共查询到20条相似文献，搜索用时 46 毫秒
1.
The shipping noise properties in the deep ocean are studied. Shipping noise exhibits the strong dual-horned directionality features in the flat-seabed ocean, and its directional density can be modeled by a Von Mises distribution. With the explicit expression for the directional density function, the spatial coherence functions of shipping noise are also derived, and the relative features are studied. The research result shows that the properties of shipping noise are different from the ambient noise of other sources, and it can be used for the sonar array design. The model is well matched with the experimental result, and it can be extended to the situations when the ambient noise exhibits the dual-horned structure. 相似文献
2.
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
3.
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
4.
《Oceanic Engineering, IEEE Journal of》2006,31(2):413-420
The robustness of the coherence of waveguide propagation to environmental uncertainty becomes an important consideration for systems that seek to exploit coherence for gain. Examples include matched field processing for passive localization and time reversal mirrors (TRMs) for active systems. Here, efficient normal mode representations of midfrequency time-domain propagation using the narrowband and adiabatic approximations are used to explore the deterioration of coherent active system predictability and performance in the presence sound-speed perturbations in the water column. Results show that for TRMs the reverberation level at the focal range is increased, and the scattering from an illuminated object is reduced for ensembles over oceanographic uncertainty. Results are obtained analytically as formal averages and are believed to represent a lower limit on the deterioration of TRM performance in the presence of environmental uncertainty for shallow-water waveguides. 相似文献
5.
A basic formalism is developed to treat the vertical spatial coherence of backscatter from wind-generated microbubbles beneath the ocean surface. This formalism treats signals multiply scattered by the sea surface and the subsurface scatterers, as well as absorption in the bubble layer. Approximate solutions are obtained for the case of narrow beamwidth sources and are applied to study the influence of measurement system and environmental parameters on coherence. Using bubble densities derived from acoustic backscatter data, the coherence is found to depend strongly on source frequency and beam pattern. The primary environmental effect is due to the increase in both bubble density and penetration depth below the surface that occurs with increasing windspeed. At high wind speeds, the vertical coherence is sufficiently dependent on the scatterer depth distribution to provide a viable means of studying this phenomenon. 相似文献
6.
7.
The author addresses the spatial coherence of high-frequency acoustic signals that have been forward scattered from the sea surface. The Fresnel-corrected Kirchhoff approximation is applied to derive closed-form expressions for the spatial coherence. These expressions are used to study the influence of geometrical and environmental factors on the coherence. An application of the theory involving the rejection of the surface image of a source by a vertical adaptive line array is presented. The author concludes that the environment has a strong impact on the array processing of surface-scattered fields through its influence on both vertical and horizontal spatial coherence 相似文献
8.
A statistical theory of signal coherence 总被引：1，自引：0，他引：1
A periodic signal can be perfectly predicted far into the future since it perfectly repeats every period. There is always some variation in the waveform over time for signals which are labeled as periodic but which are not truly deterministic. A formal definition is presented in this paper for such a varying periodic signal and the properties of such a class of signals are exploited. A measure called a signal coherence function of the amount of random variation in each Fourier component of the signal is defined and its statistical properties are developed. This signal coherence function is very different from the coherence function between two stationary signals. The method is applied to a digitized record of an acoustic signal generated by a boat in a bag in the Baltic Sea south of Stockholm, Sweden 相似文献
9.
The authors examine the subject of space-time processing and review fundamental environmental effects and their influence on arrays in the deep ocean sound channel. Space-time transforms are reviewed to demonstrate the analogy between spatial and temporal properties to stress the importance of convolution and matched field processing. A criterion is presented by which the resolution of such measurement systems could be calculated. The static source-receiver case is shown to be influenced by the randomness in signal phase due to scattering. Calculations and data are used to show the importance of multipath effects on the relative gain of line array measurement systems and the difficulties encountered for the determination of coherence lengths. Single path coherence lengths were found to be large and predictable using an environmental parameter and the Beran-McCoy mutual coherence functional form. However, multipath effects appeared to be dominant. The temporal fluctuation problem is briefly introduced to stress the fact that for relative source-receiver speeds of 1.5 m/s (3 knots) or greater, the fluctuations are dominated by the changes in the multipath arrivals 相似文献
10.
在集合数据同化中,协方差局地化（covariance localization,CL）方法的使用存在限制。集合转换卡尔曼滤波（ensemble transform Kalman filter,ETKF）作为集合平方根滤波的变种方法,是一种应用较广、计算高效的数据同化方法。本文分析了CL方法应用于ETKF方法的困难,从而改进CL方法使其可以适用于ETKF方法。另外,结合浅水方程,利用Askey函数作为多元局地化函数,提出了一种适用于多元数值模型的CL方法。通过具体实验验证,得到了较好的分析结果。 相似文献
11.
Badiey M. Yongke Mu Simmen J.A. Forsythe S.E. 《Oceanic Engineering, IEEE Journal of》2000,25(4):492-500
Coherence of broad-band acoustic waves for mid-to-high frequencies (0.6-18 kHz) is obtained for a very shallow-water (15-m-deep) waveguide over a wide band of environmental conditions and for a source-receiver range of 387 m. Temporal behavior is sampled at two different rates: one that resolves at fractions of a second over intermittent periods of 40 s and another that resolves at 10 min over periods of several days. Spatial behavior is sampled in the vertical by hydrophones with spacings of the order of meters. The effect of environmental variability on coherence, in particular, soundspeed fluctuations in the water column and wind-induced modulations of the air-sea interface, is noted as a function of acoustic frequency and ray path. Analysis of the acoustic fluctuations over short time scales more accurately resolves the temporal decorrelation of the received signal due to sea surface waves. The vertical sampling of the received signal permits an analysis of arrival-angle fluctuations. The dependence of coherence on the number of surface bounces is studied by comparing arrivals that have zero, one, two, and three surface bounces 相似文献
12.
Canonical correlation analysis is employed as a multiaspect feature extraction method for underwater target classification. The method exploits linear dependence or coherence between two consecutive sonar returns, at different aspect angles. This is accomplished by extracting the dominant canonical correlations between the two sonar returns and using them as features for classifying mine-like objects from nonmine-like objects. The experimental results on a wideband acoustic backscattered data set, which contains sonar returns from several mine-like and nonmine-like objects in two different environmental conditions, show the promise of canonical correlation features for mine-like versus nonmine-like discrimination. The results also reveal that in a fixed bottom condition, canonical correlation features are relatively invariant to changes in aspect angle. 相似文献
13.
The moment generating function and the probability function for non-deterministic wave forces on a small diameter fixed vertical pile are derived for linear, Gaussian waves. The dimensionless force covariance function is obtained by taking the expectation of the dimensionless force. An indefinite integral is obtained which may be conveniently solved using Laplace transforms. These results are compared with previously published covariance expressions. 相似文献
14.
Reliability-Based Design for Jacket Platform Under Extreme Loads 总被引：7，自引：0，他引：7
In this paper, reliability analysis for the offshore jacket platform with the interaction of structure- pile- soil under extreme environmental loads is carried out. The inherent uncertainties of the environmental load, foundation soil, platform itself, and calculating models are evaluated. The action of extreme loads on the offshore platform is modeled as a function of extreme wave height. The system capacity of the whole platform is determined by nonlinear pushover analysis, and the relevant probability property is obtained by the simulation method. The reliability model for the whole jacket platform is described as the relationship between the load and resistance based on the offshore design codes. The reliability of whole platform is calculated by the analytical method and the importance sampling method on the basis of a case study for a tripod jacket platform. 相似文献
15.
Cheolsoo Park Woojae Seong Gerstoft P. Siderius M. 《Oceanic Engineering, IEEE Journal of》2003,28(3):468-478
An inversion method using a towed system consisting of a source and two receivers is presented. High-frequency chirp signals that have been emitted from the source are received after multiple penetrations and reflections from the shallow water sub-bottom structure and are processed for geoacoustical parameter estimation. The data are processed such that a good resolution and robustness is achieved via matched filtering, which requires information about the source signal. The inversion is formulated as an optimization problem, which maximizes the cost function defined as a normalized correlation between the measured and modeled signals directly in the time domain. The very fast simulated reannealing optimization method is applied to the global search problem. The modeled time signal is obtained using a ray approach. An experiment was carried out in the Mediterranean Sea using a towed source and receiver system. The inversion method is applied to the experimental data and results are found to be consistent with previous frequency-domain analyses using measurements from a towed horizontal array of receivers and measurements on a vertical array. 相似文献
16.
In this paper, we study the temporal resolution of a time-reversal or passive-phase conjugation process as applied to underwater acoustic communications. Specifically, we address 1) the time resolution or the pulse width of a back-propagated time-compressed pulse as compared with the original transmitted pulse; 2) the effectiveness of temporal focusing as measured by the peak-to-sidelobe ratio of the back-propagated or phase-conjugated pulse (both pulse elongation and sidelobe leakages are causes of intersymbol interference and bit errors for communications); 3) the duration of temporal focusing or the temporal coherence time of the underwater acoustic channel; and 4) the stability of temporal focusing as measured by the phase fluctuations of successive pulses (symbols). Binary phase-shift keying signals collected at sea from a fixed source to a fixed receiver are used to extract the above four parameters and are compared with simulated results. Mid-frequency (3-4-kHz) data were collected in a dynamic shallow-water environment, exhibiting high temporal fluctuations over a scale of minutes. Despite this, the channel is found to be highly coherent over a length of 17 s. As a result, only one probe signal is used for 17 s of data. The bit error rate and variance of the symbol phase fluctuations are measured as a function of the number of receivers. They are of the same order as that calculated from the simulated data. The agreement suggests that these two quantities could be modeled for a communication channel with high coherence time. The phase variance can be used to determine the maximum data rate for a phase-shift keying signal for a given signal bandwidth and a given number of receivers. 相似文献
17.
This paper deals with the problem of coherent radar detection of targets embedded in clutter modeled as a compound-Gaussian process. We first provide a survey on clutter mitigation techniques with a particular emphasis on adaptive detection schemes ensuring the constant false-alarm rate (CFAR) property with respect to all of the clutter parameters. Thus, we propose a novel decision rule based on a recursive covariance estimator, which exploits the persymmetry property of the clutter covariance matrix. Remarkably, the devised receiver is fully CFAR in that its threshold can be set independently of the clutter distribution as well as of its covariance, even if the environment is highly heterogeneous; namely, the disturbance distributional parameters vary from cell to cell. At the analysis stage, we compare the performance of the novel detector with some classical radar receivers such as that of Kelly and the adaptive matched filter both in the presence of simulated as well as on real radar data, which statistical analysis has shown to be compatible with the compound-Gaussian model. The results show that the new receiving structure generally provides higher detection performance than the others and, for a fluctuating target, it uniformly outperforms the counterparts. We also provide a discussion on the CFAR behavior of the analyzed receivers as well as on their computational complexity. 相似文献
18.
The authors explore the resolving power of an inversion algorithm which estimates five parameters of the seafloor covariance function from a single swath of multibeam echosounding data. The resolving power is evaluated as a function of the swath length, the orientation of ship track with respect to topographic grain, and the response width of the sounding system. The analysis is conducted by inverting sets of synthetic data with known statistics. The mean and standard deviation of the inverted parameters can be directly compared with the input parameters and the standard errors output from the inversion. Experiments show that resolution of the covariance parameters is strongly dependent on the number of characteristic lengths which are sampled. Root-mean-square seafloor height can be estimated to within ~15%, and anisotropic orientation to within ~5% (for a strong lineation), using track lengths as short as three characteristic lengths 相似文献
19.
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
20.
Carey W.M. Cable P.G. Siegmann W.L. Lynch J.F. Rozenfeld I. 《Oceanic Engineering, IEEE Journal of》2002,27(4):841-852
The experiment, The Acoustic Characterization Test III, was conducted in the oceanographically complex Strait of Korea to accurately measure the sound transmission under known environmental conditions. Geoacoustic profiles derived from geophysical measurements, measured bathymetry, and sound-speed profiles were the basis for range dependent parabolic equation (PE) calculations. Agreement between measured and calculated transmission loss was obtained with an attenuation profile in the near water-sediment interface layer with a dependence on frequency to the 1.8 power consistent with measurements in other sand-silt areas. Since the environment was oceanographically complex and the shipping noise levels were high, the coherency of the sound transmission was estimated using relative signal gain (RSG). RSG was taken as the difference between the gain calculated with PE and measured with the array and at longer ranges and higher frequencies was found to be approximately -2 dB with a signal gain coefficient of variation of 5%. This RSG degradation, attributed to the random signal phase fluctuations resulting from scattering from the surfaces and volume of the waveguide, yielded using a Gaussian coherence function a spatial coherence length of 30/spl lambda/ @ 400 Hz-40 km. In addition, high resolution imaging of five targets with two bottom mounted arrays illustrate the achievable performance of low-to-mid frequency active sonar in this environment. 相似文献