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1.
《Oceanic Engineering, IEEE Journal of》2005,30(2):267-274
A theoretical model for the vertical directionality and depth dependence of high frequency (8 to 50 kHz) ambient noise in the deep ocean is developed. The anisotropic noise field at a variety of depths and frequencies is evaluated and displayed. It was found that at high frequencies and deep depths, a bottom-mounted hydrophone receives the maximum noise energy from overhead rather than from the horizontal. This leads to the consideration of an oblate hydrophone receiving response pattern for underwater tracking ranges that would provide a constant signal-to-noise ratio (SNR) for an acoustic source located anywhere in a circular area centered above the hydrophone. Two of the desirable characteristics of this type of pattern are the increase in receiving range of a bottom-mounted sensor and the decrease of the dynamic range of signals that a signal processor must handle. 相似文献
2.
An operational passive sonar is required to detect signals from sources, which are subject to spatial and temporal coherence losses via modifications by the ocean environment. Furthermore, these signals are to be detected in the presence of frequency-dependent correlated noise fields. For a system which employs splitbeam cross-correlation processing, the spatial and spectral properties of the signal and noise are of significant import. Therefore, the exact probability density and cumulative distribution functions of the N-sampled correlator outputs of a splitbeam broadband passive sonar are derived for the case of Gaussian inputs which are described by arbitrary cross-spectral density matrices. The validity of approximating the exact probability density function (pdf) as a Gaussian distribution is investigated. The effect of signal coherence loss and noise correlation on the detection performance is considered and the associated processing loss is expressed as a degradation factor within the detection threshold equation 相似文献
3.
A rotated coordinates inversion algorithm is used on subsets of the Inversion Techniques 2001 Geoacoustic Workshop data, to which white Gaussian noise is added. The resulting data sets are equivalent to noisy broad-band signals received on a horizontal line array (HLA) during a single integration time interval. The inversions are performed using a technique called systematic decoupling using rotated coordinates (SDRC), which expands the original idea of rotated coordinates by using multiple sets of rotated coordinates, each corresponding to a different set of bounds, to systematically decouple the unknowns in a series of efficient simulated annealing inversions. The cost function minimized in the inversion is based on the coherent broad-band correlation between data and model cross spectra, which increases the coherence gain of the signal relative to incoherent noise. Using the coherent broad-band cost function with sparse HLA-like data sets, the SDRC inversion method yields good estimates for the sensitive environmental parameters for signal-to-noise ratios as low as -15 dB. 相似文献
4.
Spatial processing, including beamforming and diversity combining, is widely used in communications to mitigate intersymbol interference (ISI) and signal fading caused by multipath propagation. Beamforming suppresses ISI (and noise) by eliminating multipath (and noise) arrivals outside the signal beam. Beamforming requires the signals to be highly coherent between the receivers. Diversity combining combats ISI as well as signal fading by taking advantage of the independent information in the signal. Classical (spatial) diversity requires that signals are independently fading, hence are (spatially) uncorrelated with each other. In the real world, the received signals are neither totally coherent nor totally uncorrelated. The available diversity is complex and not well understood. In this paper, we study the spatial processing gain (SPG) as a function of the number of receivers used, receiver separation, and array aperture based on experimental data, using beamforming and multichannel combining algorithms. We find that the output symbol signal-to-noise ratio (SNR) for a multichannel equalizer is predominantly determined by the array aperture divided by the signal coherence length, with a negligible dependence on the number of receivers used. For a given number of receivers, an optimal output symbol SNR (OSNR) is achieved by spacing the receivers equal to or greater than the signal coherence length. We model the SPG in decibels as the sum of the noise suppression gain (NSG, equivalent to signal-to-noise enhancement) and the ISI suppression gain (ISG, equivalent to signal-to-ISI enhancement) both expressed in decibels; the latter exploits the spatial diversity and forms the basis for the diversity gain. Data are interpreted using the modeled result as a guide. We discuss a beam-domain processor for sonar arrays, which yields an improved performance at low-input SNR compared to the element-domain processor because of the SNR enhancement from beamforming many sensors. 相似文献
5.
The testing of a synthetic aperture technique, the ETAM algorithm, is extended and its performance for CW pseudorandom signals and broadband ship noise is examined. The results show the limitations of the technique and are of special interest for operational systems development. In the CW experiments, the transmitted signal was generated with high temporal coherence, and loss of the spatial and temporal coherence of the received signal was introduced only by the medium and the stability of the towed array. In the experiments that included the pseudorandom signal and the ship noise, the temporal coherence of the transmitted signals was deliberately chosen to be poor in order to study the effects and the performance of this algorithm with broadband signals. The related experimental results show that for received signals, which have their segments over the synthesizing period highly cross-correlated, a synthetic aperture array gain was achieved that corresponds to the length of an equivalent fully populated array 相似文献
6.
A special-purpose definition is proposed for phase fluctuations to overcome the obstacle of unpredictable dynamic changes in the phase angle. This definition implies a specific time history for each phase sample and any deviation is termed a phase fluctuation. Its application to acoustic data led to the development of a technique for temporally aligning the phase angles of the acoustic pressure phasors. This alignment process transforms the signal phasors to the real half-space of a rotated complex plane, while the corresponding noise is distributed with random phase angles. Signal processing conducted in the rotated plane improves the temporal coherence of the signals without significantly altering the incoherence of the noise. Coherent attenuation and cancellation of signals is common with temporal coherence and vector averaging. These were eliminated when the aligned-phase angles were substituted for the original unaligned phase angles. Thus, the transformation produces a net temporal coherence gain. Furthermore, it significantly improves the robustness of the signal processor to source and receiver motion. An automatic identifier of signals in the transformed plane also is introduced. Signal identification is based on aligned-phase angle temporal coherence, which significantly improves identification of signals. Results are included for both ocean and atmosphere acoustic data. 相似文献
7.
An algorithm that synthesizes apertures in the beam domain using FFT transformations and performs coherent processing of subaperture signals at successive time intervals is presented. Experimental tests of the algorithm show that for ocean environments with spatial coherence longer than the synthetic aperture length and for signals with temporal coherence longer than the required acquisition time, a synthetic array gain is achieved which roughly corresponds to the length of an equivalent fully populated array. In the experiments, transducer generated CW with phase stability and pseudorandom signals were used. Limitations on the spatial and temporal coherence were introduced only by the medium, the temporal coherence of the pseudorandom signal, and the shape and stability of the line array used 相似文献
8.
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
9.
Ocean noise is characterized by both temporal and spectral properties. Time-domain statistics are often non-Gaussian, and spectra are typically non-white. Both attributes can be simulated at the same time by applying a spectral shaping filter to random numbers having a specified kurtosis. A method for generating random numbers with arbitrary kurtosis is presented 相似文献
10.
Acoustic vector-sensor correlations in ambient noise 总被引：3，自引：0，他引：3
Most array-processing methods require knowledge of the correlation structure of the noise. While such information may sometimes be obtained from measurements made when no sources are present, this may not always be possible. Furthermore, measurements made in-situ can hardly be used to analyze system performance before deployment. The development of models of the correlation structure under various environmental assumptions is therefore very important. In this paper, we obtain integral and closed form expressions for the auto- and cross-correlations between the components of an acoustic vector sensor (AVS) for a wideband-noise field, under the following assumptions concerning its spatial distribution: 1) azimuthal independence; 2) azimuthal independence and elevational symmetry; and 3) spherical isotropy. We also derive expressions for the cross-covariances between all components of two spatially displaced AVSs in a narrowband-noise field under the same assumptions. These results can be used to determine the noise-covariance matrix of an array of acoustic vector sensors in ambient noise. We apply them to a uniform linear AVS array to asses its beamforming capabilities and localization accuracy, via the Cramer-Rao bound, in isotropic and anisotropic noise 相似文献
11.
西沙北部海域海洋环境噪声频谱特性 总被引：2，自引：1，他引：1
Ambient noise is very important in the prediction system of a sonar performance, because it determines the detection ranges always in a passive sonar and usually in an active sonar. In the uncertainty issue for the so-nar performance, it is necessary to know this factor＇s statistical characteristics that are only obtained by data processing from the underwater ambient noise measurements. Broad-band ambient noise signals from 16 hydrophones were amplified and recorded for 2 min every 1 h. The results show that the ambient noise is essentially depth independent. The cross correlation of the ambient noise levels （1, 6 and 12 h average） with a wind speed is presented. It was found that the correlation is excellent on the upper frequency band and the noise levels correlate better with high wind speed than with low wind speed. 相似文献
12.
基于潜标测量的海洋环境噪声谱特性分析 总被引：3，自引：1，他引：2
利用海洋环境噪声测量潜标系统对南海典型海域开展了为期3个月的海洋环境噪声测量,16通道海洋环境噪声测量系统每小时测量两分钟噪声信号。数据处理结果表明,800~5 000Hz范围内,噪声谱与风速相关性最好,且风速越大相关性越好,噪声谱与风速的相关性好于与浪高的相关性。风关噪声谱级在海水中部基本不随接收深度发生变化,但由于测量水听器阵长度未能覆盖整个水深,因此未给出海面和海底处谱级变化规律。在400Hz以上的高频段整个风速范围内噪声谱级都随风速发生变化,且噪声谱级与对数风速具有很好的线性关系。 相似文献
13.
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
14.
In this paper, we address the problem of detecting an inhomogeneity in shallow water by observing changes in the acoustic field as the inhomogeneity passes between an acoustic source and vertical line array of receivers. A signal processing scheme is developed to detect the perturbed field in the presence of the much stronger primary source signal, and to estimate such parameters as the time when the inhomogeneity crosses the source-receiver path, its velocity, and its size. The effectiveness of incoherent, coherent, and partially coherent spatial processing of the array signals is evaluated using models and data obtained from experiments in a lake. The effect of different bottom types is also considered, and it is shown that partially coherent processing can have a significant advantage depending on the bottom type. Estimates of the minimum input signal-to-noise ratios (SNRs) for which the diffracted signal can be observed are presented. 相似文献
15.
Total sea surface temperature (SST) in a coupled GCM is diagnosed by separating the variability into signal variance and noise
variance. The signal and the noise is calculated from multi-decadal simulations from the COLA anomaly coupled GCM and the
interactive ensemble model by assuming both simulations have a similar signal variance. The interactive ensemble model is
a new coupling strategy that is designed to increase signal to noise ratio by using an ensemble of atmospheric realizations
coupled to a single ocean model. The procedure for separating the signal and the noise variability presented here does not
rely on any ad hoc temporal or spatial filter. Based on these simulations, we find that the signal versus the noise of SST
variability in the North Pacific is significantly different from that in the equatorial Pacific. The noise SST variability
explains the majority of the total variability in the North Pacific, whereas the signal dominates in the deep tropics. It
is also found that the spatial characteristics of the signal and the noise are also distinct in the North Pacific and equatorial
Pacific. 相似文献
16.
Sonar generated acoustic signals transmitted in underwater channel for distant communications are affected by numerous factors like ambient noise, making them nonlinear and non-stationary in nature. In recent years, the application of Empirical Mode Decomposition （EMD） technique to analyze nonlinear and non-stationary signals has gained much attention. It is an empirical approach to decompose a signal into a set of oscillatory modes known as intrinsic mode functions （IMFs）. In general, Hilbert transform is used in EMD for the identification of oscillatory signals. In this paper anew EMD algorithm is proposed using FFT to identify and extract the acoustic signals available in the underwater channel that are corrupted due to various ambient noises over a range of 100 Hz to 10 kHz in a shallow water region.Data for analysis are collected at a depth of 5 m and 10 m offshore Chennai at the Bay of Bengal. The algorithm is validated for different sets of known and unknown reference signals. It is observed that the proposed EMD algorithmidentifies and extracts the reference signals against various ambient noises. Significant SNR improvement is alsoachieved for underwater acoustic signals. 相似文献
17.
Bjerrum-Niese C. Bjorno L. Pinto M.A. Quellec B. 《Oceanic Engineering, IEEE Journal of》1996,21(2):143-149
The paper discusses the development of a simulation tool to model high data-rate acoustic communication in shallow water. The simulation tool is able to generate synthetic time series of signals received at a transducer array after transmission across a shallow-water communication channel. The simulation tool is suitable for testing advanced signal processing techniques for message recovery. A channel model has been developed based on the physical aspects of the acoustic channel. Special emphasis has been given to fluctuations of the signal transmission caused by time-varying multipath effects. At shorter ranges, the temporal variations are dominated by acoustic scattering from the moving sea surface. Therefore, the channel model produces a coherence function which may be interpreted as a time-varying reflection coefficient for the surface scattered acoustical path. A static, range-independent ray model identifies the significant multipaths, and the surface path is modulated with the time-varying reflection coefficient. The advantages and limitations of the channel model are discussed and assumptions necessary to overcome the limitations are emphasised. Based on the assumptions, an algorithm has been developed and implemented to model how a binary message will be modulated when transmitted by a transducer, is distorted in the channel and finally is received by a transducer array 相似文献
18.
一个水声扩频通信系统设计与实现 总被引：2，自引：0，他引：2
严重的多途衰落、多普勒频偏是水声通信中引起误码的主要原因。低功耗、远距离、高隐蔽性、低信噪比检测、高可靠性的数据传输是水声通信的一个发展方向。设计并实现了一个水声扩频通信系统,有效地解决了以上问题,并采用快速相关算法,代替传统的矢量与矩阵相乘运算,极大地减少了程序的运行量,从而实时地处理接收信号。通过湖试和海试,验证了此通信系统的优良性能。 相似文献
19.
The Cramer-Rao lower bound is used to assess the potential localization accuracy of a horizontal array observing a narrowband moving target. The narrowband signal received by the array is assumed to have only partial temporal coherence, which is modeled by taking the signal to be completely coherent over a data block but with an unknown absolute phase from block to block. A numerical example for a linear array illustrates the improvement in localization accuracy caused by an increase in the signal coherence time. The effect of target/array geometry is also studied 相似文献
20.
Ship noise received on a horizontal array towed behind the ship is shown to be useful as a potentially diagnostic tool for estimating local acoustic bottom properties. In numerical simulations, tow-ship noise which bounces off the bottom is processed on a beamformer that shows the arrival angles; the beamformer output is readily interpreted by relating it to the Green's function of the acoustic wave equation. Simple signal processing is shown to be sufficient to extract the propagation angles of the "trapped" (i.e., propagating) modes of the acoustic waveguide. By relating the trapped modes to a basic geophysical model of the bottom, one can predict acoustic-propagation conditions for a particular bottom-interacting ocean acoustic environment. 相似文献