修正双尺度模型在非高斯海面散射中的应用

基于海浪在水平及垂直方向上的倾斜效应,应用修正的双尺度模型求解非高斯海面的电磁散射,在经典的一阶微扰散射系数上添加了一个附加散射系数项,它与非高斯海面的双谱函数成正比,且该项反映了海面后向散射系数在逆风和顺风向上观测结果的不对称性.在修正模型中还考虑了遮蔽函数、曲率效应等因素对散射结果的影响.最后应用修正的双尺度模型数值计算并讨论了非高斯海面后向电磁散射特性,修正模型下的计算结果与实验数据有较好的吻合.     

二维分形海面的全极化散射模型

采用二维分形海面模型模拟海面状态,根据惠根斯原理,把海面近似为无数克希霍夫散射面,并根据驻相原理导出散射面法线的方向导数,推导出笛卡儿坐标系下椭圆极化入射时海面的散射场和局部坐标系下的散射系数矩阵,建立了二维分形海面的全极化双站雷达散射模型,并数值计算了全极化海面雷达截面.与经典单极化散射模型相比,该模型不仅保留共极化(HH极化VV极化)散射信息,还包含了多极化SAR海浪谱反演过程中必不可少的交叉极化(VH极化HV极化)信息.数值结果表明用该模型计算的共极化雷达截面与其他文献计算的结果能很好的吻合.同时给出了二维分形海面的不同接收方位雷达截面的数值模拟结果.     

A scattering model for snow-covered sea ice

The special properties of a robust radiative transfer model for scattering from layers of inhomogeneous rough-boundary slabs are presented. The model is applied to backscattering from saline and desalinated ice. Comparisons are made at single and multiple frequencies with some of the most complete sets of measurement data available, using measured physical and electrical characteristics of the ice as inputs to the model where possible. The results show close agreement. For example, for the saline ice backscatter data set, which consisted of measurements at two like and two cross polarizations at 5 and 13.9 GHz, the agreement with model predictions is within 2 dB except at 13.9-GHz cross polarization. Backscattering from >15-cm-thick saline ice is generally dominated by scattering from the top surface while backscattering from <8-cm-thick saline ice can be strongly influenced by returns from the ice/water interface, particularly at frequencies less than about 5 GHz     

A numerical model for electromagnetic scattering from sea ice

A numerical model for scattering from sea ice based on the finite difference time domain (FDTD) technique is presented. The sea ice medium is modeled as consisting of randomly located spherical brine scatterers with a specified fractional volume, and the medium is modeled both with and without a randomly rough boundary to study the relative effects of volume and surface scattering. A Monte Carlo simulation is used to obtain numerical results for incoherent υυ backscattered normalized radar cross sections (RCSs) in the frequency range from 3 to 9 GHz and for incidence angles from 10° to 50° from normal incidence. The computational intensity of the study necessitates an effective permittivity approach to modeling brine pocket effects and a nonuniform grid for small scale surface roughness. However, comparisons with analytical models show that these approximations should introduce errors no larger than approximately 3 dB. Incoherent υυ cross sections backscattered from sea ice models with a smooth surface show only a small dependence on incidence angle, while results for sea ice models with slightly rough surfaces are found to be dominated by surface scattering at incidence angles less than 30° and by scattering from brine pockets at angles greater than 30°. As the surface roughness increases, surface scattering tends to dominate at all incidence angles. Initial comparisons with measurements taken with artificially grown sea ice are made, and even the simplified sea ice model used in the FDTD simulation is found to provide reasonable agreement with measured data trends. The numerical model developed ran be useful in interpreting measurements when parameters such as surface roughness and scatterer distributions lie outside ranges where analytical models are valid     

An improved geometrical optics model for bistatic GPS scattering from the ocean surface

This paper is concerned with the properties of bistatic microwave scattering from a randomly rough surface, and specifically its application to the study of global positioning system (GPS) satellite signals reflected from the ocean. We present a discussion of some recent refinements of Kirchhoff-type models based on second-order iterations of the surface-current integral equation, and the relationship between these models and their high-frequency (geometric optics) limit. In particular, we show that use of these refinements can extend the domain of applicability of the standard geometrical optics (GO) model. It is found that GO can be nearly as accurate as a Kirchhoff-based model provided that the wavenumber cutoff, at which the surface wave spectrum must be filtered in computing the required slope moments, depends on the roughness of the ocean surface (i.e., wind speed) as well as the incident angle and frequency of the radiation. We use a GO model refined in this way to analyze GPS surface reflection data collected from an aircraft equipped with two down-looking antennas for receiving both left- and right-hand circularly polarized reflections. Concurrent measurements of the local wind and wave conditions were collected from a nearby research vessel. Measured waveforms and mean Doppler widths at both polarizations are compared with predictions from our refined GO model, and discussion is given concerning the sensitivity of the reflected radiation to various geophysical parameters and the utility of GPS reflections for remote sensing applications.     

基于并行双尺度射线追踪的海面电磁散射计算

利用一种基于双尺度模型(Two Scale Model, TSM)的射线追踪(Ray Tracing, RT)算法(TSM-RT)快速计算电大尺寸海面电磁散射, 与传统的射线追踪算法相比, 该算法能够有效减少射线与面元的求交次数, 提高了计算效率.同时, 为了进一步减少计算时间, 利用图形处理单元(Graphics Processing Unit, GPU)强大的并行处理能力对TSM-RT算法进行加速.计算结果表明:基于GPU的并行TSM-RT算法与基于CPU的串行TSM-RT算法相比计算时间有了很大程度的减少, 获得了很好的加速效果.     

A study of the mechanism of sea surface scattering

The mechanism by which electromagnetic waves are scattered from the sea surface is studied herein. Principal applications for such studies occur in over-water communication channels and radar. Predictions based on rigorous formulations for diffraction from a (moving) time-space periodic surface are correlated with the experimental results of other authors. Such predictions explain the existence of strong coherent scattering from the sea surface in nonspecular directions (grating lobes) and also anticipate the appearance of well-defined, harmonically related, frequency shifts (time-space harmonics) in scattered signals observed in these experiments. On the basis of such manifestations of the periodic nature of the sea surface, a model is formulated for use in calculation of the angular power spectrum of scattered electromagnetic waves. This model, accounting for spatial coherence effects, assumes for the fluctuation of heights on the water surface a Gaussian distribution with a narrow band-pass (nearly sinusoidal) spatial spectrum. Calculations of the angular, scattered spectrum due to this model show strong predominantly coherent wave summation in directions corresponding precisely to the grating lobes of a purely periodic structure of period equal to the center period of the band-pass spectrum. Spectral (angular) spreading increases with increased bandwidth of the spatial spectrum of the surface. Calculations for the present study are made on the basis of a perfectly reflecting boundary and hence do not display the proper distinction of polarization expected for a completely realistic model. The results, however, display the spectral charactral characteristics necessary to explain strong scattering in nonspecular directions by a random surface.     

分形海面电磁散射特性的基尔霍夫近似

利用二维Weierstrass带限函数模拟了粗糙海面的形状,讨论了风速、粗糙度等参量对海面形状的影响;针对模拟的分形海面形状,从亥姆霍兹（Helmholtz）积分出发,利用基尔霍夫近似推导了二维分形海面的电磁散射系数,并进行了数值模拟,同时对数值模拟结果进行了分析,讨论了海面形状对电磁散射的影响,所得结论可用于目标探测技术的研究。     

Incoherent bistatic scattering from the sea surface at L-band

A bistatic electromagnetic wave scattering model for the sea surface is developed to examine its wind dependence property over a wide range of incident angles along the specular direction. This is done by combining an existing scattering model with a sea spectrum recently reported in the literature. In general, electromagnetic wave scattering from a rough surface is dependent on the Fourier transform of the nth power of its height correlation function which can be computed numerically from the surface spectrum. This transform relation indicates that scattering is sensitive not only to the surface spectrum but also to its convoluted properties. Generally, surface scattering is sensitive only to a portion of the surface correlation measured from the origin. The size of this portion is a function of three variables (the incident angle, the surface height standard deviation, and the exploring wavelength) and the rate of decay of the correlation function. The decay rate near the origin of the sea surface correlation is very small, so much so that at L-band this portion is too wide for a two-term approximation of the correlation function. This is true in spite of the fact that the sea surface has a very large rms height. Thus, a scattering model based on geometric optics is generally not applicable at L-band especially at large angles of incidence. An additional finding is that in specular scattering wind dependence is stronger at larger angles of incidence for incident angles between 0 and 70° over the wind speed range of 4 m/s-20 m/s     

An iterative algorithm for signal reconstruction from bispectrum

The author presents a procedure to reconstruct a (minimum or nonminimum phase) discrete-time signal from its bispectrum. The algorithm is iterative and based on using the method of projections onto convex sets (POCS) in an inner product space in which the vector space consists of real sequences, and vector addition, is defined in terms of the convolution operation. Prior information, such as an energy bound on the signal, can be incorporated into this algorithm. An algorithm that employs energy information was found to produce better results in simulation than one that ignores energy constraints     

Digital model of radiolocation scattering by sea surface when it is probed by coastal radiolocation station

It is synthesized on a heuristic level a simplified variant of digital model of radiolocation scattering of a signal, when sea surface is probed. This model takes into account technical parameters and real geometric dimensions of coastal radiolocation station situation.     

粗糙海面蒸发波导传播和近掠入射散射分析

讨论在蒸发波导条件下粗糙海面对电波传播特性的影响。推导粗糙海面的表面反射系数,通过实验检验某粗糙海面电波传播预测模型在我国海域的适用性,并给出实验结果和分析。给出了不同条件下波导高度和传播损耗、最大探测距离间的关系。数据分析表明:冬季的粗糙海面情况下模型计算的传播损耗值与实测值比较接近,而秋季时两者相差较大,说明在海况较好时该模型适用于我国近海海域,而当风浪较大时,将导致表面粗糙度增大,影响模型的精度,使其低估传播损耗。     

各向异性粗糙海面对掠入射激光的散射特性研究

针对目前粗糙海面的激光散射场模型并不适用于激光束掠入射的情况以及忽略风速的影响,基于Cox-Munk模型及基尔霍夫近似（KA）模拟各向异性粗糙海面,建立海面对掠入射激光束的方向反射率分布模型,仿真分析了方向反射率随风速、风向、入射角、入射方位角、反射角、反射方位角的变化规律,并通过水池实验进行了验证。结果表明:粗糙海面对激光束的散射能量主要分布在以被照射海面为顶点的圆锥形角空域,此空域角方向位于镜面反射方向附近,且随着风速的增加,空域角逐渐变大,对于掠入射的激光束,由于海浪的遮蔽效应,空域角方向会偏离镜面方向。该模型可以为激光半主动制导导弹对海上小目标射击研究以及利用激光束照射粗糙海面布设激光诱饵研究提供一定理论依据。     

粗糙海面上舰船Kelvin尾迹的电磁散射仿真

基于点源扰动方法建立了舰船的Kelvin尾迹模型, 模拟了不同船速下的Kelvin尾迹.实际中, 舰船尾迹除了受到舰船自身参数影响外, 还会受到风驱海浪的调制.为了建立更为符合实际的Kelvin尾迹模型, 借助PM(Pierson-Moscowitz)海谱生成粗糙海面, 将其与Kelvin尾迹模型进行线性叠加后, 得到了粗糙海面上舰船尾迹.在该复合模型基础上, 利用物理光学法获得了不同入射情况下的单站、双站的雷达散射截面, 实现了海面舰船尾迹电磁散射的模拟仿真.这一过程的实现, 有利于更好地模拟舰船尾迹在合成孔径雷达(Synthetic Aperture Radar, SAR)中的成像仿真, 同时为SAR成像中舰船尾迹以及舰船自身的识别奠定了基础.     

Very high frequency radiowave scattering by a disturbed sea surface Part II: Scattering from an actual sea surface

A two-scale model of rough surfaces is considered which permits theoretical interpretation of the features of very high frequency (VHF) scattering from such surfaces (say, from a sea surface). The scattering surface is assumed to be a superposition of small-scale ripple and large waves (swell). Reflection from the latter may he considered by the Kirchhoff approximation. The spatial spectrum of corrugations is taken into consideration; the calculations based on this model help to give an explanation of the behavior of the scattered intensity as a function of the angle of incidencepsi, and to establish which factors affect this dependence at various values ofpsi- Theoretically predicted dependence of the scattered intensity upon radio wavelengthlambda, depolarization of the scattered signal, and other features of the scattered radiation are in good agreement with the experimental data obtained from direct measurements. Frequency spectra of the backscattered signal were also investigated (experimental measurements were carried out at wavelengths 3.2 cm, 10 cm, 50 cm, 1.5 m, and 4 m). Observed shifts of the central frequency agree with results of other authors (for the range oflambda = 3cm to 200 m). The measured values of the spectrum width appeared approximately twice those theoretically calculated. This may be explained by the influence of dissipative processes and of fluctuations of the skin-deep layer drift velocities. Space correlation of the backscattered signal was also investigated (both theoretically and experimentally). 50 percent decorrelation occurs at distances compared with dimensions of a wave slope.     

光学表面散射模型探索

文章讨论了光散射模型和表面粗糙度的统计特征。由数值解得到：入射角在决定散射曲线的行为中起着重要的作用。     

海面正置长方体目标复合散射极化特征

为分析海面上长方体目标的极化特征,通过极化特征图讨论了长方体目标尺寸参数和海面风速、风向对于复合极化散射的影响.计算结果显示入射角和目标尺寸足够大的情况下,二次散射更为突出,对应的极化特征图具有明显的二面角特点.侧风将会改变极化特征图的对称性.     

An efficient two-scale model for the computation of thermal emission and atmospheric reflection from the sea surface

An efficient implementation of the two-scale model of sea surface thermal emission and atmospheric reflection is described. The model is applied in a study of the reflection of downwelling atmospheric radiation. Results show that reflected downwelling radiation can increase azimuthal variations of total observed brightnesses.     

An empirical model for interpreting the relationship betweenbackscattering and arid land surface roughness as seen with the SAR

The nature of the surface in a flat arid zone located in the western Sahara desert was studied and its roughness parameters defined. The main geological units were compared with backscattering data from ERS-1 images. Results showed a close correlation between the backscattering coefficient, σ₀, and the maximum height, h_max, of the rocks making up the arid landscape. The simplified relationship σ₀=4.63.1og h_max-15.29 (with h_max in cm and σ₀ in dB) was established from field measurements, giving an r² of about 88%. This empirical relationship is confirmed by another close relation between σ₀ and the statistical parameter s, the height standard deviation. The advantage of h_max is that it can be extracted for any site, whatever its roughness may be. The spatial distribution of the rock facets and the geometric characteristics of the incidence angle of the radar signal seem to explain the results