共查询到20条相似文献，搜索用时 453 毫秒
1.
The spectral structure of oceanic incoherent Doppler sonar data is analyzed with Fourier and two finite-parameter modeling methods (Spectral Moment Estimation (SME) and AutoRegressive (AR)). Fourier spectral analysis, applied to long-range segments of echo return data, validates a theoretical point scattering model. It is demonstrated that Fourier spectral techniques, applied to data from incoherent sonar systems, cannot be used to probe oceanic spectral and velocity fields on the scales that frequently are important in oceanographic applications. The finite-parameter methods model the spectral structure on closely spaced range intervals. The SME method calculates the first moment of the Doppler spectrum at each range interval and the AR method provides spectral estimates at these same intervals. Trade-offs in estimating scatterer radial velocity using the various algorithms are considered. It is shown that a frequency-integrated AR technique has velocity estimation performance comparable to the SME method. In addition, the AR technique reveals that several regions of data possess asymmetric spectral structure. The implications of this spectral structure for oceanic velocity estimates are discussed. 相似文献
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Jianjun Zhang Eric W. Gill 《Oceanic Engineering, IEEE Journal of》2006,31(4):779-796
An algorithm is developed for the inversion of bistatic high-frequency (HF) radar sea echo to give the nondirectional wave spectrum. The bistatic HF radar second-order cross section of patch scattering, consisting of a combination of four Fredholm-type integral equations, contains a nonlinear product of ocean wave directional spectrum factors. The energy inside the first-order cross section is used to normalize this integrand. The unknown ocean wave spectrum is represented by a truncated Fourier series. The integral equation is then converted to a matrix equation and a singular value decomposition (SVD) method is invoked to pseudoinvert the kernel matrix. The new algorithm is verified with simulated radar Doppler spectrum for varying water depths, wind velocities, and radar operating frequencies. To make the simulation more realistic, zero-mean Gaussian noise from external sources is also taken into account 相似文献
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Local and far-field surface elevation around a vertical cylinder in unidirectional steep wave groups
The problem of diffraction of a unidirectional incident wave group by a bottom-seated cylinder is considered. We assume the amplitude of the incoming wave to be small in comparison with other linear scales of the problem, and develop the corresponding second-order perturbation theory. We use the Fourier transform to treat time variation and separate spatial variables when solving the non-homogeneous second-order problem. The resulting set of non-homogeneous Bessel equations is solved numerically.Solutions for various types of incoming wave spectrum are obtained including the Gaussian spectrum and the Pierson–Moskowitz spectrum. To validate the method, problems with gradually decreasing bandwidth of Gaussian spectrum are solved and it is shown that the corresponding solution approaches that for the monochromatic case. The Pierson–Moskowitz spectrum with a set of realistic physical parameters is used as an example of extreme wave interaction with an offshore structure. The corresponding first- and second-order solutions are obtained and the effect of non-linearity on the solution is discussed with the emphasis on the growth of maximum free-surface elevation on the cylinder’s surface and generation of high frequency free radiated waves. 相似文献
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Stewart W.I. Chu D. Malik S. Lerner S. Singh H. 《Oceanic Engineering, IEEE Journal of》1994,19(4):599-610
Bathymetry and backscatter measurements from a 120-kHz phase-difference sonar are analysed in terms of statistical and spectral characteristics. Data from a multisensor, multiscale survey of the Juan de Fuca Ridge are compared across three distinct geological provinces: sediment pond, ridge flank, and axial valley. The detrended bathymetry follows a Gaussian distribution; the power spectral density can be approximately described by a power law. The composite multiscale power spectrum demonstrates a similar slope spanning a spatial frequency range from about 0.005 to 50 cycles/m, corresponding to a range of geological features from a few hundred meters down to several centimeters. The backscattering strength and grazing-angle dependencies agree with previous empirical studies; data from a sediment-pond region are shown to match theoretical predictions of the composite-roughness model. Histograms of the echo amplitude are characterized by a multimodal Rayleigh probability density function. For all analyses, the data show distinct differences among the three provinces 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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Several detection statistics are compared in the frequency domain based on the asymptotic probability of detection (APD) criterion. They include second-order, fourth-order, normalized fourth-order, and kurtosis estimates. The results show that for randomly occurring signals which can be characterized as non-Gaussian, the fourth-order, normalized fourth-order, and kurtosis estimates can have higher asymptotic probability of detection levels compared with second-order estimates. But only for the normalized fourth-order and kurtosis estimates do the results seem significant. Moreover, if a second-order estimate of the noise is available to normalize a fourth-order estimate of signal and noise, the resultant normalized fourth-order estimate has higher asymptotic probability of detection levels even for Gaussian signals. This result holds only when there is a significant positive covariance between the numerator and the normalizing noise sample in the denominator. On the other hand, if an independent noise sample is used to normalize a second-order or fourth-order estimate, the overall performance based on the asymptotic probability of detection will be degraded compared with the unnormalized second-order or fourth-order estimates, respectively. 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 precise ocean bottom map for ocean surveying and dredging is desired. Especially in dredging, it is essential to know the seabed topography in real time without being affected by scatterers (for example floating sand and mud) in the seawater during work. To meet these requirements, the multi-narrow-beam sonar system (MBSS) has been developed. The MBSS forms beams with the use of the complex fast Fourier transform (CFFT) algorithm. In the MBSS, arithmetic mean processing is employed to eliminate echo from scatterers and the measurement error due to the oblique incident angle is reduced by peak value detection processing. By using these processes, an ocean bottom map can be accurately obtained. It is both theoretically and experimentally shown that the distribution of echo intensity from scatterers is approximated by the Rayleigh probability density function. The arithmetic mean of four to eight successively received echoes from scatterers reduces the variance of the echo intensity distribution by 6 to 12 dB. 相似文献
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We report on the testing of a single-beam 1.7-MHz coherent Doppler sonar. The system is PC-controlled, using a digital signal processor (DSP) to acquire and extract the velocity and backscatter amplitude data. Results from a series of tow-tank calibration tests demonstrate an accuracy in the order of 5 mm s-1 for data rates of 10 profiles/second over a 1-2 m range with 1.5-cm range bins. An expression for system accuracy is developed which allows generalization to other pulse-to-pulse coherent Doppler systems. We present data showing the systematic decorrelation of backscatter signals due to particle advection: increased decorrelation is seen in the transducer near-field. Example observations of velocity profiles in laboratory-generated waves are presented 相似文献
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This paper presents an evaluation of second, third, and fourth-order moments for the passive detection of transient signals in both simulated Gaussian noise and measured noise. The measured noise was recorded by a vertical array located near the San Diego, CA, harbor and is dominated at low frequencies by ship-generated noise. The detectors assume neither noise nor signal stationarity and can use single or multiple channels of data. Simulation results indicate that the fourth-order moment detector often performs better than the energy detector in the correlated measured noise, with increasing channel contributions to the moment function, resulting in increased gain. The results in simulated Gaussian noise likewise favor the fourth-order moment detector, at least for the signals with significant fourth-order moments, but the ability of the higher order detector to discriminate against correlated noise is evident. Analysis over a 30-min segment of the measured noise with selected signals demonstrates that fourth-order detection gains can be reliably expected as the noise statistics change. 相似文献
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《Oceanic Engineering, IEEE Journal of》2009,34(4):444-450
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《Applied Ocean Research》2004,26(3-4):154-161
A frequency-domain cumulant spectral analysis method is developed in this study to estimate the higher-order statistics of the linear oscillator responses driven by Morison wave force. The fourth-order cumulant function of the nonlinear drag force is formulated in terms of the autocorrelation functions of water particle velocity. Price's theorem is applied to evaluate the associated higher-order joint moments. Three-dimensional Fourier Transforms are employed to obtain the trispectra of Morison force and oscillator responses. The estimated force and response kurtosis are in good agreement with those obtained from time-domain simulations; while the proposed method is found to be much more efficient. The numerical results also show that the drag force cubicization based on the least square approximation results in an overestimation of the kurtosis values; in addition, it is necessary to include the joint moments of order higher than eighth. 相似文献
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This paper focuses on estimating the two-dimensional (2-D) target-speed vector (course and speed) using a multistatic sonar that consists of one monostatic sonar and one bistatic receiver. The speed and course estimates are obtained after a single transmission. The theory on bistatic Doppler and 2-D target-speed vector estimation is first considered and then applied to simulated and real data. The results can be used to improve classification algorithms or to feed speed information to tracking algorithms, for example. 相似文献
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In order to enhance the efficiency of the interpretation of surface images obtained with a side scanning sonar, it is proposed to supplement the standard processing software with a program for obtaining acoustic stereo images. Examples of such images synthesized with this program using the data of a bottom sonar survey with a side scanning sonar and an echo sounder are presented. The cases are considered when the information on the bottom relief contained in sonar images obtained with a standard side scanning sonar or its modifications can be used instead of the data of an echo sounding survey. 相似文献
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小波函数对侧扫声纳图像滤波效果的影响分析 总被引：1，自引：0，他引：1
侧扫声纳技术应用日益广泛，已成为海洋测量的重要工具，而去除噪声处理是对侧扫声纳图像进行正确判读的前提。利用小波函数滤波处理的方法，分别采用Haar、Daubechies、Coiflets、Symlets、Discrete Meyer、Biorthogonal、Reverse Biorthogonal等小波函数与中值滤波函数对侧扫声纳图像进行处理，并以平滑指数和边缘保持指数为评价指标，对滤波效果进行定量比较。试验表明，小波函数可以有效地平滑声纳图像，并能保持其较好的边缘效果。 相似文献
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An active sonar is described that adaptively changes its location and configuration in response to the echoes it observes in order to locate an object, position it at a known location, and identify it using features extracted from the echoes. The sonar consists of a center transmitter flanked by two receivers that can rotate and is positioned at the end of a robot arm that has five degree-of-freedom mobility. The sonar operates in air using Polaroid transducers that are resonant at 60 kHz with a nominal wavelength equal to 6 mm. The emitted pulse has a short duration with a useful bandwidth extending from 20 to 130 kHz. Using binaural information, the transmitter rotates to position an echo-producing object on its axis to maximize the acoustic intensity incident on the nearest echo-producing feature. The receivers rotate to maximize the echo amplitude and bandwidth. These optimizations are useful for differentiating objects. The system recognizes a collection of ball bearings, machine washers, and rubber O-rings of different sizes ranging from 0.45 to 2.54 cm, some differing by less than 1 mm in diameter. Learning is accomplished by extracting vectors of 32 echo envelope values acquired during a scan in elevation and forming a data base. Recognition is accomplished by comparing a single observed echo vector with the data base to find the least squared error match. A bent-wire paper clip illustrates the recognition of an asymmetric pose-dependent object 相似文献