Spectral characteristics of a microwave radar signal backscattered by the sea surface at small incidence angles

We present a theoretical analysis of the Doppler-spectrum properties of a microwave radar signal scattered by the sea surface at small incidence angles. The dependences of Doppler-spectrum width and shift on the wind velocity and wave development stage and their azimuthal dependence are analyzed. The case of mixed sea (wind wave plus swell) is also considered. The JONSWAP spectrum model is used to describe sea waves. The study shows that Doppler-spectrum parameters are sensitive to variation of sea-surface state; for example, for the case of developed sea waves, an increase in wind velocity by 1 m/sec leads to increases in the Doppler-spectrum width and shift by 15 Hz and 3 Hz, respectively. It is shown that for the case of a moving radar the Doppler spectrum remains sensitive to variation of sea- surface state with a sufficiently narrow radar directivity pattern. Estimates show that in the case of a single sea- wave system on the surface, using Doppler-spectrum parameters we can, in principle, determine wave type (developing wind wave, developed wind wave, or swell), dominant wavelength, wave propagation direction, and wave height; wind velocity, direction, and acceleration distance can be determined for wind waves.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 39, No. 5, pp. 517–526, May, 1996.This paper was supported by the Russian Foundation for Fundamental Research (project 93-02-15892).     

Application of the method of equivalent edge currents to composite scattering from the cone-cylinder above a dielectric rough sea surface

Compared with scattering from a rough surface only, composite scattering from a target above a rough surface has become so practical that it is a subject of great interest. At present, this problem has been solved by some numerical methods which will produce an enormous calculation amount. In order to overcome this shortcoming, the reciprocity theorem (RT) and the method of equivalent edge currents (MEC) are used in this paper. Due to the advantage of RT, the difficulty in computing the secondary scattered fields is reduced. Simultaneously, MEC, a high-frequency method with edge diffraction considered, is used to calculate the scattered field from the cone-cylinder target with a high accuracy and efficiency. The backscattered field and the polarization currents of the rough sea surface are evaluated by the Kirchhoff approximation (KA) method and physical optics (PO) method, respectively. The effects of the backscattering radar cross section (RCS) and the Doppler spectrum on the size of the target and the windspeed of the sea surface for different incident angles are analysed in detail.     

Single-scattering spectroscopy for extremely dense colloidal suspensions by use of a low-coherence interferometer

Single-scattering spectroscopy by use of a low-coherence interferometer is introduced to measure the power spectra of light scattered from extremely dense colloidal suspensions. The power spectrum of a heterodyne component can be obtained by subtraction of the power spectrum of a homodyne component from the measured power spectrum. The heterodyne power spectrum for light scattered from the medium is shown to coincide with the single-scattering spectrum to a depth of up to a few times the mean-free path length. Therefore single-scattering spectroscopy is newly proposed as a means by which to analyze the characteristics of extremely dense colloidal suspensions.     

Interference noise spectrum above a rough sea surface at small glancing angles

The interference noise spectrum due to sea clutter has been investigated using statistical scattering theory. The scattered field was calculated in the tangent-plane approximation. The bearings of a motionless source are taken in the bistatic case. Taking the function of shadows on a spatially inhomogeneous random surface into account, we were able to explain the nonmonotonic dependence of the width of the spectrum of arrival angle fluctuations on the distance as well as the presence of maxima in the spectrum at frequencies substantially higher than that of the maximum of a rough sea surface. The theoretical results are compared with data from a full-scale experiment.Radioastronomical Institute, Ukrainian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, No. 10, pp. 1294–1302, October, 1994.     

二维粗糙海面散射回波多普勒谱频移及展宽特征

基于二维线性海面模型及粗糙面电磁散射的一阶小斜率近似公式,研究了海面回波各阶多普勒谱的频移特性,得到了多普勒谱频移所对应的理论公式,与经典多普勒频移公式相比考虑到了风速、大尺度波浪轨道运动等因素对多普勒谱频移的影响;同时根据粗糙面的双尺度模型,基于多普勒谱频移的物理机理,给出了求解散射场多普勒谱展宽的理论公式.最后将理论公式计算所得结果与模拟结果及实测数据进行了比较,讨论了风速、入射波频率及入射角等因素对多普勒频移及多普勒谱展宽的影响.通过比较可以发现,给出的理论公式可以对多普勒谱频移和展宽进行一定程度的预测. 关键词： 粗糙海面 电磁散射 多普勒谱     

Interference in light scattering from slightly rough dielectric layers

The scattering of light from a slightly rough surface overlying a reflecting surface is investigated. It is shown that the long-scale component of the roughness spectrum plays a critical role in the scattering patterns obtained. The scattered interference patterns are critically dependent on small variation of the rms height of the long-scale component of the roughness. Conventional perturbation theory is found to be invalid in cases in which interference phenomena in the scattering are of importance. A model is proposed that quantitatively describes the measured angular intensity distributions.     

Numerical simulation of Bragg scattering of sound by surface roughness for different values of the Rayleigh parameter

Numerical simulation methods are described for the spectral characteristics of an acoustic signal scattered by multiscale surface waves. The methods include the algorithms for calculating the scattered field by the Kirchhoff method and with the use of an integral equation, as well as the algorithms of surface waves generation with allowance for nonlinear hydrodynamic effects. The paper focuses on studying the spectrum of Bragg scattering caused by surface waves whose frequency exceeds the fundamental low-frequency component of the surface waves by several octaves. The spectrum broadening of the backscattered signal is estimated. The possibility of extending the range of applicability of the computing method developed under small perturbation conditions to cases characterized by a Rayleigh parameter of ≥1 is estimated.     

The scattering of He atoms from an ordered layer of Xe atoms on Cu(001) with energy analysis

The scattering of He atoms from an ordered layer of Xe atoms deposited on Cu(001) is described. Energy analysis was carried out using a LiF crystal analyser. Energy gain and loss events were observed, the values of which were independent of the parallel momentum transfer. Results are shown for varying degrees of coverage where at large coverages a change in the energy spectrum of the scattered atoms was observed. Adsorption of Xe on a Cu surface damaged by ion bombardment yielded no inelastic information — only an elastic component was detected.     

Methods of calculating the band structure and low-energy secondary electron spectroscopy of iridium

A theoretical interpretation is put forward for the fine structure of the secondary electron emission spectra of Ir normal to the (111) surface and the total current spectrum of an Ir polycrystal. The calculations took into account the energy dependence of the broadening of the energy band levels, the electron-electron and electron-plasmon contributions to the nonequilibrium electron distribution function, and the isotropic component of the current from the electrons scattered at the surface. It is shown that the fine structure of the secondary electron emission spectrum and the total current spectrum is mainly attributable to the electron structure of the final states into which the electrons enter or from which they are emitted so that the characteristics of the band configuration in the energy band structure can be reconstructed directly from the experimental data. This method can be used to separate bulk effects from surface effects in the secondary electron emission and total current spectra. It is confirmed that the fine structure of the secondary electron emission and total current spectra depends on the geometric structure and the degree of ordering of the crystals. A reduction in the intensity of the fine structure serves as a measure of the defect structure in the surface region of the sample which can be successfully used to monitor the surface state during treatment. Zh. Tekh. Fiz. 69, 94–96 (June 1999)     

Frequency response of 1D dispersive sea surface by the ADE-FDTD algorithm

The frequency response of a one-dimensional (1D) sea surface was investigated using the finite-difference time-domain (FDTD) algorithm with pulsed wave excitation. Where the sea water is regarded as an isotropic and dispersive medium, it satisfies the single-pole Debye model. The auxiliary differential equation (ADE) technique is incorporated into the FDTD scheme to deal with the dispersive sea surface. To ensure the feasibility of our present method, the frequency response of dispersive sea surface by the ADE-FDTD method is compared with the result by the conventional method of moments (MOM), which requires an individual calculation for every frequency point of interest. Finally, the scattering characteristic of dispersive sea water is investigated for different wind speed, temperature, and salinity. The representative far zone scattered field versus time is also analyzed.     

Study of the frequency distribution of the fluorescent light induced by monochromatic radiation

The frequency distribution of the fluorescent light induced by monochromatic dye laser radiation was investigated. To exclude the influence of the Doppler width a strongly collimated atomic beam was used. The spectrum was measured by means of a piezoelectrically tunable spherical Fabry Perot. The interaction region between the laser light and the atomic beam was placed into the center of the interferometer. Thus the observed fluorescence spectrum was considerably more intense than in the case where the interferometer is used separately from the beam. The fluorescence spectrum was observed for different directions of polarization of the incident laser beam. In the case of weak excitation the spectrum consists of a sharp component essentially due to elastically scattered light. At high intensities a structure of three components is observed which is in agreement with theoretical predictions when circularly polarized light is used for excitation.     

Study of the frequency distribution of the fluorescent light induced by monochromatic radiation

The frequency distribution of the fluorescent light induced by monochromatic dye laser radiation was investigated. To exclude the influence of the Doppler width a strongly collimated atomic beam was used. The spectrum was measured by means of a piezoelectrically tunable spherical Fabry Perot. The interaction region between the laser light and the atomic beam was placed into the center of the interferometer. Thus the observed fluorescence spectrum was considerably more intense than in the case where the interferometer is used separately from the beam. The fluorescence spectrum was observed for different directions of polarization of the incident laser beam. In the case of weak excitation the spectrum consists of a sharp component essentially due to elastically scattered light. At high intensities a structure of three components is observed which is in agreement with theoretical predictions when circularly polarized light is used for excitation.     

Time-resolved spectrum for resonant light scattering in an exciton-phonon system

A formula is given for the time-resolved spectrum of the resonantly- scattered radiation from an exciton-phonon system, and it is applied to the calculation of two-LO-phonon assisted spectrum. Both the Raman component associated with virtual intermediate states and the luminescence with real intermediate states are obtained. A Raman-like component with an oscillatory behavior can be seen, when the exciton relaxation time is shorter than the excitation pulse duration. It is proposed that the energy dependence of exciton relaxation rate can be directly measured from the time decay of a hot luminescence spectrum.     

Backscattering of light from a dielectric layer on a reflecting substrate

Scattering of light from rough dielectric films is studied experimentally. It is shown that the interference pattern of the scattered field depends critically on the power spectrum of the roughness, especially on its long-scale component. When the height of roughness is small compared with the wavelength, the backscattering peak (if it exists) is due to the interference of the singly scattered fields; hence the properties of the backscattered peak are rather unusual. In particular, the width of the peak is determined by the thickness of the film and is independent of the parameters of disorder. The intensity of the peak increases with an increase of the rms height of the surface roughness and becomes independent of the rms when the roughness is of the order of the wavelength.     

Features of the scattering of focused terahertz radiation from the probe of a terahertz near-field microscope

The scattering of pulsed terahertz radiation from metallic probes in the form of thin cylinders and cones with a small opening angle, which are used in apertureless terahertz near-field microscopes, has been investigated. The extrema of the waveform of pulsed terahertz radiation scattered from a free probe are linearly shifted with a change in the vertical position of the probe, and the spectral distribution is characterized by an inversely proportional frequency dependence. In the presence of a reflecting surface under the probe, when new excitation and detection directions appear, the spectrum of scattered terahertz radiation does not differ from the spectrum of the incident radiation. The experimental data are in mutual agreement with the theoretical results obtained within the model of the generation of diffraction edge waves at the interface of inhomogeneous excitation between the excitation region and shadow region.     

Analysis of multiple scattering from two-dimensional dielectric sea surface with iterative Kirchhoff approximation

<正>An iterative method in the Kirchhoff approximation is proposed for high frequency multiple electromagnetic scattering from two-dimensional dielectric sea surface.The multiple interaction of the scattering field is characterized with the corrected electromagnetic currents of the wind-driven sea surface.The actual surface currents are approximated with the iterative solution of the corrected currents.A newly developed sea spectrum,Elfouhaily spectrum,is utilized to build the sea surface model.The shadowing correction is improved by the Depth-Buffer algorithm.The validity of the iterative Kirchhoff approximation is verified by the agreement of backscattering coefficients with the measured data.     

浅海涌浪对表面声道声传播的影响

受海面强风和海-气相互作用影响,表面声道普遍存在于冬季海洋环境中,是一种天然有利于声传播的波导.但是海面波浪使得海表形成粗糙界面,会严重破坏这种优良性能.本文利用南海北部海区的一次冬季声传播实验数据,研究表面声道声传播特性.研究表明,海底底质对表面声道内声传播的影响较弱,当海面风较小时,涌浪造成的影响为主要原因.实验数据显示,考虑涌浪后的粗糙海面给70km远处带来了10dB的传播损失增长.因此在考察南海北部海区冬季声场特性时,不仅要考虑海面风浪的影响,更需要考虑周围海域传来的涌浪的影响.研究涌浪存在时的声传播特性对提升声纳设备在海况较差时的使用性能具有重要意义.     

有冰海区油膜光谱特征研究

近些年有冰海区石油的勘探、开发以及运输频度逐年上升,这增加了冰区溢油发生的风险。试验通过测量海水、碎冰、整冰等不同背景条件下轻柴油和原油油膜的可见光-近红外光谱反射率曲线,并与洁净的海水、碎冰和平整冰光谱曲线进行比较,得到能够有效识别冰区溢油的波段。测量结果表明：同一油种在不同背景下油膜的光谱反射率曲线有所差异,同一油种同一背景下由于分布形态不同光谱反射率曲线也有所差异。但同一油种的油膜仍呈现出许多区别于背景的共同特征：以海水、碎冰或整冰为背景的轻柴油油膜的光谱反射率曲线呈现“先低后高”、部分“相伴而行”的特点;海水、碎冰和整冰中原油油膜的光谱反射率曲线在750～770 nm区间出现楔状反射峰的典型特点。根据以上特点可以将溢油污染海水/海冰与洁净海水/海冰很好的区分开来,从而为遥感监测冰区溢油时的波段选择和溢油识别提供参考和依据。     

Scattering from canonical objects above a sea-like one-dimensional rough surface from a rigorous fast method

This paper is devoted to scattering from a canonical object above a sea-like one-dimensional rough surface (two-dimensional problem). Such a problem involves a high number of unknowns and cannot be solved easily with a Method of Moments by using a direct LU inversion. Thus, a recently developed fast numerical method called Extended-PILE combined with FB-SA is used. First, a parametrization is proposed for the FB-SA by considering the sea surface only. Then, the convergence of E-PILE+FB-SA is tested for the scattering from a cylinder, a plate and a cross located above a rough surface obeying a Gaussian process with the Elfouhaily et al. spectrum. After generating several surface realizations, a Monte Carlo process is applied on the scattered fields in order to discuss on the Normalized Radar Cross-Section (NRCS) for a maritime application at microwave frequencies. Eventually, the study focuses on the case of low grazing incidence.