Active Target Detection Research Articles

Underwater target detection based on fourth-order cumulants beamforming

Obtaining the target echo data of high signal to reverberation ratio (SRR) or signal to noise ratio (SNR) in the complex ocean environment is a key problem to improve the detection capability of underwater targets. In this paper, combined with the expansion characteristics of fourth-order cumulants to the array aperture and the structure of the smallest redundant array, a robust and high-resolution fourth-order cumulants beamforming method based on the smallest redundant array is presented. The target direction-of-arrival estimation (DOA) is achieved by forming a narrow space beam, while the reverberation interference is reduced. For the broadband detection system, combined with fractional Fourier transform (FRFT), the fourth-order cumulants beamforming method is extended to the DOA estimation of LFM signal. According to the signal characteristics difference between the target echo and the reverberation, the reverberation is filtered out in the FRFT domain, which achieves the secondary suppression of reverberation interference in the spatial domain and the transform domain, and further improves the SRR of the target echo signal. The validity of the research method is verified by the processing results of the active sonar target detection experiment.

Automatic active acoustic target detection in turbulent aquatic environments

AbstractThere is no established approach for dealing with the active acoustic detection of biological targets in highly dynamic aquatic environments where intense physical interference means that standard techniques are unsuitable. This is a particular problem in ecologically important environments with emerging industrial significance such as marine energy extraction sites. We developed an automatic processing method which allows effective target detection with high sensitivity throughout variable acoustic conditions. The method is based on scale‐dependent adaptive filtering of data and morphological analysis of short‐scale backscatter contributions for the exclusion of intense turbulent features and isolation of biological targets. Echosounder platform deployments around marine energy infrastructure in a tidal channel provide test data which demonstrate the effectiveness of the proposed approach. Target validation and assessment is carried out by the analysis of multifrequency characteristics and direct inspection. The results deliver effective, quantitative, and repeatable assessment of ecological interactions and target distributions with clear implications for environmental assessment in high energy sites and promising applications in other contexts.

Fractional 푸리에 변환을 이용한 능동소나 표적탐지

Many studies in detection and classification of the targets in the underwater environments have been conducted for military purposes, as well as for non-military purpose. Due to the complicated characteristics of underwater acoustic signal reflecting multipath environments and spatio-temporal varying characteristics, active sonar target detection technique has been considered as a difficult technique. In this paper, we describe the basic concept of Fractional Fourier transform and optimal transform order. Then we analyze the relationship between time-frequency characteristics of an LFM signal and its spectrum using Fractional Fourier transform. Based on the analysis results, we present active sonar target detection method. To verify the performance of proposed methods, we compared the results with conventional FFT-based matched filter. The experimental results demonstrate the superiority of the proposed method compared to the conventional method in the aspect of AUC(Area Under the ROC Curve).

Orienting of visuo-spatial attention in complex 3D space: Search and detection.

The ability to detect changes in the environment is necessary for appropriate interactions with the external world. Changes in the background go more unnoticed than foreground changes, possibly because attention prioritizes processing of foreground/near stimuli. Here, we investigated the detectability of foreground and background changes within natural scenes and the influence of stereoscopic depth cues on this. Using a flicker paradigm, we alternated a pair of images that were exactly same or differed for one single element (i.e., a color change of one object in the scene). The participants were asked to find the change that occurred either in a foreground or background object, while viewing the stimuli either with binocular and monocular cues (bmC) or monocular cues only (mC). The behavioral results showed faster and more accurate detections for foreground changes and overall better performance in bmC than mC conditions. The imaging results highlighted the involvement of fronto‐parietal attention controlling networks during active search and target detection. These attention networks did not show any differential effect as function of the presence/absence of the binocular cues, or the detection of foreground/background changes. By contrast, the lateral occipital cortex showed greater activation for detections in foreground compared to background, while area V3A showed a main effect of bmC vs. mC, specifically during search. These findings indicate that visual search with binocular cues does not impose any specific requirement on attention‐controlling fronto‐parietal networks, while the enhanced detection of front/near objects in the bmC condition reflects bottom‐up sensory processes in visual cortex. Hum Brain Mapp 36:2231–2247, 2015. © 2015 The Authors Human Brain Mapping Published by Wiley Periodicals, Inc.

Combination of time‐reversal focusing and nulling for detection of small targets in strong reverberation environments

Time reversal has attracted considerable research interest because of its potential for signal enhancement. In the past, target focusing and reverberation nulling are separately investigated as two complementary aspects of time-reversal processing. In essence, nulling is regarded as one kind of focusing with minimising the energy incident on a location. When time-reversal operation is regarded as inversed beamforming, it is available to simultaneously realise time-reversal focusing and nulling under an optimum beamforming framework. The feasibility of the combination processing is evaluated in detection of a small target in the presence of strong reverberation. The acoustic energy focused on a target of interest is maintained constant while reverberation is regarded as an interference to be suppressed by the nulling technique. The experimental results have verified the effectiveness of optimum time-reversal processing for active target detection in a shallow water environment.

Tests of Micro-Pattern Gaseous Detectors for active target time projection chambers in nuclear physics

Active target detection systems, where the gas used as the detection medium is also a target for nuclear reactions, have been used for a wide variety of nuclear physics applications since the eighties. Improvements in Micro-Pattern Gaseous Detectors (MPGDs) and in micro-electronics achieved in the last decade permit the development of a new generation of active targets with higher granularity pad planes that allow spatial and time information to be determined with unprecedented accuracy. A novel active target and time projection chamber (ACTAR TPC), that will be used to study reactions and decays of exotic nuclei at facilities such as SPIRAL2, is presently under development and will be based on MPGD technology. Several MPGDs (Micromegas and Thick GEM) coupled to a 2×2mm2 pixelated pad plane have been tested and their performances have been determined with different gases over a wide range of pressures. Of particular interest for nuclear physics experiments are the angular and energy resolutions. The angular resolution has been determined to be better than 1° FWHM for short traces of about 4cm in length and the energy resolution deduced from the particle range was found to be better than 5% for 5.5MeV α particles. These performances have been compared to Geant4 simulations. These experimental results validate the use of these detectors for several applications in nuclear physics.

Multiple Target Localization Using Wideband Echo Chirp Signals

Active target detection and localization is a classical signal processing problem that arises in various military and biomedical applications. A novel method for the detection and estimation of the range, velocity and direction of arrival (DOA) of multiple far-field targets using wideband chirp signals is proposed in this paper. Using the plane wave representation of the signals reflected from far-field targets, a modal preprocessing procedure is designed for the echo signals received at a concentric circular array. Following that, the parameter estimation method for the multiple targets present is developed based on (i) the fractional Fourier transform which projects a signal onto an orthonormal basis formed by chirps and (ii) the Root-MUSIC algorithm which is a high resolution DOA estimation method originally proposed for narrowband signals received at a uniform linear array. There is no limit on the number of targets detectable by the proposed method as long as the echo signals are separable in the chosen fractional Fourier domain. Simulation results show that the proposed method demonstrates good performance, with low root-mean-square errors in the parameters estimated under difficult conditions such as closely spaced targets and low signal-to-noise ratio.

Robust time reversal processing for active detection of a small bottom target in a shallow water waveguide

With the spatial-temporal focusing of acoustic energy, time reversal processing (TRP) shows the potential application for active target detection in shallow water. To turn the ‘potential’ into a reality, the TRP based on a model source (MS) instead of a physical probe source (PS) is investigated. For uncertain ocean environments, the robustness of TRP is discussed for the narrowband and broadband signal respectively. The channel transfer function matrix is first constructed in the acoustic perturbation space. Then a steering vector for time reversal transmission is obtained by singular value decomposition (SVD) of the matrix. For verification of the robust TRP, the tank experiments of time reversal transmission focusing and its application for active target detection are undertaken. The experimental results have shown that the robust TRP can effectively detect and locate a small bottom target.

Reductions in the N1 and P2 Auditory Event-Related Potentials in First-Hospitalized and Chronic Schizophrenia

The N1 auditory event-related potential (ERP) is reduced in chronic schizophrenia, as is the P2 to attended tones. N1 reduction may be endophenotypic for schizophrenia, being reduced in twins of schizophrenic patients and showing heritability. Results in family members, however, are equivocal, with abnormally small N1 (consistent with an endophenotype) and abnormally large N1 (inconsistent with an endophenotype) reported. P2 has been little studied in schizophrenia or family members. One crucial step in establishing endophenotypes is to rule out causal chronicity factors. We examined schizophrenia patients within 1 year of first hospitalization (most within 2 wk), chronically ill patients, and matched controls to examine N1 and P2 reductions and disease stage. Two active target detection oddball tasks were used, one with 97-dB tones against 70-dB white masking noise, the second with 97-dB tones without noise. Results from 8 samples are reported: first-hospitalized patients and matched controls and chronic patients and matched controls for the 2 tasks. N1 and P2 were measured from the standard stimuli. N1 and P2 were significantly reduced in chronic patients, as expected, and reduced in first-hospitalized patients. Because N1 and P2 are reduced even at the first hospitalization for schizophrenia, they may serve as viable electrophysiological endophenotypes for the disorder. However, deficit early in the disease is necessary but not sufficient to establish these ERPs as endophenotypes. Deficits must next be demonstrated in at least a subset of unaffected family members, a crucial criterion for an endophenotype.

Perception of emotional speech in Parkinson's Disease

Nonmotor symptoms in Parkinson's disease (PD) involving cognition and emotionality have progressively received attention. The objective of the present study was to investigate recognition of emotional prosody in patients with PD (n = 14) in comparison to healthy control subjects (HC, n = 14). Event-related brain potentials (ERP) were recorded in a modified oddball paradigm under passive listening and active target detection instructions. Results showed a poorer performance of PD patients in classifying emotional prosody. ERP generated by emotional deviants (happy/sad) during passive listening revealed diminished amplitudes of the mismatch-related negativity for sad deviants, indicating an impairment of early preattentive processing of emotional prosody in PD.

Environmentally adaptive reverberation nulling using a time reversal mirror

Backscattering from the rough water-bottom interface can serve as a surrogate probe source in time reversal. A time-gated portion of the reverberation then is refocused to the bottom interface at the corresponding range [Lingevitch et al., J. Acoust. Soc. Am. 111, 2609–2614 (2002)]. In this paper, reverberation nulling is investigated to enhance active target detection without a priori knowledge of the environment. The basic idea is to minimize the acoustic energy incident on the corresponding scattering interface by applying an excitation weight vector on the time reversal mirror which is in the complementary subspace orthogonal to the focusing vector. Numerical simulations illustrate the potential of reverberation nulling using a time reversal mirror.

Experimental demonstration of adaptive reverberation nulling using a time reversal mirror

Boundary reverberation (e.g., due to bottom roughness) can serve as a surrogate probe source in time reversal. A time-gated portion of the reverberation is then refocused to the bottom interface at the corresponding range [Lingevitch et al., J. Acoust. Soc. Am. 111, 2609–2614 (2002)]. In this paper, reverberation nulling is investigated to enhance active target detection. The basic idea is to minimize the acoustic energy incident on the corresponding interface by applying a weight vector on the time reversal mirror which is in the orthogonal subspace to the focusing vector. The feasibility of the reverberation nulling is demonstrated in a recent time reversal experiment at 850 and 3500 Hz.

High-frequency volume and boundary acoustic backscatter fluctuations in shallow water.

Volume and boundary acoustic backscatter envelope fluctuations are characterized from data collected by the Toroidal Volume Search Sonar (TVSS), a 68 kHz cylindrical array capable of 360 degrees multibeam imaging in the vertical plane perpendicular to its axis. The data are processed to form acoustic backscatter images of the seafloor, sea surface, and horizontal and vertical planes in the volume, which are used to attribute nonhomogeneous spatial distributions of zooplankton, fish, bubbles and bubble clouds, and multiple boundary interactions to the observed backscatter amplitude statistics. Three component Rayleigh mixture probability distribution functions (PDFs) provided the best fit to the empirical distribution functions of seafloor acoustic backscatter. Sea surface and near-surface volume acoustic backscatter PDFs are better described by Rayleigh mixture or log-normal distributions, with the high density portion of the distributions arising from boundary reverberation, and the tails arising from nonhomogeneously distributed scatterers such as bubbles, fish, and zooplankton. PDF fits to the volume and near-surface acoustic backscatter data are poor compared to PDF fits to the boundary backscatter, suggesting that these data may be better described by mixture distributions with component densities from different parametric families. For active sonar target detection, the results demonstrate that threshold detectors which assume Rayleigh distributed envelope fluctuations will experience significantly higher false alarm rates in shallow water environments which are influenced by near-surface microbubbles, aggregations of zooplankton and fish, and boundary reverberation.

Active target detection in the ocean: Optimizing performance and cost

The goal of this work is to design a processor for underwater target detection with superior performance to the commonly used matched filter without the efficiency penalties of the optimal filter. The theoretically optimal filter for active detection in the ocean is a correlator between the received signal and the convolution of the transmitted signal and the impulse response of the ocean. This filter is seldom employed because of the considerable computational overhead. The practically preferred method is a standard matched filter that correlates the transmitted signal to the received signal. It is computationally efficient but analytically suboptimal because it ignores the effects of the propagation medium on the transmitted signal. The standard filter is also preferable because of its attractive property of portability across environments, this feature stemming from deliberate disregard of the physics of the propagation medium. Here, the potential for improving detection performance is explored through modified matched filters that retain the portability property but do not resort to the exhaustive multiparameter grid searches of the optimal filter. [Work supported by ONR Ocean Acoustics, through Grant No. N000149710600.]

Reverberation rejection via modeforming with a vertical line array

The concept of modeforming with a vertical line array for active target detection in a reverberation-limited shallow-water environment is explored. The concept is based on the hypothesis that, in certain environments, the reverberation field is due to subbottom scattering and the reverberant energy is carried by the higher order modes; on the other hand, the targets in the water column are contained in the lower order modes. Under these conditions, isolation of a low-order mode, or a selected set of modes, with a vertical array could substantially increase the target-to-reverberation ratio. The optimum mode depends greatly on the environmental conditions and the depth of the target. Modeforming is achieved by deriving a spatial filter function that depends on the receiving array and the local environment.

Optimum target detection in an uncertain reverberation-limited environment

In a reverberation-limited environment, can imperfect knowledge of the spatial correlation properties of the underlying seafloor geomorphology impact active target detection performance? This work seeks to address this question by examining the dependence of the joint probability density function of the scattered acoustic field at low to moderate incidence angles on the seafloor roughness correlation function. The objective is the development of an optimum decision theoretic approach to the problem of high-frequency active detection of a target suspended in the water column in the vicinity of the seafloor. Acoustic modeling is a geologically meaningful seafloor scattering environment is performed by applying the 3-D Helmholtz–Kirchhoff formulation to the anisotropic, power-law description of seafloor relief due to Goff and Jordan. The generality of the Bayesian approach permits uncertainty with respect to geoacoustic environmental properties to be defined probabilistically and incorporated into the detection algorithm. The method attempts to exploit the spatial coherence of the scattered sound field in constructing the joint pdf of the scattered pressure field measured at an N element array of sensors. Upper bounds on detection performance in terms of ROC curves are obtained as a function of environmental uncertainty. [Work supported by ONR, Ocean Acoustics.]

The critical interval in dolphin echolocation: what is it?

A backward masking function relating target detection to masker delay was generated for a bottlenosed dolphin in an active echolocation target detection task. The masker was triggered by each outgoing echolocation click and could be temporally adjusted from coincidence with the target echo to delays of 700 microseconds. The animal reported target condition using a go/no-go response procedure. A modified method of constants was used to present the four masking delay intervals. Results indicated that 700- and 500-microseconds delays had little effect on target detection. However, as the delay was reduced to 100 microseconds, detection dropped to chance performance. The calculated 70% detection threshold corresponded to a delay of 265 microseconds. The results are discussed in support of the view that time separation pitch (TSP) may be an analytic mechanism used by the dolphin to discern various within-echo target attributes rather than an analytic mechanism for determining target range.