双尺度动态分形粗糙海面的电磁散射及多普勒谱研究

利用更为符合海面实际散射场计算的双尺度法分析并计算了一维动态分形粗糙海面的后向散射截面，同时与微扰法有关结果进行了比较.研究了动态分形海面后向散射信号的多普勒谱分形特征，分析了不同极化状态下后向散射截面和多普勒谱谱宽随入射角及分维的变化规律，最后详细讨论了多普勒谱中心频率与海面Bragg谐振频率间的关系. 关键词： 双尺度模型 分形海面 电磁散射 多普勒谱     

分形海面的微波电磁散射计算模型

利用2维Weierstrass带限函数建立了模拟粗糙海面形状的模型,讨论了分形维数、频率幅度尺度因子等分形参数对海面形状的影响。以粗糙海面形状模型为基础,针对模拟的分形海面形状,从亥姆霍兹积分出发,利用基尔霍夫近似推导出2维分形海面的电磁散射系数,并进行了数值模拟。对微波电磁散射特性随分形维数、频率幅度尺度因子、入射波入射角变化的规律做了进一步讨论分析。在低掠射角时电波会受海面的遮挡,用遮挡函数对散射系数进行修正。研究表明:随着分形维数的增大,散射峰分布变均匀。频率幅度尺度因子越大,散射也越分散。随着入射角的增大,后向散射也逐渐增强,而前向散射逐渐减弱。     

基于小斜率近似的深海海面混响

收发设备在海面附近的深海混响实验中,多途时延使得最先到达水听器的为海面混响信号,且不受海底散射声场的干扰.本文利用射线理论描述深海声传播的格林函数,采用粗糙界面一阶小斜率近似方法描述全角度海面散射,给出海面混响声场的表达式.同时考虑了海面表层气泡散射的贡献,获得了海面混响理论.通过数值仿真数据和深海实验数据的比较对海面混响模型进行验证,分析了不同接收深度、频率下的海面混响强度衰减趋势.结果表明:低海况条件下,低频海面混响由粗糙界面散射主导,气泡散射可以忽略,随频率升高,气泡散射对海面混响的贡献逐渐增大,海面附近收发深度的小幅变化对混响衰减曲线的影响不明显.基于该模型提出一种反演海面粗糙界面谱参数的方法,数值计算结果验证了该模型能够在风速已知的前提下,通过海面混响数据提取海面粗糙界面谱参数.     

覆盖泡沫粗糙海面的激光散射特性研究

对潜通信及对潜探测中蓝绿激光将穿透海水表面,大气/海水界面由于风速及其他外力作用而呈现复杂状况,将影响蓝绿激光在界面的传输特性。针对泡沫-海面复合模型,采用矢量辐射传输理论、米氏理论以及粗糙面散射理论对泡沫海面的散射特性进行了分析。采用基尔霍夫近似针对JONSWAP海谱,讨论了泡沫海面在蓝绿激光波段的后向散射系数与入射角、风速、风区以及海水温度、含盐度等参量的关系。结果表明:随着海面上方风速、风区及介电常数的增大,泡沫层对海面的激光散射具有相当大的影响,尤其是在入射角度较大的状态下。     

起伏海面环境下水声信道特性及估计方法*

为了研究随机起伏海面的散射对水声信道特性的影响,以及不同信道估计算法在该信道条件下的性能问题,该文从利用Monte Carlo方法模拟的基于PM海浪谱的二维随机起伏海面出发,提取了与风向不同夹角的一维随机起伏海面,并利用Kirchhoff近似法计算了其散射强度。将得到的声散射特性和海面环境信息融入到信道模型中,建立了基于PM海浪谱的随机信道模型。通过仿真分析了不同风速、传播距离条件下,声波的传播损失和信道冲激响应变化,并利用l_0-最小均方误差法、匹配追踪算法和正交匹配追踪算法进行了信道估计。给出了不同环境条件下,三种算法估计性能的综合比较结果,验证了算法针对该信道模型的有效性。     

二维粗糙海面的光散射及其红外成像

首先根据JONSWAP海面功率谱模型数值模拟出二维粗糙海面，采用几何光学近拟与基尔霍夫（Kirchhoff）标量近似计算了二维海面的光散射，计算中将每一面元看成一具有微粗糙度的粗糙面而不是近似地当作平面，并利用投影法与射线追踪法数值计算了一定入射角和散射角下的遮挡函数，有效地提高了海面光散射计算的精确性。最后利用太阳光的光谱辐照度数值模拟了海面的3μm-5μm红外散射图像，对于红外探测器抑制海面反射太阳光造成的亮带干扰具有一定的参考价值。     

小掠射角下高斯谱粗糙海面反射损失建模

在散射能量基本为前向散射且集中在“镜面反射”方向的情况下,粗糙海面反射损失建模是声呐信号传播建模必不可少的一部分,尤其对于中远距离下浅海或者存在表面声道的水声环境,小掠射角(10°以内)下的粗糙海面反射损失建模尤为重要。首先基于高斯谱粗糙海面模型,通过高海况下的声传播试验数据处理分析了粗糙海面边界条件下的Ramsurf声传播模型的有效性,进而以Ramsurf声传播模型为基准,在小掠射角下,比较分析了Kirchhoff近似(KA)海面反射损失模型和小斜率近似(SSA)海面反射损失模型,数值计算结果表明,在小掠射角下SSA海面反射损失模型与Ramsurf计算结果较为吻合,是比较精确的海面反射损失模型。      

基于数值方法的激光海面漫反射通信能量分布特性

 针对激光点对点通信方式的不足,提出了利用海面作为激光漫反射媒介进行组网通信的设想,采用前后向迭代的数值方法结合Green函数谱积分加速算法(FBM/SAA)对激光海面漫反射通信的能量分布特性进行了研究。通过对激光光束入射海面后产生的散射场的分析、计算及实验验证,得出了较为准确的2维激光海面双站散射系数,并对激光光束入射海面后的散射场进行了分析。研究结果表明：入射激光束经过粗糙海面散射后,能量大部分集中在前向散射区域上,而后向散射强度很弱,且在散射场的边缘处能量迅速衰减,说明了激光海面漫反射组网通信方法的可行性。通过与前后向迭代法和Kirchhoff近似方法的计算结果的比较,说明了FBM/SAA是一种高效、准确的计算方法。     

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     

二维导体粗糙面电磁散射的分形特征研究

采用二维归一化带限Brown分形函数来模拟二维分形粗糙面,主要利用基尔霍夫近似给出了该导体粗糙面的电磁散射场.导出了平均散射场、平均散射系数和散射强度方差的计算公式.讨论了散射场分布与分维的关系,获得了散射场波峰拟合线的斜率与分维D满足线性关系这一重要结论. 关键词：     

Study of the Electromagnetic Scattering from the Very Rough Fractal Sea Surface

In this paper, based on the fundamental formulae of the first-order and second-order Kirchhoff approx-imation mad with consideration of the shadowing effect, the backscattering enhancement of the one-dimensional very rough fractal sea surface with Pierson-Moskowitz spectrum is studied under the second-order Kirchhoff approximation at microwave frequency. The numerical results are compared with those of the first-order Kirchhoff approximation and integral equation method. The dependencies of the bistatic scattering cross section and the backscattering enhancement on the incident angle, fractal dimension, and windspeed over the sea surface are analyzed in detail.     

Study of the Electromagnetic Scattering from the Very Rough Fractal Sea Surface

In this paper, based on the fundamental formulae of the first-order and second-order Kirchhoff approx-imation and with consideration of the shadowing effect, the backscattering enhancement of the one-dimensional veryrough fractal sea surface with Pierson-Moskowitz spectrum is studied under the second-order Kirchhoff approximationat microwave frequency. The numerical results are compared with those of the first-order Kirchhoff approximation andintegral equation method. The dependencies of the bistatic scattering cross section and the backscattering enhancementon the incident angle, fractal dimension, and windspeed over the sea surface are analyzed in detail.     

Electromagnetic backscattering from one-dimensional drifting fractal sea surface Ⅱ:Electromagnetic backscattering model

Sea surface current has a significant influence on electromagnetic(EM) backscattering signals and may constitute a dominant synthetic aperture radar(SAR) imaging mechanism. An effective EM backscattering model for a one-dimensional drifting fractal sea surface is presented in this paper. This model is used to simulate EM backscattering signals from the drifting sea surface. Numerical results show that ocean currents have a significant influence on EM backscattering signals from the sea surface. The normalized radar cross section(NRCS) discrepancies between the model for a coupled wavecurrent fractal sea surface and the model for an uncoupled fractal sea surface increase with the increase of incidence angle,as well as with increasing ocean currents. Ocean currents that are parallel to the direction of the wave can weaken the EM backscattering signal intensity, while the EM backscattering signal is intensified by ocean currents propagating oppositely to the wave direction. The model presented in this paper can be used to study the SAR imaging mechanism for a drifting sea surface.     

Electromagnetic backscattering from one-dimensional drifting fractal sea surface I: Wave-current coupled model

To study the electromagnetic backscattering from a one-dimensional drifting fractal sea surface, a fractal sea surface wave-current model is derived, based on the mechanism of wave-current interactions. The numerical results show the effect of the ocean current on the wave. Wave amplitude decreases, wavelength and kurtosis of wave height increase, spectrum intensity decreases and shifts towards lower frequencies when the current occurs parallel to the direction of the ocean wave. By comparison, wave amplitude increases, wavelength and kurtosis of wave height decrease, spectrum intensity increases and shifts towards higher frequencies if the current is in the opposite direction to the direction of ocean wave. The wave-current interaction effect of the ocean current is much stronger than that of the nonlinear wave-wave interaction. The kurtosis of the nonlinear fractal ocean surface is larger than that of linear fractal ocean surface. The effect of the current on skewness of the probability distribution function is negligible. Therefore, the ocean wave spectrum is notably changed by the surface current and the change should be detectable in the electromagnetic backscattering signal.     

Performance comparison of detection methods for weak target based on two-dimensional fractal sea surface

A detection method of the weak radar target is studied by applying fuzzy theory and multifractal correlation theory based on a two-dimensional fractal sea surface model. Firstly, a two-dimensional fractal sea surface model and its backscattering coefficient are introduced, the backscattering coefficient is a universal model affected by seawater permittivity, electromagnetic wave incidence angle, incident frequency, wind speed and wind direction factors. A novel two-dimensional wideband radar echo model, which is considered as a time-domain convolution of the stepped frequency signal radiated by airborne radar and the backscattering coefficient, is derived. Secondly, multifractal correlation theory is elaborated and a computation method of a membership degree of multifractal correlation spectrum is proposed, fuzzy theory and the AdaBoost algorithm are applied to the target detection. Finally, several target detection methods are compared with CA-CFAR and works of the predecessors. The results of the comparative study show its rationality of the two-dimensional wideband radar echo model and the superiority of wideband radars in detection performance, it is also seen that the multifractal correlation spectrum outperforms the multifractal spectrum in the probability of detection.     

Optical backscattering measurements from fractal surfaces and their possible relevance to radar scattering from the sea surface

In spite of recent progress, existing theories of electromagnetic wave scattering by rough surfaces cannot yet explain several important features of microwave sea echo, particularly in grazing incidence geometries. To explain them it is necessary to develop scattering models that can reproduce the features in a robust way. The aim of this paper is to report laboratory optical frequency experiments designed to aid achievement of this objective. In particular, polarization sensitive measurements of the backscattered intensity are reported for a range of characterized fractal surfaces using visible laser radiation. The appearance of enhanced backscattering peaks are observed even quite close to grazing incidence for some types of surfaces and indicate the importance of multiple scattering. It is also shown that the dependence of the backscattering cross-section on incidence angle bears some gross similarities to that obtained from microwave scattering from the sea surface for high index fractal surfaces that exhibit intermittency.     

Investigation of EM scattering from electrically large sea surface with breaking wave at low grazing angles

According to the high-resolution radar data, backscattering scattering from sea surfaces may give rise to super events of strong scattering and large Doppler offsets for horizontal (HH) polarization at low grazing angles (LGA), which are usually considered to be caused by breaking waves. In this paper, the mesoscale breaking wave model is introduced to account for the contribution of breaking waves, which can be responsible for sea spikes and high Doppler shift. The backscattering radar cross-section (RCS) from a single breaking wave is computed and the scattering from sea surfaces is analyzed by a composite surface scattering model, in which the phase factor of every facet is modified by the capillary waves. By using the slope criterion, the spatial distribution of the breaking waves is obtained. At the same time, the RCS of sea surfaces with breaking waves is calculated and the results agree with measured data well at LGA for HH polarization. Considering the Bragg phase velocity, orbital motion of facets and wind drift, the Doppler spectrum of sea surface with breaking wave is simulated, and when compared with the result without breaking wave, there is a good improvement for HH polarization, and for the vertical polarization, the revision is little, which are agreeable with the results of recent researches.     

Radar backscattering from Gerstner's sea surface wave

In the framework of a two-scale scattering model, radar backscattering from the rough sea surface was considered. The sea surface was modelled as a superposition of a nonlinear, large-scale Gerstner's wave and small-scale resonant Bragg scattering ripples. The zero-order diffracted field was found by a geometrical optics approach, with shadowing taken into account, and by an 'exact' solution of the diffraction problem obtained numerically. For vertical and horizontal polarizations, the spatial distribution of specific scattering cross sections along the large-scale wave was obtained. The spatially averaged specific backscattering cross sections, as well as the mean Doppler frequency shifts at both polarizations, obtained by the geometrical optics approach are compared with those obtained by using the 'exact' solution of the large-scale diffraction problem. The roles of shadowing and multiple wave scattering processes are discussed, and qualitative explanations of the difference between these two approaches are given.     

Doppler spectrum of polarimetric scattering field from two-dimensional time-varying nonlinear sea surfaces

Polarimetric scattering model of second-order small-slope approxi-mation combined with “choppy wave" model (CWM) for describing nonlinear hydrodynamic interactions between ocean waves is utilized in this paper to investigate the influence of sea surface nonlinearities on backscattering coefficient as well as Doppler spectrum signatures including Doppler shift and spectral bandwidth. Simulation results show that at moderate to large incidence angles the Doppler shift and spectral bandwidth of the CWM nonlinear sea surfaces are significantly larger than those of linear sea surfaces, in particular at low grazing angles. In addition, Doppler signatures show distinct polarization dependence, and most importantly the cross-polarized Doppler signatures significantly differ from the co-polarized ones. It is also indicated that co-polarized Doppler shift increases obviously with wind speed increasing, whereas the cross-polarized Doppler shift looks less sensitive to wind speed variations. The difference of Doppler signatures between co- and cross-polarization is potentially valuable for ocean remote sensing applications, especially for observing very high winds.