Summary of image theory expressions for the quasi-static fields of antennas at or above the earth's surface

Expressions for the quasi-static range (the measurement distance is much less than a free-space wavelength) field components produced by infinitesimal dipole antennas located at or above that of the earth have been derived by employing finitely conducting earth theory techniques. Formulas for the field components produced by an elevated, finite-length, horizontal electric antenna are also presented. Image theory is employed because most of the resulting integrals encountered cannot be evaluated analytically. The image theory and previously derived analytical (or in some cases, numerical integration) results are in agreement throughout the quasi-static range.     

Transient response of a dissipative earth by use of image theory

Simple, closed form formulas are derived for the transient response between dipoles at arbitrary locations above a dissipative earth. Both early time and late time responses are displayed. The case of an homogeneous earth is treated, using a single image for early times and an additional set of continuous images for late times. The results are checked favorably against published work. Multiple images are used for the case of a two-layered earth.     

Mutual coupling between loops on layered earth using images

The problem of mutual coupling between wire loops on or above the surface of a layered lossy earth is solved by using the image theory. It is shown that the image solution, besides its simplicity, is accurate both when the wavelength inside the earth is much larger or much smaller than the distances involved. In the intermediate frequency range when these distances are comparable to the wavelength or skin depth inside the earth, accuracy of the solution is retained by the addition of a set of continuous images to complement the single image representation. Numerical examples relevant to the prediction of the depth of a subsurface layer inside the ground are given.     

Error analysis of image representations for sources near to a dissipative earth

Various image representations for sources above a dissipative earth are introduced. A criterion for assessing their accuracy is given. Some of these image representations comprise purely discrete images, a purely continuous image or a mixture of both types. It is shown that the error in any representation is a function of the height of the field point from the earth's surface. A mixture of a discrete image and a simple continuous image can lead to a negligibly small error for field calculations over a wide range of the height of the field point. This means that the image theory can offer simplicity and accuracy in many application such as geophysical prospecting of the earth.     

ELF Earth Return Current Coupling

The theory for predicting the coupling between long line earth-return circuits and extremely low-frequency earth-return sources is reviewed. This theory is based on a uniform earth model. The electric fields predicted by this theory are then compared to a sampling of experimental data derived from long probe wire measurements.     

Image theory for electric dipoles above a conducting anisotropic earth

New image representations for vertical electric dipoles (VED) above an imperfectly conducting and axially anisotropic earth are developed. These include multidiscrete images at different depths below the air-earth interface and multipole image sources. It is shown that, in contrast with the available image representations in the literature, the developed ones predict the correct behavior of the fields in the far zone along the earth's surface. Extension to a layered earth's model is made. The theory is also extended to the horizontal electric dipole with similar conclusions to the case of the vertical dipole.     

Assessment of image theory for field evaluation over a multilayer Earth

The complex image theory is used to predict the electromagnetic fields excited by a line source above a horizontally stratified earth. It is shown that the image solution for fields in the air is merely an approximation of one part of the complete rigorous solution. This part is the main branch cut which dominates in the far zone, while other branch cuts and the trapped waveguide modes are completely ignored. It is thus suggested that the image solution be complemented by the trapped waveguide modes which can dominate at high frequencies and in a certain intermediate range of transmitter-receiver spacing.     

高速几何校正卡设计与实现

机载航拍摄影时,地球曲率和飞行器的姿态变化会引起传感器倾斜,传感器倾斜拍摄使图像产生几何畸变。研究了基于坐标变换与重采样的几何校正方法,有效地矫正了图像的几何畸变,并设计了适用于机载航拍图像几何校正的集成了8片TS201芯片的高速处理板卡。经工程验证,该图像校正板卡充分满足图像几何校正对运算速度和存储容量的要求,能够快速准确地实现几何校正。     

机载星敏感器成像的仿真技术研究

根据天文导航的基本原理,综合考虑了星敏感器自身噪声、星云星团、大气折射、大气扰动和气动、地球遮挡等因素对机载星敏感器成像的影响,实现了复杂环境下的星光在机载CCD星敏感器上的成像仿真。在仿真过程中,重点考虑了大气折射的影响,分析了载体在大气层内飞行时星光角距、载体姿态和折射角之间的关系,推导了星敏感器成像点位置偏移的计算公式,为进一步研究机载天文导航技术打下了基础。     

Exact image theory for the calculation of fields transmitted through a planar interface of two media

Exact image theory, recently developed for the calculation of fields reflected from a planar interface between two homogenous media, is extended for transmitted field calculation. The image of a point source is found to be a line source located in complex space for proper convergence. Unlike for the reflection image, the transmission image line is seen to be curved in complex space for efficient computation. Expressions are derived for the transmission image of the most general three-dimensional electric and magnetic sources and for media with bothepsilonandmudiscontinuous across the interface. The new theory is verified with several asymptotic tests and properties of the transmission image function are discussed. As an application, radiation from a resonant horizontal loop antenna into the ground is calculated and results are seen to agree with those given in the literature.     

粗糙表面的多路径散射模型研究

要提高雷达低角跟踪目标的精度,必须建立精确的多路径反射模型.采用小平面模型描述自然散射表面,利用基尔霍夫近似理论建立了适用于任意粗糙表面的低空多路径散射模型,考虑了阴影和多次散射对多径回波信号的影响.基于此模型仿真研究了多路径回波信号与波长及粗糙表面参量间的各种关系;分析了雷达接收到的直达信号和反射信号之间的相关性.     

Physical optics polarization scattering matrix for a top hatreflector

The polarization scattering matrix for a cylinder on a circular disk is derived subject to the physical optics (PO) approximation. The multiple scattering between the cylinder and the circular disk ground plane is obtained by invoking image theory, and is expressed as a bistatic return from the cylinder and its image, due to the image field. Results for a particular case are evaluated and compared with measurements     

地球红外辐射背景成像方法研究

为模拟生成地球红外辐射背景图像,研究其成像方法。首先将地球表面划分为若干地表区域,考虑大气传输特性、地表类型、地袁经纬度及地袁海拔高度等因素影响,根据地袁类型,建立地球袁面区域温度模型和红外辐射亮度模型．然后运用数值计算方法得到地球表面温度和红外辐射亮度分布。根据红外探测器视角与视点．利用OpenGL实现了红外探测器在太空中对地球背景红外辐射亮度的三维图像显示。结果表明,模拟生成的地球红外背景图像效果逼真,可用于红外目标识别和仿真应用。     

地球红外辐射背景成像方法研究

为模拟生成地球红外辐射背景图像, 研究其成像方法。首先将地球表面划分为若干地表区域,考虑大气传输特性、地表类型、地表经纬度及地表海拔高度等因素影响, 根据地表类型, 建立地球表面区域温度模型和红外辐射亮度模型, 然后运用数值计算方法得到地球表面温度和红外辐射亮度分布。根据红外探测器视角与视点,利用OpenGL 实现了红外探测器在太空中对地球背景红外辐射亮度的三维图像显示。结果表明, 模拟生成的地球红外背景图像效果逼真, 可用于红外目标识别和仿真应用。     

Studies in Overhead Wire--Goubau Line Above Ground

The transition theory which Kikuchi proposed first was improved and expanded in the range of the Sommerfeld line above ground and even the Goubau line (G line) above ground. Then it was proved by the experiment with the aid of the G line. The solution for the electromagnetic field produced by an overhead wire is derived using Maxwell's equations and treating the situation as a boundary value problem. In particular, the integrals which are caused by the finite conductivity of the earth and which are responsible for the distribution of the fields in the neighborhood of the surface of the earth are evaluated by means of the saddle-point method. Based on the field theory described above, the primary transmission line constants of the G line above ground (R, L, C, and G) and the secondary transmission line constants (gamma/sub 0/ = alpha/sub 0/ + j beta/sub 0/ and Z/sub 0/) were obtained, and then the equivalent circuit for the G line above ground was given. The behavior of the line from an engineering standpoint is now completely determined by the usual simple circuit theory. The transition of the G line from a ground return transmission line of a surface-wave transmission line was proved experimentally.     

A Broad-Band Directional Coupler for Both Dielectric and Image Guides (Short Papers)

A four-port directional coupler is presented for use in dielectric and image guides. Simple design equations are derived and the subsequent results are in good agreement with theory. Broad-band performance has been achieved over the entire conventiortal waveguide bandwidth (26-40 GHz).     

分布式小卫星SAR多普勒解模糊成像

该文结合实际卫星轨道及地球自转特性,研究利用分布式小卫星丰富的空域信息解多普勒模糊,从而解决宽场景成像和方位高分辨之间的矛盾。提出建立子孔径坐标系,解决地球自转引起的星间回波不一致性并简化椭圆轨道的几何复杂性。建立子孔径坐标系包括两方面:一是将成像几何模型转换到地球固定坐标系下分析,此时地面场景静止,卫星轨道等效旋转;二是分孔径处理,针对每个子孔径建立坐标几何,经过误差补偿后小卫星轨道构型等效为固定基线且平行。对每个子孔径回波数据分别进行空域滤波解模糊处理后再进行孔径拼接,同时结合传统星载SAR成像算法实现大场景高分辨成像。最后以CARTWHEEL模型为例进行仿真,验证了该方法的有效性。     

Robust velocity estimates, stream functions, and simulatedLagrangian drifters from sequential spacecraft data

The maximum cross-correlation (MCC) method has been used to compute both oceanic and cloud velocity vectors from sequences of satellite data [e.g., advanced very high resolution radiometer (AVHRR), coastal zone color scanner (CZCS), geostationary observing earth satellite (GOES)]. Unfortunately, the two-dimensional cross-correlation functions used in the computation often contain saddlepoints, which can give rise to large magnitude and direction uncertainties in the derived velocity estimates. This paper develops a numerical iterative procedure that combines image analysis methods and dynamical constraints to minimize these difficulties. The resultant velocities are both physically realistic and numerically stable. Thus, it is also possible to compute stream functions and simulated Lagrangian drifters. The validity of these results are confirmed with independent oceanic observations. Finally, the advective-diffusive equation is solved for a few oceanic applications (e.g., prediction of sea-surface temperature, dispersal of anchovy eggs and larvae) using the derived velocities     

离轴三反大视场空间相机像移速度场模型

大视场空间相机在轨成像期间,由于地球自转、卫星姿态机动和颤振等因素导致焦面像速场呈非线性各向异性分布。为此提出了一种新的基于刚体运动学的像移速度场建模方法,考虑离轴角参数,推导了离轴三反大视场空间相机的像速场解析式。以某大视场空间相机为例,分析了侧摆成像时同速与异速像移速度匹配模式对相机成像质量的影响。分析结果表明:以传函下降5%为约束,侧摆15成像时,当积分级数大于10级时应采用异速匹配模式,积分级数为32级时,异速匹配相比于同速匹配使焦面动态MTF从0.340 8提高到0.970 2。当积分级数确定为16级时,侧摆角在12.3以内时可采用同速匹配模式。在轨成像结果证明了像移速度场模型的准确性,可为大视场空间相机像移补偿提供可靠依据。     

Operator-valued transmission lines as models of a horizontally layered earth under transient two-dimensional electromagnetic excitation

A new method of computing the transient behavior of a horizontally layered earth under two-dimensional electromagnetic excitation is presented. The earth's conductivity, magnetic permeability, and electric permittivity are assumed to vary only with depth, but those variations are allowed to be quite arbitrary. Displacement current is not neglected. After discretization of Maxwell's equations, the earth is modeled by infinite RLC electrical networks. No truncations of those networks are invoked during the computational procedure. Instead, those networks are viewed as infinite ladder networks of Hilbert ports, that is, as operator-valued lumped transmission lines, and the computations are based upon the recently devised theory of such systems. This analysis yields a new surface impedance operator, which provides a considerably more comprehensive representation of the earth's behavior than does the Tikhonov-Cagniard surface impedance. For example, the Tikhonov-Cagniard surface impedance is applicable only when the electric or magnetic fields vary slowly or linearly with respect to horizontal displacements. The surface-impedance operator derived herein does not require those restrictions; only quadratic summability and Laplace transformability are needed.