太赫兹低频段随机粗糙金属板散射特性研究

该文提出了一种高效混合近似算法计算太赫兹频段无限薄金属板的电磁散射特性。在太赫兹低频段,金属目标可以被视为具有微粗糙表面的理想导体,散射场可以分为相干场和非相干场。该文采用物理光学法结合截断劈增量长度绕射系数法和微扰法来计算金属板的电磁散射分布。基于蒙特卡洛方法,分别利用多层快速多极子和提出的混合算法计算太赫兹低频段金属板的雷达散射截面,仿真结果表明该文提出的混合算法能够高效快速地给出太赫兹低频段金属板的电磁散射特性。      

Transport of the mutual coherence functions and the intensity of a backscattered pulse in a random medium

Starting with a Helmholtz wave equation, the transport equations for the coherent and incoherent parts of the two-frequency two-position mutual coherence function in the transform domain can be derived. These equations have their respective analogs in the conventional radiative transfer theory. By solving the equations for the coherent part and the incoherent part, the incoherent scattered intensity is expressed as an infinite series, each term of which can be interpreted as an "nth order" scattered contribution. A procedure is given to compute the effective wavenumber and the total cross section, starting from a proper form of the mean Green's function. The intensity of a backscattered pulse is next investigated. A series expression is obtained. In order to be able to localize the position of pulse returns, "higher order" scattering terms must not be important when compared with the "single" scattered pulse. Numerical examples, applicable to the microwave scattering, show that the backscattered intensity is reduced when it is compared with the Booker-Gordon formula, even when the mean-square dielectric fluctuation is very small. This reduction is attributed to the multiple scattering effects, especially the multiple large-angle scatterings.     

The Contribution of Wedge Scattering to the Radar Cross Section of the Ocean Surface

This letter considers the contribution to the radar cross section of the ocean surface due to scattering from edges for which the local radius of curvature is small compared with the radar wavelength. An analytic expression based on the method of equivalent currents is given for such scattering and is evaluated for several assumed sets of parameters. This contribution is shown to augment the Bragg scattering cross section in regions where the latter underestimates the measured radar cross section, while remaining smaller than the Bragg component elsewhere.     

Bistatic scattering by arbitrarily shaped objects with rough surface at optical and infrared frequencies

The scattering cross sections for arbitrarily shaped dielectric objects with rough surface are determined for optical and infrared frequencies using the Kirchhoff approximation. The formula of the coherent scattering cross section is derived, and numerical method of incoherent scattering cross section is given. As a specific example, the infrared laser scattering cross sections of rough spheres are calculated at 1.06 μm.     

Radar Volume Backscatter From Spatially Extended Geophysical Targets in a “Slice” Approach

This paper presents an assessment of the radar backscatter from a spatially extended geophysical target (SEGT) based on a semiempirical (SE) model. An SEGT is any geophysical object that is at least semitransparent to radar illumination (clouds, rain, snowfall in the atmosphere, thick snow cover of the ground). The existing SE model does not take into account the statistical properties of the SEGT's media. To improve the SE model, a so-called “slice” approach is applied. In this approach, the particles located close to the wavefront of the radar illumination are assumed to produce backscatter that is mainly coherent. This method allows the contribution of the microphysical parameters of the scattering media to the volume component of the radar cross section to be described more comprehensively than the SE model based on the incoherent approach. It is shown that the slice concept results in the original SE model in the particular case when the particle number fluctuation within the slices pertains to the Poisson law.      

Estimation of Incoherent Scattered Field by Multiple Scatterers in Random Media

This paper proposes a method to estimate directly the incoherent scattered intensity and radar cross section (RCS) from the effective permittivity of a random media. The proposed method is derived from the original concept of incoherent scattering. The incoherent scattered field is expressed as a simple formula. Therefore, to reduce computation time, the proposed method can estimate the incoherent scattered intensity and RCS of a random media. To verify the potential of the proposed method for the desired applications, we conducted a Monte‐Carlo analysis using the method of moments; we characterized the accuracy of the proposed method using the normalized mean square error (NMSE). In addition, several medium parameters, such as the density of scatterers and analysis volume, were studied to understand their effect on the scattering characteristics of a random media. The results of the Monte‐Carlo analysis show good agreement with those of the proposed method, and the NMSE values of the proposed method and Monte‐Carlo analysis are relatively small at less than 0.05.     

The effect of ionospheric reflected noise on the performance of anorbital-debris radar system

An orbital-debris radar system was designed to detect the presence of small objects in low Earth orbit by reflecting radio waves off the objects. The author provides a rigorous derivation of the ionospheric reflected noise power and provides an integration over the complete pulse period. Free electrons and ions contained in the ionosphere cause incoherent scatter of the radar signal. This ionospheric reflection tends to increase the noise at the terrestrial radar receiver. A parameter called the ionospheric scattering cross section per unit volume, which is a function of altitude, is useful for computing the power of the ionospheric reflection signal. The Doppler frequency speed of the ionospheric reflected signal is also a function of altitude. The ionospheric noise of a 9-GHz orbital-debris radar receiver is computed using these concepts. Annual and diurnal variations of the noise are included     

低频太赫兹准目标雷达散射截面的实验研究

以研究太赫兹雷达散射截面的特性为目的,选用所搭建低频太赫兹雷达测试系统,并借助于标准目标法开展了有关太赫兹雷达粗糙铝盘散射截面的实验研究工作.实验结果表明:在小角度散射中太赫兹雷达散射截面随散射角的增大变化比较明显,在散射角超过5后太赫兹雷达散射截面随散射角的变化趋向缓慢,但当散射角超过12后探测信号的强度已衰减到无法测量,在太赫兹雷达散射截面的测试中没有出现微波雷达散射截面的大小随散射角的变化而剧烈振荡的现象;将测试结果与同尺寸微波、激光雷达散射截面的结果进行了对比,得到结论:在0附近太赫兹雷达散射截面的数值比同尺寸微波雷达散射截面的数值要小两个数量级,但比同尺寸激光雷达散射截面的数值要高一个数量级.     

曲靖非相干散射雷达在空间碎片探测中的应用

目前,空间碎片探测方面的研究越来越受重视,曲靖非相干散射雷达的建成进一步加快了我国空间碎片探测的步伐.本文基于曲靖非相干散射雷达基本特性,首先利用Mie理论研究了498MHz、500MHz和502MHz三个频率理想球形目标的散射特性并进行了分析,结果表明目标大部分散射能量分布在前向及其附近方向;然后以编目为14209的空间碎片为例,通过该雷达的探测得到其后向雷达散射截面（RCS）为0.0043m²,并给出了该雷达的最小可探测目标,这说明了曲靖非相干散射雷达在空间碎片探测方面具有优良的性能,目标电磁散射截面按500MHz计算正确有效,并能够满足工程需要.最后,以西安7.3m和1m天线为例分析了该结论对非相干散射雷达优化布站以及组网探测空间碎片具有参考价值.     

情报雷达对隐身目标侦察的数据特点分析

隐身目标具有雷达截面（RCS）随雷达照射姿态异常分布的特征,其雷达散射截面积小,电磁隐蔽性强,使得对空情报雷达的探测距离大大减小,难以可靠、连续地检测目标。结合某隐身飞机模型的散射特性,对情报雷达侦察隐身目标的数据特点进行了分析,并通过仿真得出：雷达站只能在部分姿态下测得隐身目标的断续点迹,不能对其进行连续有效的跟踪。     

Bistatic scatter from rain

An experimental investigation of bistatic scatter from rain was conducted using a 143 km scatter path at frequencies of 4.5 and 7.7 GHz. The ratio of transmitted to received power (transmission loss) was measured for scattering angles ranging from6degto130deg. Simultaneous weather radar observations were made at a frequency of 1.3 GHz. Transmission loss estimates for the bistatic scatter path were computed using the weather radar data, the bistatic radar equation, and a model for the scattering cross section per unit volume of rain based upon Rayleigh scattering by an ensemble of water spheres. The measured and estimated transmission loss values were compared to test the use of the scattering model for the estimation of interference. The averaged ratio of measured-to-calculated transmission loss for the 4.5 GHz data is 1.2pm 0.4dB. The averaged ratio for the 7.7 GHz data is -1.6pm 0.5dB. Both these values are within the combined calibration uncertainties of each measurement system. The results show that the use of the simplified Rayleigh scattering cross section model for an ensemble of water spheres adequately describes bistatic scatter for a wide range of scattering angles and frequencies below 7.7 GHz for the hydrometeor types (rain, snow, and mixed rain and snow) encountered in New England.     

平板缝隙阵列天线的雷达截面积分析

本文用矩量法、物理光学法及消奇点技术,对大型平板缝隙阵列天线的散射进行了研究,计算了典型阵列的雷达截面积(RCS),并进行了实验验证,两者趋势基本一致。本文计算结果表明,平板对散射场的贡献(结构项)远大于缝隙对散射场的贡献(模式项)。     

Backscatter from gated fluctuating regions: A Bremmer series approach

In many monostatic configurations, the effective scattering volume is defined longitudinally by the radar gating and transversely by either the antenna beamwidth or the extent of the scattering region. Often the radar signal must traverse fluctuating transmission regions on its way to and from the range cell. Consequently, the return signal is perturbed not only by events within the range cell but contains statistical characteristics ascribable to the fluctuating transmission path as well. The limit of large-scale fluctuations (which is of interest in the current context) is dominated by strong incoherent small angle scattering. A systematic analytic sequence resembling that pursued by Bremmer [1] (though significantly complicated by the three dimensionality and temporal dependence of the fluctuations) constitutes the main subject matter of this study. The proposed iterative scheme, while properly accounting for multiple small angle scatter, orders successive iterates by the number of large angle scattering events they represent. While the discussion is carried within the physical framework of an electromagnetic (radar) application, an extension to the acoustical (sonar) analog is simple. Furthermore, the extension of the presently investigated monostatic configuration to bistatic ones is possible but is by no means trivial. The reported work has initially been motivated by an attempt to interpret on board radar cross section and spectre measurements associated with an ionized wake.     

Rapid hydrometeor bistatic scatter calculations using non‐orthogonal function expansion of reflectivity profiles

A non‐orthogonal Gaussian function expansion of vertical reflectivity profiles is proposed. In conjunction with a generalization of a previously published formula this allows bistatic radar cross‐sections to be calculated rapidly for hydrometeor scatter problems involving arbitrary reflectivity profiles. In particular, it allows scattering from melting band and ice regions to be included in the cross‐section calculation. Copyright © 2000 John Wiley & Sons, Ltd.     

一维Weierstrass分形粗糙表面光散射的基尔霍夫近似

采用Weierstrass分形函数模拟实际粗糙表面,从简谐光波满足的亥姆荷兹方程出发,利用基尔霍夫近似推导出一维Weierstrass分形粗糙表面的相干散射场,数值计算并分析了相干雷达散射截面的角分布特性。     

粗糙金属和介质目标的太赫兹散射特性分析

太赫兹频段金属和介质粗糙目标的散射特性是研究太赫兹雷达目标特性的重要基础。当目标表面的主曲率半径远远大于入射波长,且粗糙表面高度起伏与斜率起伏远小于入射波长时,根据稳定相位法和标量近似法,可获得粗糙金属和介质目标的相干散射截面和非相干散射截面。基于稳定相位法,任意目标的相干散射截面可退化为粗糙导体、光滑介质和粗糙介质目标的相干散射。该文分析了电大尺寸光滑金属铝和介质白漆球的散射截面,与Mie理论计算的介质球的散射特性吻合,散射截面误差小于0.1 dBm2。采用朗伯定理,验证了粗糙介质球的太赫兹非相干散射精确解,当目标表面剖分精度越高,非相干散射的计算精度越高。该文数值计算了粗糙介质球的太赫兹相干和非相干散射特性,分析了表面粗糙度和表面材料对散射特性的影响,为电大尺寸空间目标太赫兹散射特性分析提供了理论基础。      

Scattering, transmission, and absorption by a rough resistive sheet - E polarization

Motivated by the use of thin conducting textiles in absorbers, the interaction of electromagnetic waves with a rough resistive sheet is examined. Expressions for the fields above and below the sheet are derived in terms of an equivalent current using Gaussian beam illumination. These field expressions are then used with equations derived from the conservation of energy to find the distribution of scattered, transmitted, and absorbed power using an exact numerical solution and the Kirchhoff approximation. As a consequence of the lack of refraction, the Kirchhoff single scattering solution produces strong coherent transmitted intensities at the forward angles with an attendant minimum in the incoherent intensity. A second minimum in the incoherent intensity is observed in the antispecular direction. Fields arising from multiple scattering interactions are observed independent of the Kirchhoff single scatter contributions at these angles for surfaces of large root-mean-square slope. An increase in the absorption in the sheet is also observed as surface roughness is increased. The increase in absorption is believed due to multiple scattering interactions on the surface.     

A numerical study of scattering from an object above a rough surface

A numerical model is applied in a Monte Carlo study of scattering from a three-dimensional penetrable object above a lossy dielectric rough interface. The model is based on an iterative method of moments solution for equivalent electric and magnetic surface current densities on the rough interface and equivalent volumetric electric currents in the penetrable object. Both time-and frequency-domain results are investigated to illustrate the relative importance of coherent and incoherent scattering effects in the sample problem considered. Results show that a four-path model using a reduced-reflection coefficient can be reasonable for coherent scattering predictions and that incoherent object/surface interaction effects can make significant contributions to received cross sections.     

不均匀非磁化等离子体覆盖三维目标的FDTD分析

针对典型的目标模型,采用RC-FDTD算法计算不均匀非磁化等离子体覆盖三维导体球的双站和单站雷达散射截面（RCS）。分析了等离子体电子密度分布和等离子体碰撞频率对目标RCS缩减的影响。数值结果表明,等离子体隐身技术显著;双站结果显示,不均匀非磁化等离子体能在较大双站角范围内减小目标的RCS;单站结果说明,等离子体包层能在较大的频率范围内对目标RCS的缩减起作用。选择合适的等离子体参数可以加强隐身效果。