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1.
The simulation of active sonar reverberation time series has traditionally been done using either a computationally intensive point-scatterer model or a Rayleigh-distributed reverberation-envelope model with a time-varying power level. Although adequate in scenarios where reverberation arises from a multitude of scatterers, the Rayleigh model is not representative of the target-like non-Rayleigh reverberation or clutter commonly observed with modern high-resolution sonar systems operating in shallow-water environments. In this paper, techniques for simulating non-Rayleigh reverberation are developed within the context of the finite-number-of-scatterers representation of K-distributed reverberation, which allows control of the reverberation-envelope statistics as a function of system (beamwidth and bandwidth) and environmental (scatterer density and size) parameters. To avoid the high computational effort of the point-scatterer model, reverberation is simulated at the output of the matched filter and is generated using efficient approximate methods for forming K-distributed random variables. Finite impulse response filters are used to introduce the effects of multipath propagation and the shape of the reverberation power spectrum, the latter of which requires the development of a prewarping of the K distribution parameters to control the reverberation-envelope statistics. The simulation methods presented in this paper will be useful in the testing and evaluation of active sonar signal processing algorithms, as well as for simulation-based research on the effects of the sonar system and environment on the reverberation-envelope probability density function. 相似文献
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Active sonar detection in shallow water using the Page test 总被引：1，自引：0，他引：1
The use of active sonar in shallow water results in received echoes that may be considerably spread in time compared to the resolution of the transmitted waveform. The duration and structure of the spreading and the time of occurrence of the received echo are unknown without accurate knowledge of the environment and a priori information on the location and reflection properties of the target. A sequential detector based on the Page test is proposed for the detection of time-spread active sonar echoes. The detector also provides estimates of the starting and stopping times of the received echo. This signal segmentation is crucial to allow further processing such as more accurate range and bearing localization, depth localization, or classification. The detector is designed to exploit the time spreading of the received echo and is tuned as a function of range to the expected signal-to-noise ratio (SNR) as determined by the transmitted signal power, transmission loss, approximate target strength, and the estimated noise background level. The theoretical false alarm and detection performance of the proposed detector, the standard Page test, and the conventional thresholded matched filter detector are compared as a function of range, echo duration, SNR, and the mismatch between the actual and assumed SNR. The proposed detector and the standard Page test are seen to perform better than the conventional thresholded matched filter detector as soon as the received echo is minimally spread in time. The use of the proposed detector and the standard Page test in active sonar is illustrated with reverberation data containing target-like echoes from geological features, where it was seen that the proposed detector was able to suppress reverberation generated false alarms that were detected by the standard Page test 相似文献
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A new coherent reverberation model developed at the Naval Research Laboratory, Washington, DC, and the Supreme Allied Commander Atlantic Undersea Research Centre, La Spezia, Italy, is exercised in the 17-750-Hz band to estimate the degree of non-Rayleighness of shallow-water reverberation envelopes as a function of waveguide multipath, system bandwidth, directivity, and frequency. Findings suggest that reverberation from diffuse, but non-Gaussian, scatterer distributions is significantly more Rayleigh for multipath environments than for equivalent environments excited by a single or small number of modes or for broadside receiver array processing that extracts narrow angles of reception. These findings suggest that the problem of non-Rayleigh reverberation in shallow-water waveguides can be ameliorated through the use of tuned ensonification and reception schemes, which retain high probabilities of detection while reducing the associated probability of false alarm. 相似文献
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Conventional detection in active sonar involves comparing the normalized matched filter output power to a fixed preset threshold. Threshold crossings from contacts of interest are labeled as detections and those from undesired clutter echoes as false alarms. To maintain a constant false-alarm rate (CFAR) in the presence of strong transient clutter, the system can either increase the threshold or apply some function that suppresses this background down to an acceptable level. The latter approach leads to a more consistent background on the display, which enables operator-assisted detection. Background clutter suppression should not come at the expense of contact detection; to maximize the probability of detection (PD) for a given probability of false alarm (PFA), the likelihood ratio test (LRT) is used. However, the LRT does not address display issues, since the threshold that achieves a desired PFA varies with the input distribution. Ideally, the LRT output is monotonically transformed using a "statistical normalizer" (SN) that returns a consistent CFAR background without degrading the optimized PD. Within the radar community, clutter suppression is proposed using a LRT tuned to a K-distributed spherically invariant random vector (SIRV) model. However, this model does not lend itself to SN, as a closed-form expression for the LRT output density does not exist. In contrast, the proposed SIRV clutter model, with Pareto distributed power, leads to a closed-form density from which the SN function is readily derived. This combined Pareto-LRT/SN detector nearly matches the optimized PD performance of the K-distributed LRT and maintains a consistent CFAR background for display purposes. 相似文献
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Active sonar systems operating in shallow-water environments are often faced with high numbers of false alarms, generically referred to as clutter, arising from among other sources bottom scattering that results in heavy tails in the matched filter envelope probability density function compared with the Rayleigh distribution. In this paper, the effect of multipath propagation on the envelope statistics (i.e., the disparity from the Rayleigh distribution) is modeled through the use of the -distribution where the shape and scale parameters are formed from the autocorrelation function of the transmit waveform, the multipath structure, and the strength and spatial density of the bottom scatterers. Use of the -distribution is justified by showing that it is the limiting distribution of the sum of independent but not identically distributed -distributed random variables, which is representative of multipath when the bottom produces -distributed backscatter. The shape parameter, which drives the clutter statistics, is seen to be inversely proportional to bandwidth at bandwidths low enough that the multipath is not resolved and again at bandwidths high enough that all of the paths are resolved. As has been previously reported by LePage [IEEE J. Ocean. Eng., vol. 29, no. 2, pp. 330-346, 2004], multipath is shown to make clutter statistics more Rayleigh-like, which in this analysis equates to an increase in the -distribution shape parameter. The model is used to evaluate the effect on clutter statistics of varying environmental characterizations and system configurations where it is seen that, for a constant sound-speed profile, increasing the vertical aperture of the sonar, the center frequency, or surface roughness can lead to less multipath and, therefore, a reduction in the -distribution shape parameter and an increase in the probability of false alarm. 相似文献
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The detection of a target echo in a sonar image is usually a difficult task since the reverberation, consisting of a large number of spurious echoes, generates a lot of false alarms. In this paper, we propose two new detectors derived from image processing algorithms. These detectors are respectively based on a morphological and a statistical contrast. Each detector only requires setting a few parameters. This setting is done using some prior knowledge about the data (shape of the emitted signal and the used antenna, characteristics of the reverberation). Nevertheless, an extensive statistical study of the detection performances proves that the proposed methods are robust and that even an imprecise setting of the parameters leads to satisfactory results. Applied to the real data, these detectors and their sequential combination lead to a significant improvement on the performances: The false alarm rate is drastically reduced while the detection probability is preserved. Based on different contrasts, these detectors have complementary behaviors. Therefore, a further improvement is achieved by a fusion of the different results to classify the remaining echoes as whether spurious or true detection. 相似文献
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The synchronization problem over fading dispersive channels is discussed. Although stated in general terms, the results are equally applicable to sonar and undersea problems in which the propagation medium has significant multipath spread and fluctuation rate. Optimum detection over such channels requires a perfect knowledge of the channels' scattering functions, which completely characterize the behavior of fading dispersive channels. Lack of such knowledge may result in performance degradation. A closed-form expression for the probability of false alarm and probability of detection is given, and a set of curves is provided to demonstrate the amount of degradation due to mismatch conditions. A parallel structure to achieve acquisition is suggested, and its performance is investigated. A performance measure for the synchronizer in this mode is defined. Upper and lower bounds on the performance measure are derived. The effect of various parameters, such as the signal-to-noise ratio, pulse width and modulation, number of pulses, spread of the channel, and the size of the resolution cell on the performance of the synchronizer is investigated. After achieving acquisition, the system needs to switch to the tracking mode. An optimum closed-loop symbol synchronizer is developed 相似文献
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Detection in the presence of reverberation is often difficult in active sonar, due to the reflection/diffusion/diffraction of the transmitted signal by the ocean surface, ground, and volume. A modelization of reverberation is often used to improve detection because classical algorithms are inefficient. A commonly used reverberation model is colored and nonstationary noise. This model leads to elaborate detection algorithms which normalize and whiten reverberation. In this paper, we focus on a more deterministic model which considers reverberation as a sum of echoes issued from the transmitted signal. The Principal Component Inverse (PCI) algorithm is used with this model to estimate and delete the reverberation echoes. A rank analysis of the observation matrix shows that PCI is efficient in this configuration under some conditions, such as when the transmitted signal is Frequency Modulated. Both methods are validated with real sonar surface reverberation noise. We show that whitening has poor performance when reverberation and target echo have the same properties, while PCI maintains the same performance whatever the reverberation characteristics. Further, we extend the algorithms to spatio-temporal data. We propose a new algorithm for PCI which allows better echo separation. This new method is shown to be more efficient on real spatio-temporal data 相似文献
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Fenghua Li Jianjun Liu Renhe Zhang 《Oceanic Engineering, IEEE Journal of》2004,29(4):1060-1066
A bottom-scattering model based on sediment small scatterers, single scattering approximation is presented, which is combined with a normal-mode-based reverberation model. The combined model (for total reverberation) is compared with measurements of shallow-water reverberation from the 2001 Asian Sea International Acoustic Experiment (ASIAEX) in the East China Sea. Reverberation intensities as a function of time and frequency are compared with theoretical predictions with reasonable agreement. The effects of the rough sea surface on the reverberation are also discussed. 相似文献
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《Oceanic Engineering, IEEE Journal of》2009,34(4):575-585
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《Oceanic Engineering, IEEE Journal of》2009,34(4):451-458
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A method is described by which reverberation data gathered on a cardioid array can be processed to produce a 'scatterer map' of an area. Such a map shows the locations of strong seabed scatterers and gives a qualitative estimate of their strength. The method cannot give absolute values for scattering strength but its output is nonetheless useful in describing the locations of strong scatterers in an area. Low-Frequency Active Sonar (LFAS) data gathered on the Malta Plateau are presented and the physical causes of observed strong scatterers are described, along with an assessment of the degree of certainty to which these causes are known. 相似文献
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基于风险的海洋结构物无损检测功能的分级 总被引：1，自引：1，他引：0
提出了基于风险的海洋结构物无损检测功能分级方法,采用检测概率、错误识别概率和裂纹出现概率度量检测功能,并修正了错误识别概率的取值范围。根据风险值对无损检测功能进行了分级,研究了风险值随检测概率、错误识别概率和裂纹出现概率的变化规律,分析了风险值对检测费用、维修费用及失效费用的敏感性。在此基础上,将基于风险的海洋结构物无损检测功能分级方法应用于优化检测策略,用算例证明了文中所提方法的有效性。 相似文献
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A computationally efficient fast maximum-likelihood (FML) estimation scheme, which makes use of the shape of the surface of the compressed likelihood function (CLF), is proposed. The scheme uses only multiple one-dimensional searches oriented along appropriate ridges on the surface of the CLF. Simulations indicate that the performances of the proposed estimators match those of the corresponding maximum-likelihood estimators with very high probability. The approach is demonstrated by applying it to two different problems. The first problem involves the estimation of time of arrival and Doppler compression of a wideband hyperbolic frequency modulated (HFM) active sonar signal buried in reverberation. The second problem deals with estimating the frequencies of sinusoids. A threshold analysis of the proposed scheme is carried out to predict the signal-to-noise ratio (SNR) at which large estimation errors begin to occur, i.e., the threshold SNR, and its computational complexity is discussed 相似文献
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在高斯白噪声背景下,匹配滤波器作为线性调频信号的最优检测器,在水声信号处理中被广泛应用。 当发射信号为线性调频信号时,由水下目标径向速度引起的多普勒频移会造成回波和样本之间失配, 使匹配滤波器的检测性能下降,增加了目标速度估计的难度。 利用分数阶傅里叶变换对于线性调频信号的聚焦特性,提出了应用分数阶傅里叶变换的水下运动目标线性调频回波检测算法,完成对目标速度的估计, 推导目标运动速度与分数阶傅里叶变换阶数之间的关系,并对测量结果进行误差分析。 仿真测试表明,该算法可有效地估计混响背景下的目标径向速度,且具有良好的估计性能。 相似文献
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This paper examines the fluctuations in low-grazing-angle 100-kHz backscatter from a rocky limestone sea bed near Copenhagen, Denmark, at horizontal ranges up to 420 m. The sea-bed reverberation was characterized by strong short spatial-scale variations in scattering strength and statistical parameters. The measured areal backscatter strengths were in the range from -50 to -24 dB at grazing angles less than 3/spl deg/, showing a strong local variability and grazing-angle dependence definitely not in accordance with Lambert's law. The observed echo-amplitude distributions varied between log-normal and Rayleigh models, with more Rayleigh-like probability density functions having higher scintillation indices and skewness (approaching values of 1.0 and 0.63, respectively). The scintillation index and skewness parameters were found to increase mildly with both horizontal range and water-current magnitude. A simple model using the coherent superposition of multiple scatterers was proposed to explain the observed scattering statistics. This model is based on the assumption that the sea bed is effectively immobile, with water-borne scintillation and micro-multipaths providing fluctuations in scatterer phase. This simple model shows that echo-amplitude fluctuations can deviate from the Rayleigh model through two mechanisms: 1) decreasing the levels of water-borne phase fluctuation and 2) increasing the nonuniformity of the sea-bed scatterer amplitudes. 相似文献
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A model for the matched filter response to continuous reverberation from the transmission of broadband waveforms is developed. The application is for reverberation from a rough interface, based on perturbation theory. The model is developed for both the stationary rough bottom and the moving ocean surface interfaces. The mean reverberation is predicted as a function of the Doppler speed of the matched filter replica. Application is made to the design of waveforms with comb-like spectra. A uniform train of impulses produces a comb spectrum that is shown to significantly reject reverberation for a certain range of Doppler speeds. A similar low-reverberation response is produced from a continuous source emitting a wavetrain composed of adjacent hyperbolic-frequency-modulated (HFM) pulses. A waveform design technique is demonstrated to ensure continuity of the entire HFM wavetrain. Finally, waveforms with geometrically spaced comb spectra are considered. A new geometric comb waveform with constant amplitude is specified. However, this waveform requires a large bandwidth which may be difficult to obtain with practical high-power sources. Hard and soft-clipped versions of the comb spectra waveform are considered which provide useful compromises between the amount of reverberation suppression, the transmitted energy efficiency, and the utilization of available bandwidth. 相似文献