Aperture Sonar Research Articles

Echolocating bats detect but misperceive a multidimensional incongruent acoustic stimulus

Coherent perception relies on integrating multiple dimensions of a sensory modality, for example, color and shape in vision. We reveal how different acoustic dimensions, specifically echo intensity and sonar aperture (or width), are important for correct perception by echolocating bats. We flew bats down a corridor blocked by objects with different intensity-aperture combinations. To our surprise, bats crashed straight into large (aperture) walls with weak echo intensity as if they did not exist. The echolocation behavior of the bats indicated that they did detect the wall, suggesting that crashing was not a result of limited sensory sensitivity, but of a perceptual deficit. We systematically manipulated intensity and aperture by changing the materials and width of different reflectors, and we conclude that a coherent echo-based percept is created only when these two acoustic dimensions have certain relations which are typical for objects in nature (e.g., large and intense or small and weak reflectors). Nevertheless, we show that these preferred relations are not innate. We show that young pups are not constrained to these relations and that new intensity-aperture associations can also be learned by adult bats.

Array directivity enhancement by leveraging angle-dependent scattering.

The quality of a sonar array's localization capabilities, often expressed as directivity, is limited by the sonar's aperture, that is, the length of the sonar array. Previous attempts to improve directivity, without increasing array size, have been moderately successful. Wave scattering within a nontraditional array, such as an array fabricated from a non-homogeneous material, could provide additional information to the localization calculations and improve array directivity without increasing the size of the array. An investigation of array directivity improvement through wave scattering is performed. This paper modifies existing localization and directivity calculations to consider the scattered waves and uses the derived equations to explain why previous proposed scattering was incapable of increasing directivity. A scattering relationship capable of enhancing array localization without increasing array size is proposed, and the directivity improvement claims are verified with beamform plot comparisons and directivity index calculations.

High-frequency side-scan sonar fish reconnaissance by autonomous underwater vehicles

A dichotomy between depth penetration and resolution as a function of sonar frequency, draw resolution, and beam spread challenges fish target classification from sonar. Moving high-frequency sources to depth using autonomous underwater vehicles (AUVs) mitigates this and also co-locates transducers with other AUV-mounted short-range sensors to allow a holistic approach to ecological surveys. This widely available tool with a pedigree for bottom mapping is not commonly applied to fish reconnaissance and requires the development of an interpretation of pelagic reflective features, revisitation of count methods, image-processing rather than wave-form recognition for automation, and an understanding of bias. In a series of AUV mission test cases, side-scan sonar (600 and 900 kHz) returns often resolved individual school members, spacing, size, behavior, and (infrequently) species from anatomical features and could be intuitively classified by ecologists — but also produced artifacts. Fish often followed the AUV and thus were videographed, but in doing so removed themselves from the sonar aperture. AUV-supported high-frequency side-scan holds particular promise for survey of scarce, large species or for synergistic investigation of predators and their prey because the spatial scale of observations may be similar to those of predators.

Approximation of a Warship Passive Sonar Signal Using Taylor Expansion

함정의 수동소나는 여러 개의 지향성과 무지향성 센서로 구성되어 있다. 함정 소나에 수신되는 음향 신호를 모의할 때, 일반적으로 임의의 소음원으로부터 소나에 장착된 모든 센서간의 음파 전달 모델링이 필요하다. 그러나 모든 센서에 대한 통합적인 계산은 시간이 많이 소모되며 소나 시뮬레이터의 성능을 저하시킨다. 본 연구에서는 음선 정보가 알려진 기준 센서가 존재한다고 할 때 그에 인접한 센서 위치에서의 소나 신호를 추정하는 근사적인 방법을 제안한다. 이 방법은 음선의 도달 시간에 대한 테일러 급수를 이용하여 개발되었으며 소나 개구면에 대한 Fraunhofer와 Fresnel 근사와 유사하다. 제안된 기법을 검증하기 위해 수동 소나에 대해 여러 수치실험이 수행되었다. 2차 항까지 테일러 근사를 적용한 근사법이 보다 우수한 결과를 보였다. 추가적으로 각각의 근사 해에 대한 오차 한계가 제시되었다. A passive sonar of warship is composed of several directional or omni-directional sensors. In order to model the acoustic signal received into a warship sonar, the wave propagation modeling is usually required from arbitrary noise source to all sensors equipped to the sonar. However, the full calculation for all sensors is time-consuming and the performance of sonar simulator deteriorates. In this study, we suggest an asymptotic method to estimate the sonar signal arrived to sensors adjacent to the reference sensor, where it is assumed that all information of eigenrays is known. This method is developed using Taylor series for the time delay of eigenray and similar to Fraunhofer and Fresnel approximation for sonar aperture. To validate the proposed method, some numerical experiments are performed for the passive sonar. The approximation when the second-order term is kept is vastly superior. In addition, the error criterion for each approximation is provided with a practical example.

Modeling of bio-inspired broadband sonar for high-resolution angular imaging

Echolocating mammals perceive images of targets with hyper-resolution and navigate seamlessly through obstacles in complex acoustic environments. The biological solution to imaging with sound is vastly different from man-made sonar. The most prominent difference is that instead of imaging with narrow beams, bats ensonify a large spatial region and exploit broadband echo information to acoustically focus with about one degree of angular resolution. Angular localization may therefore be redefined as a spectral pattern matching problem. By imaging with wider beams, this remarkable performance requires only a single broadband transmitter and two receive elements. Our computational modeling work has led to new insight into the salient spatial information encoded by the bat’s auditory system. Although theoretically not required, spatial localization performance increases with the aid of highly complex baffle structures such as those found in biological sonar. Replicating these bio-inspired baffle structures and acoustic processing techniques in man-made systems can reduce sonar array aperture requirements by a factor of 100 or more for a variety of both aerial and underwater acoustic sensing applications. Recent modeling results are presented along with progress toward the design of a compact bio-inspired sonar system for high-resolution imaging. [Work supported by ONR and NUWC Newport.]

Size constancy in bat biosonar? Perceptual interaction of object aperture and distance.

Perception and encoding of object size is an important feature of sensory systems. In the visual system object size is encoded by the visual angle (visual aperture) on the retina, but the aperture depends on the distance of the object. As object distance is not unambiguously encoded in the visual system, higher computational mechanisms are needed. This phenomenon is termed “size constancy”. It is assumed to reflect an automatic re-scaling of visual aperture with perceived object distance. Recently, it was found that in echolocating bats, the ‘sonar aperture’, i.e., the range of angles from which sound is reflected from an object back to the bat, is unambiguously perceived and neurally encoded. Moreover, it is well known that object distance is accurately perceived and explicitly encoded in bat sonar. Here, we addressed size constancy in bat biosonar, recruiting virtual-object techniques. Bats of the species Phyllostomus discolor learned to discriminate two simple virtual objects that only differed in sonar aperture. Upon successful discrimination, test trials were randomly interspersed using virtual objects that differed in both aperture and distance. It was tested whether the bats spontaneously assigned absolute width information to these objects by combining distance and aperture. The results showed that while the isolated perceptual cues encoding object width, aperture, and distance were all perceptually well resolved by the bats, the animals did not assign absolute width information to the test objects. This lack of sonar size constancy may result from the bats relying on different modalities to extract size information at different distances. Alternatively, it is conceivable that familiarity with a behaviorally relevant, conspicuous object is required for sonar size constancy, as it has been argued for visual size constancy. Based on the current data, it appears that size constancy is not necessarily an essential feature of sonar perception in bats.

Size constancy in bat biosonar

Perception and encoding of object is an important feature of sensory systems. In the visual system object is encoded by the visual angle (visual aperture) on the retina, but the aperture depends on the distance of the object. As object distance is not unambiguously encoded in the visual system, higher computational mechanisms are needed. This phenomenon is termed size It is assumed to reflect an automatic re-scaling of visual aperture with perceived object distance. Recently, it was found that in echolocating bats, the 'sonar aperture', i.e., the range of angles from which sound is reflected from an object back to the bat, is unambiguously perceived and neurally encoded. Moreover, it is well known that object distance is accurately perceived and explicitly encoded in bat sonar. Here, we addressed constancy in bat biosonar, recruiting virtual-object techniques. Bats of the species Phyllostomus discolor learned to discriminate two simple virtual objects that only differed in sonar aperture. Upon successful discrimination, test trials were randomly interspersed using virtual objects that differed in both aperture and distance. It was tested whether the bats spontaneously assigned absolute width information to these objects by combining distance and aperture. The results showed that while the isolated perceptual cues encoding object width, aperture, and distance were all perceptually well resolved by the bats, the animals did not assign absolute width information to the test objects. This lack of sonar constancy may result from the bats relying on different modalities to extract information at different distances. Alternatively, it is conceivable that familiarity with a behaviorally relevant, conspicuous object is required for sonar constancy, as it has been argued for visual constancy. Based on the current data, it appears that constancy is not necessarily an essential feature of sonar perception in bats.

Synthetic Aperture Imaging of Surface Laid Targets by Sound

This paper discusses the design and implementation of a synthetic aperture acoustic imaging system for investigating solid objects in an outdoor environment. Measurement results are reported for a setup consisting of a rail-mounted transceiver, digital data acquisition and control system, and local storage with data post processing capabilities. This imaging system works much like typical sonar and RF synthetic aperture radar technologies but operates above ground and uses acoustic waves in air for its interrogation.

Bats’ avoidance of real and virtual objects: Implications for the sonar coding of object size

Fast movement in complex environments requires the controlled evasion of obstacles. Sonar-based obstacle evasion involves analysing the acoustic features of object-echoes (e.g., echo amplitude) that correlate with this object's physical features (e.g., object size). Here, we investigated sonar-based obstacle evasion in bats emerging in groups from their day roost. Using video-recordings, we first show that the bats evaded a small real object (ultrasonic loudspeaker) despite the familiar flight situation. Secondly, we studied the sonar coding of object size by adding a larger virtual object. The virtual object echo was generated by real-time convolution of the bats' calls with the acoustic impulse response of a large spherical disc and played from the loudspeaker. Contrary to the real object, the virtual object did not elicit evasive flight, despite the spectro-temporal similarity of real and virtual object echoes. Yet, their spatial echo features differ: virtual object echoes lack the spread of angles of incidence from which the echoes of large objects arrive at a bat's ears (sonar aperture). We hypothesise that this mismatch of spectro-temporal and spatial echo features caused the lack of virtual object evasion and suggest that the sonar aperture of object echoscapes contributes to the sonar coding of object size.

The Sonar Aperture and Its Neural Representation in Bats

As opposed to visual imaging, biosonar imaging of spatial object properties represents a challenge for the auditory system because its sensory epithelium is not arranged along space axes. For echolocating bats, object width is encoded by the amplitude of its echo (echo intensity) but also by the naturally covarying spread of angles of incidence from which the echoes impinge on the bat's ears (sonar aperture). It is unclear whether bats use the echo intensity and/or the sonar aperture to estimate an object's width. We addressed this question in a combined psychophysical and electrophysiological approach. In three virtual-object playback experiments, bats of the species Phyllostomus discolor had to discriminate simple reflections of their own echolocation calls differing in echo intensity, sonar aperture, or both. Discrimination performance for objects with physically correct covariation of sonar aperture and echo intensity ("object width") did not differ from discrimination performances when only the sonar aperture was varied. Thus, the bats were able to detect changes in object width in the absence of intensity cues. The psychophysical results are reflected in the responses of a population of units in the auditory midbrain and cortex that responded strongest to echoes from objects with a specific sonar aperture, regardless of variations in echo intensity. Neurometric functions obtained from cortical units encoding the sonar aperture are sufficient to explain the behavioral performance of the bats. These current data show that the sonar aperture is a behaviorally relevant and reliably encoded cue for object size in bat sonar.

Generalized Framework for Real Aperture, Synthetic Aperture, and Tomographic Sonar Imaging

When an underwater object is insonified over a range of aspect angles, the signals that are backscattered from the object can be sampled in space and time by an array of sensors, which can form either a real or a synthetic aperture. The spatial information contained in the backscattered signals can be processed using inverse transform methods to form an image of the 2-D spatial distribution of the object's acoustic reflectivity function. The signal measurement space (or data space) defines a sector of the polar wave number spectrum that quantifies the variation with aspect angle of the spatial frequency components that contribute to the backscattered signal. The temporal bandwidth of the incident sonar pulse determines the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">radial</i> extent of the sector, while the range of aspect angles over which the object is insonified defines the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">angular</i> extent of the sector. Equivalently, the extent of the aperture that is formed by the sensor positions relative to the object determines the angular extent of the sector. This concept provides a generalized framework that unifies sonar imaging techniques such as reconstructive tomography (image reconstruction from projections), synthetic aperture sonar, and real aperture sidescan sonar. The difference in these techniques is simply due to the azimuthal angle subtended by the aperture of the sensing array, which is typically a fraction of a degree for a real aperture sonar, several degrees for a strip map synthetic aperture sonar, tens of degrees for a spotlight synthetic aperture sonar, and 360deg for a tomographic imaging sonar. Experimental results are presented for a real aperture sidescan sonar, a synthetic aperture sonar, and a tomographic sonar that demonstrate the development of an acoustic image from a single blurred point to a clearly identifiable object as the azimuthal extent of the angle subtended by the sonar aperture is increased.

Feature extraction in speckled imagery using dynamic B‐spline deformable contours under the model

This article proposes a technique for boundary detection in speckled imagery based on active contours and on the statistical properties of speckled data. Synthetic Aperture Radar (SAR), sonar, B‐ultrasound and laser imagery is corrupted by a signal‐dependent non‐additive noise called speckle. Many statistical models have been proposed to describe this noise, aiming at the development of specialized techniques for image improvement and analysis. The distribution is a statistical model that succeeds in describing a wide range of areas as, for instance in SAR data, pastures (smooth), forests (rough) and urban (extremely rough) areas. In order to determine boundaries between different areas, dynamic B‐splines are proposed in this paper. The algorithm automatically finds seeds and regions boundaries by a decision rule based on the estimation of the parameters of the law. The proposed algorithm is tested on synthetic and real SAR images, and both the accuracy and the performance of the proposal are assessed.

Model-based approach to the detection and classification of mines in sidescan sonar.

This paper presents a model-based approach to mine detection and classification by use of sidescan sonar. Advances in autonomous underwater vehicle technology have increased the interest in automatic target recognition systems in an effort to automate a process that is currently carried out by a human operator. Current automated systems generally require training and thus produce poor results when the test data set is different from the training set. This has led to research into unsupervised systems, which are able to cope with the large variability in conditions and terrains seen in sidescan imagery. The system presented in this paper first detects possible minelike objects using a Markov random field model, which operates well on noisy images, such as sidescan, and allows a priori information to be included through the use of priors. The highlight and shadow regions of the object are then extracted with a cooperating statistical snake, which assumes these regions are statistically separate from the background. Finally, a classification decision is made using Dempster-Shafer theory, where the extracted features are compared with synthetic realizations generated with a sidescan sonar simulator model. Results for the entire process are shown on real sidescan sonar data. Similarities between the sidescan sonar and synthetic aperture radar (SAR) imaging processes ensure that the approach outlined here could be made applied to SAR image analysis.

A maximum-likelihood surface estimator for dense range data

Describes how to estimate 3D surface models from dense sets of noisy range data taken from different points of view, i.e., multiple range maps. The proposed method uses a sensor model to develop an expression for the likelihood of a 3D surface, conditional on a set of noisy range measurements. Optimizing this likelihood with respect to the model parameters provides an unbiased and efficient estimator. The proposed numerical algorithms make this estimation computationally practical for a wide variety of circumstances. The results from this method compare favorably with state-of-the-art approaches that rely on the closest-point or perpendicular distance metric, a convenient heuristic that produces biased solutions and fails completely when surfaces are not sufficiently smooth, as in the case of complex scenes or noisy range measurements. Empirical results on both simulated and real ladar data demonstrate the effectiveness of the proposed method for several different types of problems. Furthermore, the proposed method offers a general framework that can accommodate extensions to include surface priors, more sophisticated noise models, and other sensing modalities, such as sonar or synthetic aperture radar.