Some experimental results on bistatic scatter from rain

A bistatic scatter experiment was conducted on a 55.2-km scatter path in Japan, at a frequency of 6.72 GHz, continuously from August 1975 to September 1977. In this bistatic scatter experiment the forward scatter experiment and a direct path measurement were made simultaneously, and a scanning-beam experiment was performed during two rainfalls. Investigations were made on the relationship between the equivalent radar reflectivity factorZ_{e}for a 2.3-km common volume height above sea level and the surface rain rateR(mm/h) directly under the common volume, and the distribution ofZ_{e}. TheZ_{e}-Rrelationship was found to beZ_{e} = 151.4 R^{1.45}on an average for total data including all rain types. The distribution ofZ_{e}can be approximated by the lognormal distribution for a rainy period. The directivity of scattering from an ensemble of raindrops was measured by scanning the beam from a horn antenna in the vertical plane. For practical purposes the directivity was found to be similar to that of an electric dipole.     

Analytical calculation of bistatic radar cross-sections for rain scatter problems

A formula is derived for the effective bistatic radar cross-section of a rain cell with Gaussian reflectivity profile assuming antennas with Gaussian radiation patterns. The size and location of the rain cell are arbitrary, as is the bistatic scattering geometry. The expression is analytical, avoiding the need for numerical integration, and general, allowing treatment of cases with only partial beam intersection. Using a recently proposed model, sidelobe contributions and multiple cells embedded within widespread rain debris can be incorporated.     

Bistatic ISAR images from a time-domain code

Inverse-synthetic-aperture-radar (ISAR) images of radar targets are useful for target identification, visualization, and the analysis of scattering centers. The major advantage of bistatic over monostatic-ISAR imaging is the reduction, in the number of computed incident angles, from hundreds to one. This advantage has already been demonstrated for a physical theory of diffraction (PTD) code, XPATCH. The bistatic-imaging technique can be extended to scattering data obtained from any time-accurate or iterative method, including low-frequency algorithms. This paper presents images from data obtained with a finite-volume time-domain (FVTD) code. It also provides relations between (1) the range and resolution of the bistatic scattering data in the Fourier domain, and (2) the pixel resolution and image extent in the physical domain for the down-range and cross-range directions. A tapering function is applied in the Fourier domain, to dampen ringing effects. Results are shown for a trapezoidal plate, a cone-sphere, and a square-aperture cavity     

Bistatic specular scattering from rough dielectric surfaces

An experimental investigation was conducted to determine the nature of bistatic scattering from rough dielectric surfaces at 10 GHz. This paper focusses specifically on the dependence of coherent and incoherent scattered fields on surface roughness for the specular direction. The measurements, which were conducted for a smooth surface with ks<0.2 (where k=2π/λ and s is the RMS surface height) and for three rough surfaces with ks=0.5, 1.39, and 1.94, included observations over the range of incidence angles from 20° to 65° for both horizontal and vertical polarizations. For the coherent component, the reflectivity was found to behave in accordance with the prediction of the physical optics model, although it was observed that the Brewster angle exhibited a small negative shift with increasing roughness. The first-order solution of physical optics also provided good agreement with observations for hh-polarized incoherent scattering coefficient, but it failed to predict the behavior of the vv-polarized scattering coefficient in the angular range around the Brewster angle. A second-order solution is proposed which appears to partially address the deficiency of the physical optics model     

Bistatic scattering from three-dimensional layered rough surfaces

An analytical method to calculate the bistatic-scattering coefficients of a three-dimensional layered dielectric structure with slightly rough interfaces is presented. The interfaces are allowed to be statistically distinct, but possibly dependent. The waves in each region are represented as a superposition of an infinite number of up- and down-going spectral components whose amplitudes are found by simultaneously matching the boundary conditions at both interfaces. A small-perturbation formulation is used up to the first order, and the scattered fields are derived. The calculation intrinsically takes into account multiple scattering processes between the boundaries. The formulation is then validated against known solutions to special cases. New results are generated for several cases of two- and three-layer media, which will be directly applicable for modeling of the signals from radar systems and subsequent estimation of a layered medium subsurface properties, such as moisture content and layer depths.     

One-minute rain rate distributions in Japan derived from AMeDASone-hour rain rate data

By examining measured 1-min rain rate data presently available and 1-h rain rate data obtained by AMeDAS (Automated Meteorological Data Acquisition System) in Japan, a simple conversion method of 1-h rain rate data to 1-min rain rate distribution is presented. The prediction accuracy of the method is evaluated, and good accuracy is confirmed. Maps are given of the mean and standard deviation (normalized by the mean value) of 1-min rain rate for 0.01% of time based on AMeDAS data in Japan. Moreover, the effect of a record-breaking heavy rain even on long-term statistics is discussed. Since the method has been successfully applied to various locations in Japan, it can be expected to be applicable to regions of the world where 1-min rain rate data have not yet been available     

Forward scatter and crosspolarisation from spheroidal ice particles

The forward-scatter compled amplitudes of spheroidel ice and water particles are evaluated at 20 and 30 GHz by using a field a point-matching method. The crosspolarisation resulting from a group of identical equioreintated particles is then derived. It is seen that equiorientated ice particles can cause significant crosspolarisation with little attenuation, in agreement with recent measurements of crosspolarisation on satellite paths.     

多频段典型地表的双站雷达散射回波预测

双站雷达在反隐身、超低空防御方面具有独特优势,但双站测量装置较为复杂,地表参数的准确获取工作耗时耗力,且精度难以保证,地表双站雷达散射数据极其匮乏。为解决上述问题,该文以L/S/X/Ku波段裸土、水泥地和粗糙沙地后向散射实测数据为例,忽略地表的精细结构,采用等效面散射模型和遗传算法反演了各地表的等效介电常数和粗糙度参数,获取其等效参数统计特征,实现对地表双站雷达散射回波的预测。结果表明：该等效面散射模型保证了地表的后向和双站散射回波预测精度;地表双站雷达散射回波随入射波频率的增大而增大;随散射角的增大先增大而后减小,并在镜像方向出现最大值;随散射方位角的增大,地表散射回波先减小而后增大,HH极化双站散射回波的最小值一般出现在 方位角处,而VV极化双站散射回波的最小值位置随入射角的增大从 方位角向小角度方向偏移,并与入射波频率、地表湿度以及粗糙度参数相关,该双站散射特性可用于地表参数的反演以及目标的反隐身研究。     

Bistatic scattering from a vegetative medium simulated by cylinders

By considering the vegetation as a regular homogeneous distribution of individual scatterers and thus modeling it as an array of perfectly conducting finite thin circular cylinders over a ground plane, bistatic cross section is determined using the image principle and its variation with the angle of incidence, length, and diameter is studied.     

Bistatic clutter measurements

Empirical data have been obtained from which estimates of the bistatic reflection coefficientsigma_{0}have been determined. Measurements were made on two terrain types (grassy terrain and an area of tall weeds and rough ground) at low grazing bistatic angles of70deg, 75deg, 80deg, and85deg, and0degto180degazimuth angle. The terrain was illuminated at wavelengths of 3 cm and 23 cm, with horizontal polarization. Both like and orthogonal polarizations were received at each wavelength; four channels of data were recorded. The measurement technique utilized an aircraft with the 3-cm and 23-cm transmitters on board; receivers were mounted on a tower. The transmissions were pulsed, and coherent signals were recorded by utilizing a separate air-to-ground link for the receiver local oscillator signal.     

基于双基地海洋回波谱仿真数据的海浪波数谱反演

该文首次基于微扰法导出的非后向散射系数及在此基础上得到的双基地海洋回波谱仿真数据,采用Howell算法进行海浪波数谱的反演以及有效波高和风速信息的提取。文中补充了Howell算法应满足的条件,明确了Howell算法中矩阵奇异值个数r的选择原则。反演结果表明,风向趋于时,有效波高和风速反演误差会减小;根据不同海态(风速或有效波高)选择合适的工作频率可减小反演误差。在工作频率和矩阵奇异值个数选择合适时,有效波高反演误差小于3%,风速反演误差小于1%。     

Calculation of forward scatter and crosspolarisation from ice spheroids

Forward-scatter complex amplitudes for ice particles with the same eccentricities as raindrops are presented from a field point-matching calculation to a greater accuracy than in a previous letter. However, it is noted that significantly higher crosspolarisation is expected from small ice needles or plates. Arguments for the orientation of ice needles or plates in electric fields are emphasised.     

涂层目标散射的双站物理光学公式及其散射矩阵

本文从物理光学基本假设与阻抗边界条件出发，建立了涂覆雷达吸波材料（ＲＡＭ）的任意三维光滑凸型导电物体散射的基本双站公式。公式是从Ｆｒｅｓｎｅｌ反射系统及阻抗边界条件推导的。本文同时得到了涂层物体表面入射场及其同几何结构导体表面入射场之间的关系与电、磁流比系数关系。文末给出了用基本双站公式计算电大物体双站散射矩阵与双站散射截面的计算方法与计算实例。     

缝隙天线阵双站电磁散射的混合法

利用矩量法（ＭＯＭ）和等效边缘电磁流方法（ＥＥＣｓ）研究波导馈电的缝隙天线阵的双站散射问题。从理论和计算上分析,等效边缘电磁流方法可以计算有限尺寸的导体平板沿任意方向上的双站散射（包括边缘绕射场）,而矩量法可以考虑波导缝隙天线阵的散射与耦合问题,使它们混合便可以解决有限尺寸缝隙在线阵的散射问题。实际计算表明,方法是切实可行的。     

复杂自然场景双站SAR成像模拟

以单站SAR复杂场景成像模拟的映射投影(MPA)算法为基础,实现三维投影和映射的双站SAR(BISAR)成像模拟.利用条带式BISAR成像的点目标响应解析表示式,在计算得到散射系数图后,模拟BISAR原始信号的产生与压缩,以模拟BISAR图像.经过对虚拟场景的BISAR成像模拟,讨论BISAR对植被、建筑物等复杂地物观测及其与单站SAR的对比.     

双站SAR图像的极化分析

该文对双站SAR(BISAR)模拟图像不同地物的极化特征分析,发现传统单站极化特征参数(,,)在BISAR图像上不再能有效地表现地物散射的极化特征。由此,提出了统一双站极化基变换,重新定义了极化特征参数,,,使其保持原有的分离取向关联等优点。经统一双站极化基变换后,不同地物散射的极化特征更明显,重新定义的,,能反应不同散射机制,提供了BISAR图像解译和地表分类的初步手段。     

双基地合成孔径雷达

当SAR的发射和接收在同一平台上时,称为单基地SAR。近5～6年来,有多篇论文讨论了发和收分开的双基地SAR,双基地比之单基地有其他缺点和优点,由于总体技术和处理技术的进步,优点已远胜缺点,文中汇集了9篇双基地SAR,其中一篇为实验成像,其余均属理论探讨和今后5～10年内实现的打算。     

Electromagnetic scatter from small dielectric gradients in a continuous medium

A new approach is presented to the formulation and understanding of the problem of electromagnetic scatter from a slightly perturbed continuous medium. The problem is expressed in terms of periodic functions instead of conventional power series. From this formulation, specular properties are observed that are not obtainable from older considerations. The approach was verified with a laboratory experiment using microwave transmission lines with length-dependent, slowly varying dielectric cross sections. Further experiments were carried out using backscatter from a radar designed to detect clear air turbulence. The primary result shows that a turbulent medium can be replaced for the purposes of scatter calculation by a structure of finite periodicities because the medium acts as a lumped insertion loss, the stopping characteristics being determined by a spectral scale in the turbulence. The formulation defines the limits of knowledge about the medium and shows that, although it is not unique, some knowledge of the structure is obtainable. Insight is gained into the work needed to understand and interpret remote probe experiments.     

Removal of rain from videos: a review

In this paper, the algorithms for the detection and removal of rain from videos have been reviewed. Rain reduces the visibility of scene and thus performance of computer vision algorithms which use feature information. Detection and removal of rain requires the discrimination of rain and nonrain pixels. Accuracy of the algorithm depends upon this discrimination. Here merits and demerits of the algorithms are discussed, which motivate the further research. A rain removal algorithm has a wide application in tracking and navigation, consumer electronics and entertainment industries.