acoustic field in recent years. However, the existing methods do not have favourable self‐adaptability for different data because of the complex and changeable underwater environment, which easily leads to an unsatisfactory recognition effect. The concept of contrastive learning is introduced into UATR and a model named Contrastive Coding for UATR (CCU) is proposed. Based on the unsupervised contrastive learning framework, the model has been modified for the underwater acoustic field. Thus, the CCU can generate adaptable automatic features according to different data. The experimental test shows that the model is superior to other automatic encoding models and has achieved excellent recognition performance on different underwater acoustic datasets. [ABSTRACT FROM AUTHOR]

A real‐aperture high‐resolution imaging method utilising a multiple‐input multiple‐output (MIMO) sonar with two‐dimensional (2‐D) deconvolution processing is proposed. Via derivation, the authors show that the conventional beamformer (CBF) output of the MIMO sonar can be approximately expressed as the 2‐D convolution between the original angular and range distributions of scatterers and a 2‐D point spread function (PSF). Hence, to improve the imaging performance degraded by the convolution effect, the 2‐D Richardson‐Lucy (R‐L) algorithm is used to deconvolve the CBF output of the MIMO sonar. Specifically, the authors give the iteration formula of the 2‐D R‐L algorithm attached to the MIMO sonar, and take the absolute values of the CBF outputs of the virtual array as the deconvolution inputs. The analytical expression of the 2‐D PSF is also given, which is designed as the angular‐ and range‐domain (amplitude) responses of a far‐field ideal scatterer located in the normal direction of the virtual array. Meanwhile, the authors point out that the mismatch made by the approximation may degrade the imaging performance, and suggest that a small number of iterations in the 2‐D R‐L algorithm can be used to alleviate the mismatch problem. Via numerical simulations and a tank experiment, the authors show that the proposed method can simultaneously increase the angular and range resolutions and suppress the sidelobes, when compared to the existing MIMO sonar imaging method. [ABSTRACT FROM AUTHOR]

Traditional chirp scaling algorithm is often developed based on the quadratic approximation of point target reference spectrum, which neglects the higher‐order coupling between range and azimuth dimensions. Besides, the scaling operation is conducted based on quadratic term of reference range, and the residual quadratic error increases with target range far away from reference range. The analysis of phase error shows that both errors are noticeable under the case of wide‐bandwidth signal, and the imaging performance would be seriously distorted. To solve this problem, the higher‐order coupling and quadratic error are compensated by exploiting sub‐block processing method in the range dimension when the chirp scaling algorithm is exploited to reconstruct targets. Compared to traditional methods, the presented method significantly improves the focussing performance based on the simulation results. The processing of real data further shows that the performance of presented method is superior to that of traditional method. [ABSTRACT FROM AUTHOR]

In recent years, significant effort has been allocated to research on multiple‐input–multiple‐output (MIMO) sound navigation and ranging (SONAR) and RADAR systems. Most work has been conducted on the general theoretical functionality of such systems. Less effort has been applied to considerations of the real‐time MIMO capability, although this is an important factor for the application of these new algorithms in real SONAR systems. To account for this, the following work focusses, after introducing the used methodology and revisiting the general MIMO idea and considered system, on more effective permutations of the involved algorithms in the reduction of floating‐point operations. In this context, the general necessity of the algorithms utilized is shown. Furthermore, it is proven that the reduction in computational load does not affect the performance of the system. In addition, the main algorithmic parts of MIMO systems can be exchanged almost arbitrarily under given restrictions without changing the result. Therefore, the performance differences in floating‐point operations are depicted to give an estimate of the achievable degree of complexity reduction. The results for the investigated systems and algorithms are obtained by applying a system simulation of a simple underwater channel. The obtained results were also verified using a real MIMO SONAR system operating in real time. [ABSTRACT FROM AUTHOR]

Modern multibeam echosounders (MBE) employ frequency-division techniques (FDT) to ensonify multiple sectors within the same ping cycle. This leads to improved performance in coverage rate, and yaw and pitch stabilization. However, it introduces a bias among sectors because MBES systems are frequency dependent. It also reduces the maximum pulse bandwidth compared to a single-sector sonar. In this study, we consider the code-division technique (CDT) as a solution to this problem. A set of orthogonal coded pulses are received within the same frequency band, and each sector is separated with a matched filter. We assess the feasibility of the technique through two stages. 1) First, we formulate an analytical model describing the power and crosstalk budgets of any multisector MBES. The model can then be used to design transmission sequences fitting these budgets. 2) Then, we display the practical usage of the technique for MBES imaging and mapping through simulated case studies. For the same total time-bandwidth budget, we compare the performance of FDT, CDT, and multicarrier CDT (MC-CDT), a hybrid method employing CDT and FDT, which is robust to strong dynamic backscatter. This study considers only bottom detection based on signal amplitude. Our results show that it is possible to share a larger frequency bandwidth between multiple sectors while maintaining an acceptable bottom detection performance similar to FDT. Our choice of time-bandwidth product with CDT offers a crosstalk suppression of − 25 dB between sectors, but may display low-magnitude residual artefacts in the water-column data. MC-CDT provides a significant gain of pulse bandwidth while it offers interband separation performance comparable to FDT and reduces significantly the water-column artefacts. [ABSTRACT FROM AUTHOR]

A processing technique for continuous active sonar (CAS) that uses linear frequency modulation (LFM) waveforms is presented. LFM CAS employs wideband long-duration LFMs with 100% transmit duty cycle. The received signals are typically processed in sub-bands by forming a sequence of matched-filter replicas that correspond to consecutive segments of the full waveform; this increases the update rate of the sonar at the cost of decreased performance in each sub-band due to the decrease in processing bandwidth and duration (energy). The algorithm presented in this article achieves fast updates by forming a sequence of unique matched-filter replicas through a number of circular shifts to the transmitted LFM, resulting in processing across the frequency jump between pings. This approach allows increased processing duration and bandwidth (up to the full duration/bandwidth of the LFM) at the cost of reduced independence between updates. The circular-shift algorithm is mainly considered in the limit where the full duration and bandwidth of the waveform are exploited to provide partially independent detection and ranging opportunities. The update rate can be arbitrarily chosen independent of processing duration. The algorithm is validated with sonar data from the LCAS15 sea trial. [ABSTRACT FROM AUTHOR]

Sonar images are valuable in exploring underwater environmental information. As these images are generally limited by the viewing angle, sonar image mosaicking becomes an important research topic. By combining several frames consecutively acquired while the underwater vehicle is manoeuvring, an image with a wider view can be obtained. This study presents a fast sonar image mosaicking approach consisting of denoising, feature extraction, initial matching, splicing, and optimisation. Based on the Euclidean distance between initially matched points and dip angle of connection line, poorly matched feature point pairs are removed to avoid false matching. This way, the success rate of image mosaicking and quality of the resulting mosaicking is effectively improved. [ABSTRACT FROM AUTHOR]

Human echolocation is a technique used by small group of people (normally blind persons) to observe their surrounding via emitting an active signal (usually tongue click) and analyse the return echo. This skill allows them to navigate safely by 'seeing through sound'. Remarkable performances upon echolocating have attracted scholars' attention from multidiscipline field including psychophysical and engineering. Study of waveform diversity has reported the signal is wideband, consisting of multiple frequency components with exponential decay factor that probably made up the entire unique signal. As such, wideband perspective is considered during ambiguity function analysis using wide-band ambiguity function method. The waveform analysis continues and reported optimum autocorrelation function is achieved by adopting a bio-inspired technique, which incorporated gammatone-filter processing. It will be interesting to synthesise the waveform using gammatone-filter approaches prior to relay it into ambiguity function process for analysis. Therefore, it is worth to explore its output results in this paper, which hold new findings that may help to understand how these humans achieve an optimum performance upon echolocating including ambiguity function characteristic. Thus, the new knowledge explored in this paper could be beneficial in emerging concept for new development in radar and sonar system application in near future. [ABSTRACT FROM AUTHOR]

The development of a real-time 3-D underwater imaging system with a planar array is restricted by the huge hardware cost and computational burden associated with the large number of elements required. In this paper, a new design of low-complexity 3-D underwater imaging system is proposed, involving two key parts: a special sparse array with low hardware cost and an efficient beamforming with low computational burden. Especially, the simulated annealing algorithm is optimized based on distributed and parallel subarray (DPS) beamforming and results in fewer active elements than existing methods. To improve the computational efficiency especially for sparse arrays, an optimized DPS beamforming is presented with the pruning fast Fourier transform technology. To test the validity of the design, a real system based on the proposed methods was devised and employed in the lake and sea trials. The experiment results demonstrate that the low-complexity 3-D underwater imaging system can provide a satisfactory imaging quality. [ABSTRACT FROM AUTHOR]

This paper introduces a computationally intensive method for measuring distance at high measurement rates using chaotic pulse-position (CPPM) signals and single-bit signal processing in a single ultrasound sensor system. To prevent crosstalk from other ultrasonic sensor systems, a chaotic system is used, and the ultrasonic pulses are transmitted as a pulse sequence generated by CPPM. After receiving the ultrasound signal, the single-bit signal was generated by a fast Fourier transform threshold, which also functions as an environmental noise filter. Single-bit signal processing has been adopted to reduce the computational load when cross correlating the transmitted and received signals for time-of-flight calculations. Single-bit signal processing has more than 100 times memory saving effect in this paper. Additionally, as the signal-to-noise ratio increases, extension of the maximum measurable distance in the ultrasonic distance sensor system is expected. The simulation shows the crosstalk rejection performance of the proposed method, and the experimental results confirm the improvement of the measurement speed by the proposed method. [ABSTRACT FROM AUTHOR]

Impact assessments for sonar operations typically use received sound levels to predict behavioural disturbance in marine mammals. However, there are indications that cetaceans may learn to associate exposures from distant sound sources with lower perceived risk. To investigate the roles of source distance and received level in an area without frequent sonar activity, we conducted multi-scale controlled exposure experiments (n = 3) with 12 northern bottlenose whales near Jan Mayen, Norway. Animals were tagged with high-resolution archival tags (n = 1 per experiment) or mediumresolution satellite tags (n = 9 in total) and subsequently exposed to sonar. We also deployed bottom-moored recorders to acoustically monitor for whales in the exposed area. Tagged whales initiated avoidance of the sound source over awide range of distances (0.8-28 km), with responses characteristic of beakedwhales. Both onset and intensity of responsewere better predicted by received sound pressure level (SPL) than by source distance. Avoidance threshold SPLs estimated for each whale ranged from 117-126 dB re 1 mPa, comparable to those of other tagged beakedwhales. In this pristine underwater acoustic environment, we found no indication that the source distances tested in our experiments modulated the behavioural effects of sonar, as has been suggested for locations where whales are frequently exposed to sonar. [ABSTRACT FROM AUTHOR]

The probability density function of the acoustic field amplitude scattered by the seafloor was measured in a rocky environment off the coast of Norway using a synthetic aperture sonar system, and is reported here in terms of the probability of false alarm. Interpretation of the measurements focused on finding the appropriate class of statistical models (single versus two-component mixture models), and on appropriate models within these two classes. It was found that two-component mixture models performed better than single models. The two mixture models that performed the best (and had a basis in the physics of scattering) were a mixture between two K distributions, and a mixture between a Rayleigh and generalized Pareto distribution. Bayes' theorem was used to estimate the probability density function of the mixture model parameters. It was found that the K-K mixture exhibits a significant correlation between its parameters. The mixture between the Rayleigh and generalized Pareto distributions also had a significant parameter correlation, but also contained multiple modes. It is concluded that the mixture between two K distributions is the most applicable to this dataset. [ABSTRACT FROM AUTHOR]

In this study, the echolocation and flight behaviors of the Taiwanese leaf-nosed bat (Hipposideros armiger terasensis), which uses constant-frequency (CF) biosonar signals combined with a frequency-modulated (FM) sweep, are compared with those of the big brown bat (Eptesicus fuscus), which uses FM signals alone. The CF-FM bat flew through a corridor bounded by vertical poles on either side, and the inter-pole spacing of the walls was manipulated to create different echo flow conditions. The bat's flight trajectories and echolocation behaviors across corridor conditions were analyzed. Like the big brown bat, the Taiwanese leaf-nosed bat centered its flight trajectory within the corridor when the pole spacing was the same on the two walls. However, the two species showed different flight behaviors when the pole spacing differed on the two walls. While the big brown bat deviated from the corridor center towards the wall with sparse pole spacing, the Taiwanese leaf-nosed bat did not. Further, in comparison to E. fuscus, H. a. terasensis utilized different echolocation patterns showing a prevalence of grouping sounds into clusters of three. These findings indicate that the two species' distinct sonar signal designs contribute to their differences in flight trajectories in a structured corridor. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a teach‐and‐repeat path‐following method for an autonomous underwater vehicle (AUV) navigating long distances in environments where external navigation aides are denied. This method utilizes sonar images to construct a series of reference views along a path, stored as a topological map. The AUV can then renavigate along this path, either to return to the start location or to repeat the route. Utilizing unique assumptions about the sonar image‐generation process, this system exhibits robust image‐matching capabilities, providing observations to a discrete Bayesian filter that maintains an estimate of progress along the path. Image‐matching also provides an estimate of offset from the path, allowing the AUV to correct its heading and effectively close the gap. Over a series of field trials, this system demonstrated online control of an AUV in the ocean environment of Holyrood Arm, Newfoundland and Labrador, Canada. The system was implemented on an International Submarine Engineering Ltd. Explorer AUV and performed multiple path completions over both a 1 and 5 km track. These trials illustrated an AUV operating in a fully autonomous mode, in which navigation was driven solely by sensor feedback and adaptive control. Path‐following performance was as desired, with the AUV maintaining close offset to the path. [ABSTRACT FROM AUTHOR]

Sonar emits pulses of sound and uses the reflected echoes to gain information about target objects. It offers a low cost, complementary sensing modality for small robotic platforms. Although existing analytical approaches often assume independence across echoes, real sonar data can have more complicated structures due to device setup or experimental design. In this article, we consider sonar echo data collected from multiple terrain substrates with a dual-channel sonar head. Our goals are to identify the differential sonar responses to terrains and study the effectiveness of this dual-channel design in discriminating targets. We describe a unified analytical framework that achieves these goals rigorously, simultaneously, and automatically. The analysis was done by treating the echo envelope signals as functional responses and the terrain/channel information as covariates in a functional regression setting. We adopt functional mixed models that facilitate the estimation of terrain and channel effects while capturing the complex hierarchical structure in data. This unified analytical framework incorporates both Gaussian models and robust models. We fit the models using a full Bayesian approach, which enables us to perform multiple inferential tasks under the same modeling framework, including selecting models, estimating the effects of interest, identifying significant local regions, discriminating terrain types, and describing the discriminatory power of local regions. Our analysis of the sonar-terrain data identifies time regions that reflect differential sonar responses to terrains. The discriminant analysis suggests that a multi- or dual-channel design achieves target identification performance comparable with or better than a single-channel design. Supplementary materials for this article are available online. [ABSTRACT FROM PUBLISHER]

Direction-of-arrival (DOA) and range estimation is an important issue of sonar signal processing. In this paper, a novel approach using Hilbert-Huang transform (HHT) is proposed for joint bearing and range estimation of multiple targets based on a uniform linear array (ULA) of hydrophones. The structure of this ULA based on micro-electro-mechanical systems (MEMS) technology, and thus has attractive features of small size, high sensitivity and low cost, and is suitable for Autonomous Underwater Vehicle (AUV) operations. This proposed target localization method has the following advantages: only a single snapshot of data is needed and real-time processing is feasible. The proposed algorithm transforms a very complicated nonlinear estimation problem to a simple nearly linear one via time-frequency distribution (TFD) theory and is verified with HHT. Theoretical discussions of resolution issue are also provided to facilitate the design of a MEMS sensor with high sensitivity. Simulation results are shown to verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

The rich information in the power spectral density (PSD) matrix of a received signal can be extracted in different ways for various purposes in signal processing. Care, however, must be taken when distances between PSD matrices are considered because these matrices form a Riemannian manifold in the signal space. In this paper, two Riemannian distances (RD) are introduced for the measurement of distances between PSD matrices on the manifold. The principle by which the geodesics on the manifold can be lifted to a Euclidean subspace isometric with the tangent space of the manifold is also explained and emphasized. This leads to the concept that any optimization involving the RD on the manifold can be equivalently performed in this Euclidean subspace. The application of this principle is illustrated by the development of iterative algorithms to find the Riemannian means and Riemannian medians according to the two RD. These distance measures are then applied to the detection of narrow-band sonar signals. Exploration of the translation of measure reference as well as the application of optimum weighting show substantial improvement in detection performance over classical detection method. [ABSTRACT FROM AUTHOR]

The current identification algorithm using sonar signal parameters of bandwidth, frequency, duration and pulse waveform which are easy to detect and imitation, to identify the identity of sonar signal, resulting in part of the sonar signal identity is not easy to distinguish. Therefore, an algorithm based on signal feature extraction and digital watermarking is proposed to recognize the uncertain sonar signals. The algorithm embeds the digital watermark into the detection signal from the uncertain sonar. The identity of the signal is recognized by detecting whether the received signal contains watermarks. Experimental results showed that the proposed algorithm can effectively improve the recognition performance of sonar signal source. [ABSTRACT FROM AUTHOR]

Estimation of the modulation frequency of a source is important in various sonar signal processing applications. The conventional estimator of modulating frequency has been formulated under the assumption that the source and noise are Gaussian distributed. This is often inaccurate in several applications such as underwater acoustics where the signal or noise can be impulsive. We formulate novel modulation frequency estimators that are robust to impulses in the data, and outperform the conventional estimator in environments where the observed data are contaminated by impulses. We demonstrate their performance using recorded data. We characterize the performance of the methods in terms of their accuracy, harmonic distortion, and robustness to knowledge of noise statistics. We also derive the Cramer–Rao lower bound for the modulation frequency estimation problem in impulsive data. [ABSTRACT FROM AUTHOR]

Quadrature-polarimetric (quad-pol) synthetic aperture radar (SAR) is a well-established technique which has numerous applications in remote sensing. However, spaceborne quad-pol SAR systems are often constrained by severe range and azimuth ambiguities. A deep understanding of the nature of ambiguities in conventional, hybrid, and $\pm \pi $ /4 quad-pol SAR allows a correct evaluation of the ambiguity-to-signal ratio and the design of systems optimized for ambiguity suppression. In that respect azimuth phase coding and merging data available from both cross-polarized channels play an important role. The results of this paper are relevant not only for the design of future spaceborne quad-pol SAR systems, including staggered SAR ones, but also for the optimal exploitation of quad-pol SAR systems currently in operation. [ABSTRACT FROM PUBLISHER]

Two experiments were performed that investigated the effects of (1) click level and (2) continuous broadband noise on the binaural auditory brainstem response (ABR) of normal-hearing and hearingimpaired bottlenose dolphins (Tursiops truncatus). In addition to spectrally uncompensated clicks and noise, stimuli were digitally compensated to achieve "white" spectra (flat spectral density level) or "pink" spectra (spectral density level rolling off at -3 dB/octave). For experiment 1, in all spectral conditions, ABR peak latencies increased and peak amplitudes decreased with decreasing click level, but interwave intervals changed little. Latency-intensity function (LIF) slopes ranged from -3 to -11 μs/dB. The LIF slopes of ABR peaks evoked by uncompensated clicks were steeper in dolphins with hearing loss. Click level was held constant during experiment 2, and the effect of bilaterally delivered broadband masking noise on the ABR was investigated. Clicks and noise were filtered to create a pink click/noise condition and a white click/noise condition. With increasing levels of masking noise, peak latencies increased (although only P1-P4 white reached significance), peak amplitudes decreased, and interpeak intervals increased (although not significantly). These effects are compared to results reported for terrestrial mammals, and implications for auditory health assessment and biosonar function are discussed. [ABSTRACT FROM AUTHOR]

In this paper, volumetric imaging via multipass circular synthetic aperture sonar (CSAS) is demonstrated using an autonomous underwater vehicle (AUV). A multidimensional aperture is synthesized by performing a series of circular scans at varying grazing angles around targets and coherently combining the backscattering information from the set of scans to form high-resolution volumetric images. A data-driven technique for precision alignment of the individual scans comprising the multipass set enables synthesis of a multidimensional array. To beamform in the vertical dimension using the irregular and undersampled multipass aperture, a compressive-sensing-based approach is adopted which is similar to methods used in analogous synthetic aperture radar tomography applications but modified to accommodate for the wider fractional bandwidth of the synthetic aperture sonar (SAS) system. The modification exploits a joint sparsity assumption in the vertical scattering profile at different subbands and adapts a standard joint sparse solving algorithm to the relevant case in which the sparsity profile is common between solution vectors but the sensing matrices are different. Results are shown for a variety of targets, including proud and obliquely buried unexploded ordnance, a 2-1 solid aluminum cylinder, and a steel oil drum. [ABSTRACT FROM AUTHOR]

An electronic navigational chart is a major source of information for the navigator. The component that contributes most significantly to the safety of navigation on water is the information on the depth of an area. For the purposes of this article, the authors use data obtained by the interferometric sonar GeoSwath Plus. The data were collected in the area of the Port of Szczecin. The samples constitute large sets of data. Data reduction is a procedure to reduce the size of a data set to make it easier and more effective to analyse. The main objective of the authors is the compilation of a new reduction algorithm for bathymetric data. The clustering of data is the first part of the search algorithm. The next step consists of generalisation of bathymetric data. This article presents a comparison and analysis of results of clustering bathymetric data using the following selected methods: K-means clustering algorithm, traditional hierarchical clustering algorithms and self-organising map (using artificial neural networks). [ABSTRACT FROM AUTHOR]

We proposed a dual smoothing superresolution direction-of-arrival (DOA) estimation for multitarget detection with two-channel frequency modulated continuous-wave (FMCW) radar. For the conventional superresolution FMCW radar, the number of detectable targets for DOA estimation is limited by the number of RX antenna arrays. To enable DOA estimation for more targets than the number of receiving channels, we propose exploiting a dual shift invariant structure of dual smoothed matrix. We implemented two-channel FMCW radar and tested the performance of the proposed method. [ABSTRACT FROM PUBLISHER]

The development of real-time 3-D underwater imaging is restricted by the huge hardware cost and the computational burden associated with a large number of transducers. In this paper, a low-complexity real-time 3-D sonar imaging system with a cross array is proposed. The low complexity, in both hardware cost and computational load, is achieved by an innovative method of signal processing in the far field and the advantages of the cross array in minimal transducer numbers. The method consists of two parts: a multifrequency (MF) algorithm for the transmitting process and a parallel subarray (PS) algorithm for the receiving beamforming. The MF algorithm solves the real-time problem of the cross array by reducing the scanning time that is proportional to the number of transmissions. The PS algorithm improves the computational efficiency using a two-stage parallel and pipeline framework. The PS algorithm is an approximate method, and its precision is analyzed in this paper. The computational efficiency of the proposed method is compared with the direct method (DM) beamforming in both cross and planar arrays. A prototype, based on the innovative method, was designed and tested in pool and lake trials. The results demonstrate that the low-complexity system can satisfy the real-time requirement of 3-D underwater imaging applications with an acceptable imaging quality. [ABSTRACT FROM PUBLISHER]

In this paper, we discuss a novel underwater short-range sensor using electromagnetic (EM) wave attenuation. We use the revised Friis–Shelkunoff formula to calculate the EM wave attenuation underwater as a function of distance. This requires knowledge of the antenna gain underwater, which is very different from the gain in air, and also the attenuation constant which depends on the water conductivity. We calibrated the gain and attenuation in a ranging experiment and also in a 2-D localization experiment. Both methods agreed, confirming that in situ calibration of a 2-D localization experiment is feasible. The localization results show good accuracy, validating the sensor model and showing that multipath effects can be made negligible in such an experiment. [ABSTRACT FROM PUBLISHER]

Each animal population has its own acoustic signature which facilitates identification, communication and reproduction. The sonar signals of bats can convey social information, such as species identity and contextual information. The goal of this study was to determine whether bats adjust their echolocation call structures to mutually recognize and communicate when they encounter the bats from different colonies. We used the intermediate leaf-nosed bats (Hipposideros larvatus) as a case study to investigate the variations of echolocation calls when bats from one colony were introduced singly into the home cage of a new colony or two bats from different colonies were cohabitated together for one month. Our experiments showed that the single bat individual altered its peak frequency of echolocation calls to approach the call of new colony members and two bats from different colonies adjusted their call frequencies toward each other to a similar frequency after being chronically cohabitated. These results indicate that the ‘compromise’ in echolocation calls might be used to ensure effective mutual communication among bats. [ABSTRACT FROM AUTHOR]

The time and energetic costs of behavioral responses to incidental and experimental sonar exposures, as well as control stimuli, were quantified using hidden state analysis of time series of acoustic and movement data recorded by tags ( DTAG) attached to 12 sperm whales ( Physeter macrocephalus) using suction cups. Behavioral state transition modeling showed that tagged whales switched to a non-foraging, non-resting state during both experimental transmissions of low-frequency active sonar from an approaching vessel ( LFAS; 1-2 kHz, source level 214 dB re 1 μPa m, four tag records) and playbacks of potential predator (killer whale, Orcinus orca) sounds broadcast at naturally occurring sound levels as a positive control from a drifting boat (five tag records). Time spent in foraging states and the probability of prey capture attempts were reduced during these two types of exposures with little change in overall locomotion activity, suggesting an effect on energy intake with no immediate compensation. Whales switched to the active non-foraging state over received sound pressure levels of 131-165 dB re 1 μPa during LFAS exposure. In contrast, no changes in foraging behavior were detected in response to experimental negative controls (no-sonar ship approach or noise control playback) or to experimental medium-frequency active sonar exposures ( MFAS; 6-7 kHz, source level 199 re 1 μPa m, received sound pressure level [ SPL] = 73-158 dB re 1 μPa). Similarly, there was no reduction in foraging effort for three whales exposed to incidental, unidentified 4.7-5.1 kHz sonar signals received at lower levels ( SPL = 89-133 dB re 1 μPa). These results demonstrate that similar to predation risk, exposure to sonar can affect functional behaviors, and indicate that increased perception of risk with higher source level or lower frequency may modulate how sperm whales respond to anthropogenic sound. [ABSTRACT FROM AUTHOR]

When an ultrasonic motion sensor is not aligned correctly with an object's motion, the geometry of the transmitted and reflected sonar signals leads to an underestimate of velocity. This is not a large effect under most teaching laboratory conditions, but compounding such an error in the production of acceleration data can produce surprisingly large overestimates. Presented here is an analysis of the general problem for free-fall experiments, followed by experimental data that show the relative importance of these errors. [ABSTRACT FROM AUTHOR]

Passive localization of weak sources in the presence of strong interferences is generally a difficult task. In this paper, an eigenanalysis-based adaptive interference suppression (EAAIS) method is proposed for estimating the bearing and subsequently the weak source range. Assuming the target of interest (TOI) is in a known bearing sector, first we define a contribution ratio for each eigenvector of the cross-spectral density matrix (CSDM). The eigenvectors not dominated by the TOI are adaptively identified and removed for interference suppression. The remaining eigenvectors are then used to reconstruct the CSDM for TOI bearing estimation. Finally, the acoustic source range estimation is achieved with the reconstructed CSDM, by matching the beam intensity striation pattern, where it is not required to estimate the waveguide-invariant parameter. The effectiveness of the proposed TOI localization is demonstrated using several experimental data sets. [ABSTRACT FROM PUBLISHER]

This paper proposes a general model of superdirectivity to provide analytical and closed-form solutions for arbitrary sensor arrays. Based on the equivalence between the maximum directivity factor and the maximum array gain in the isotropic noise field, Gram-Schmidt orthogonalization is introduced and recursively transformed into a matrix form to conduct pre-whitening and matching operations that result in superdirectivity solutions. A Gram-Schmidt mode-beam decomposition and synthesis method is then presented to formally implement these solutions. Illustrative examples for different arrays are provided to demonstrate the feasibility of this method, and a reduced rank technique is used to deal with the practical array design for robust beamforming and acceptable high-order superdirectivity. Experimental results that are provided for a linear array consisting of nine hydrophones show the good performance of the technique. A superdirective beampattern with a beamwidth of 48.05° in the endfire direction is typically achieved when the inter-sensor spacing is only 0.09 λ ( λ is the wavelength), and the directivity index is up to 12 dB, which outperforms that of the conventional delay-and-sum counterpart by 6 dB. [ABSTRACT FROM AUTHOR]

Objective: We present a device that combines principles of ultrasonic echolocation and spatial hearing to provide human users with environmental cues that are 1) not otherwise available to the human auditory system, and 2) richer in object and spatial information than the more heavily processed sonar cues of other assistive devices. The device consists of a wearable headset with an ultrasonic emitter and stereo microphones with affixed artificial pinnae. The goal of this study is to describe the device and evaluate the utility of the echoic information it provides. Methods: The echoes of ultrasonic pulses were recorded and time stretched to lower their frequencies into the human auditory range, then played back to the user. We tested performance among naive and experienced sighted volunteers using a set of localization experiments, in which the locations of echo-reflective surfaces were judged using these time-stretched echoes. Results: Naive subjects were able to make laterality and distance judgments, suggesting that the echoes provide innately useful information without prior training. Naive subjects were generally unable to make elevation judgments from recorded echoes. However, trained subjects demonstrated an ability to judge elevation as well. Conclusion: This suggests that the device can be used effectively to examine the environment and that the human auditory system can rapidly adapt to these artificial echolocation cues. Significance: Interpreting and interacting with the external world constitutes a major challenge for persons who are blind or visually impaired. This device has the potential to aid blind people in interacting with their environment. [ABSTRACT FROM AUTHOR]

A quantitative analysis of the interspecific variability in bat biosonar beampatterns has been carried out on 267 numerical predictions of emission and reception beampatterns from 98 different species. Since these beampatterns did not share a common orientation, an alignment was necessary to analyze the variability in the shape of the patterns. To achieve this, beampatterns were aligned using a pairwise optimization framework based on a rotation-dependent cost function. The sum of the p-norms between beam-gain functions across frequency served as a figure of merit. For a representative subset of the data, it was found that all pairwise beampattern alignments resulted in a unique global minimum. This minimum was found to be contained in a subset of all possible beampattern rotations that could be predicted by the overall beam orientation. Following alignment, the beampatterns were decomposed into principal components. The average beampattern consisted of a symmetric, positionally static single lobe that narrows and became progressively asymmetric with increasing frequency. The first three "eigenbeams" controlled the beam width of the beampattern across frequency while higher rank eigenbeams account for symmetry and lobe motion. Reception and emission beampatterns could be distinguished (85% correct classification) based on the first 14 eigenbeams. [ABSTRACT FROM AUTHOR]

Real-time 3-D acoustical imaging technique is a key advance to broaden the scope and enhance the feasibility of underwater missions. In this paper, a real-time 3-D underwater acoustical imaging system to handle the actual tasks under a narrowband excitation is presented. The system consists of three parts: a transmitter, a receiving hydrophone array, and a signal processor. In this system, a distributed and parallel subarray (DPS) beamforming algorithm is proposed to process the acquired signals from a scene placed in the far field. The DPS beamforming algorithm is an approximate method for the sonar signal processing with an advantageous computational efficiency. The direct method (DM) and fast Fourier transform (FFT) beamforming are compared with DPS beamforming for the memory and computational requirements. Based on this algorithm, a prototype was developed, which has been extensively employed in the lake and sea trials. The trials demonstrate that the system can achieve the 3-D imaging of the scene and meet the real-time requirement of the underwater operations. [ABSTRACT FROM PUBLISHER]

The article analyzing how long-finned pilot whales (Globicephala melas) respond to naval sonar signals. It examines whether they changed their calls to vocally match the sonar signals used during the experiments. It is said that these whales respond to naval sonar by increasing their rate of vocalization while they were huddling in close ranks.

The mean squared error of the classical maximum likelihood time-of-flight (ToF) estimator increases dramatically when the signal-to-noise ratio falls below a certain threshold. For narrow-band signals, this well-known threshold effect occurs largely due to the biased outliers which are induced by the local maxima of the source signal autocorrelation function. In our previous work (Part I), we have described a machine learning biosonar-inspired method for reducing the effect of the outlier bias on the accuracy of a single-echo estimate. In this paper, we extend this approach by introducing a method for combining multiple echo signals into a robust ToF estimator which is resilient to outliers. The individual bias-corrected estimates are combined together using the optimal weighted averaging (OWA) scheme which takes into account the uncertainties associated with inlier and outlier measurements. As in the single-echo case, a weak classifier and a bank of phase-shifted unmatched filters are used for estimating the probabilities of appearance of a specific outlier class in a single-echo estimate. Combined with the previously introduced single-echo bias-reduction method, the OWA scheme results in significant improvement in the ToF estimation accuracy. [ABSTRACT FROM PUBLISHER]

Stealth in military sonars applications may be ensured through the use of low power signals making them difficult to intercept by the enemy. In recent years, silent sonar design has been investigated by the Department of Marine Electronic Systems of the Gdansk University of Technology. This article provides an analysis of how an intercept sonar operated by the enemy can detect silent sonar signals. To that end a theoretical intercept sonar model was developed with formulas that can numerically determine the intercept ranges of silent sonar sounding signals. This was tested for a variety of applications and water salinities. Because they are also presented in charts, the results can be used to compare the intercept ranges of silent sonar and traditional pulse sonar. [ABSTRACT FROM AUTHOR]

This paper advances a new “quasi-blind” calibration algorithm to calibrate a multi-array network (MAN) of acoustic-vector-sensors, whose component-sensors may have non-ideal gain/phase responses, incorrect orientations, and imprecise locations. This proposed calibration is “quasi-blind” in not requiring any prior knowledge/estimation of any training signal's arrival-angle. This proposed algorithm is computationally orders-of-magnitude more efficient than maximum-likelihood estimation. These advantages are achieved here by exploiting the acoustic vector-sensor's quintessential characters, to interplay between two complementary approaches of direction-finding: (1) customary interferometry between vector-sensors, and (2) “acoustic particle-velocity-field normalization” DOA-estimation within each individual vector-sensor. Monte Carlo simulations verify the proposed algorithm's efficacy in “quasi-blind” calibration and its aforementioned computational efficacy. [ABSTRACT FROM PUBLISHER]

A sodar simulator capable of producing time series data emulating sodar signals has been developed and tested. The atmospheric fields used to populate the sodar simulator are taken from output of a large-eddy simulation code. The characteristics of the sodar (number and zenith angle of beams, beamwidth, transmit frequency, range resolution, etc.) are defined by the user to allow emulation of existing systems. The range of the reflected acoustic signal is calculated based upon a temperature-dependent speed of sound. Realistic acoustic background noise is simulated using filtered white noise. The raw acoustic time series data are processed using a Fourier transform to yield acoustic Doppler spectra, from which the radial velocities are calculated. The design of the simulator allows for the testing of and comparisons between various signal-processing techniques and averaging periods. An example case of feeding the sodar simulator with large-eddy simulation data representative of a developing convective boundary layer is presented and discussed. [ABSTRACT FROM AUTHOR]

The pulse-output Direct Digital Synthesis (DDS), in which the overflow signal of the phase accumulator is used for the pulse output, can be easily implemented due to its simple hardware architecture and low algorithm complexity. This paper introduces the fundamentals for generating Linear Frequency Modulation (LFM) pulse using pulse-output DDS technique. Error introducing mechanisms that affect the accuracy of signal's duration, initial phase, and frequency are studied. Extensive analysis of round-off error is given. A modified hardware architecture for LFM pulse generation with reduced round-off error is proposed. Experiment results are given, which shows that the proposed generator is promising in applications such as sonar transmitters. [ABSTRACT FROM AUTHOR]

Eight experimentally controlled exposures to 1-2 kHz or 6-7 kHz sonar signals were conducted with four killer whale groups. The source level and proximity of the source were increased during each exposure in order to reveal response thresholds. Detailed inspection of movements during each exposure session revealed sustained changes in speed and travel direction judged to be avoidance responses during six of eight sessions. Following methods developed for Phase-I clinical trials in human medicine, response thresholds ranging from 94 to 164 dB re 1 µPa received sound pressure level (SPL) were fitted to Bayesian dose-response functions. Thresholds did not consistently differ by sonar frequency or whether a group had previously been exposed, with a mean SPL response threshold of 142 ± 15 dB (mean ± s.d.). High levels of between- and within-individual variability were identified, indicating that thresholds depended upon other undefined contextual variables. The dose-response functions indicate that some killer whales started to avoid sonar at received SPL below thresholds assumed by the U.S. Navy. The predicted extent of habitat over which avoidance reactions occur depends upon whether whales responded to proximity or received SPL of the sonar or both, but was large enough to raise concerns about biological consequences to the whales. [ABSTRACT FROM AUTHOR]