摘要:

In this paper the Rayleigh hypothesis in the theory of reflection by a cylindrical perturbation in a plane surface is investigated analytically. The hypothesis asserts that above the surface the scattered field may be expanded in terms of outward‐going wave functions. As such, it is analogous to the assumption made by Lord Rayleigh in his treatment of diffraction by a reflection grating. We show that the validity of the Rayleigh hypothesis is governed by the distribution of singularities in the analytic continuation of the exterior scattered field. Conditions are derived under which the Rayleigh hypothesis is rigorously valid. A procedure is presented that enables the validity of the Rayleigh hypothesis to be checked for a surface whose profile can be described by an analytic function. Numerical results are presente

ISSN:0048-6604    

DOI:10.1029/RS015i004p00723

年代:1980

数据来源: WILEY

摘要:

Exact expressions for the current induced on a pair of horizontal wires above the earth by a vertical electric dipole have been available for some time. It is well known that the wire currents can be expanded as a continuous spectrum current plus a sum of discrete modal currents. Each current consists of a monofilar and a bifilar component. In this paper, conditions for which the currents can be approximated either by the discrete modal currents or a simple approximation to the continuous spectrum current are found. For simplicity it is assumed that the earth is perfectly conducting and that the wires are thin and can be characterized by a surface impedance boundary condition. Using steepest descent integration, the case for grazing incidence (distance from source to observation pointRgreater than the perpendicular distance from the source to the center of the wire arrayr) is examined. It is found that either the monofilar or the bifilar component of current can be written as a single term which is proportional to 1/RifRis of the order ofror to 1/R5/2ifRis ‘very large.’ In the intermediate range the discrete modal currents dominate the solution. Transition solutions valid between these ranges are gi

ISSN:0048-6604    

DOI:10.1029/RS015i004p00733

年代:1980

数据来源: WILEY

摘要:

The electric field near the surface of the earth was measured in the vicinity of an eccentrically insulated, traveling wave antenna buried in the earth. All three mutually orthogonal components were measured as a function of both azimuthal angle and of radial range.

ISSN:0048-6604    

DOI:10.1029/RS015i004p00743

年代:1980

数据来源: WILEY

摘要:

The image formed by a side‐looking radar system of a scene characterized by a temporally and spatially varying reflectivity density effectively distributed over a plane is given. The implied spectral density transformation, with its intrinsic distortion of the scene's wave number spectral density, is noted. In the remainder it is assumed that the temporal and spatial behaviors are connected via a dispersion relation. It is shown that the image is a linear, space‐variant transformation of the scene at, say,t= 0, which can be described as the concantenation of three linear transformations: the first two are space‐invariant, one being the system transformation for a time‐invariant scene, and the third is space‐variant and gives a geometric distortion, most simply a scaling and skewing, which is present in any system using a side‐looking scan. Sufficient conditions are given for faithful imaging and spectral density mapping; those for the latter are more

ISSN:0048-6604    

DOI:10.1029/RS015i004p00749

年代:1980

数据来源: WILEY

摘要:

In this paper we examine some of the physical implications of using the extended Huygens‐Fresnel principle to calculate laser beam properties in strong turbulence. We shall demonstrate the roles played by the log‐amplitude and phase fluctuations and will show that quadratic phase structure functions lead to results incompatible with experimental data. We shall also prove that the Huygens‐Fresnel principle automatically predicts a normalized irradiance variance of unity and Gaussian field statistics in strong turbulence provided that certain conditions on the turbulence strength and spectrum are sati

ISSN:0048-6604    

DOI:10.1029/RS015i004p00757

年代:1980

数据来源: WILEY

摘要:

This work presents alternative formulations of the asymptotic, inhomogeneous wave tracking theory (IWT) proposed by Choudhary and Felsen and investigates the basic analytical features of the ensuing operators. The outstanding deviations from classical ray theory are traceable to the fact that the generalized system of equations is of the quasi‐linear, elliptical type. Consequently, representation via an initial value formulation, standardly posed by geometrical optics (and in the quasi‐homogeneous limit, by IWT) must be approached with caution, and an appropriate boundary value formulation should be considered. Questions of existence, uniqueness, stability, and locality are discussed. The following alternative, but by no means equivalent, formulations are considered: (1) first‐order, quasi‐linear system of equations cast into a canonical form, the existence and uniqueness (but not the stability) of whose solution is ensured by the Cauchy‐Kowalewsky conditions, (2) a quasi‐linear, second‐order, elliptical differential equation subject to Dirichlet boundary conditions and to the requirements of elliptical uniformity, (3) same as formulation 2, but posed as a two‐dimensional variational problem, and (4) quasi‐linear, second‐order, phase trajectory equation obtained from formulation 2 via the so‐called hod

ISSN:0048-6604    

DOI:10.1029/RS015i004p00763

年代:1980

数据来源: WILEY

摘要:

This paper presents a development for backscatter from rough surfaces and inhomogeneous media that is valid for microwave frequencies. Calculations of polarized and depolarized backscatter are presented for several values of dielectric constants and volume reflection coefficients of direct interest in active microwave remote sensing of soil moisture content. This paper confirms the finding that depolarized backscatter measurements are much less dependent on incident angle than like polarized backscatter measurements. However, most depolarized measurements reported in the literature exhibit excessive dependence on incident angle. This is an artifact of the measurement technique and not of the depolarization phenomenon.

ISSN:0048-6604    

DOI:10.1029/RS015i004p00773

年代:1980

数据来源: WILEY

摘要:

The main objective of this paper is to examine the frequency dependence of slant path rain attenuation at 15 and 35 GHz in order to determine how well the attenuation at one frequency can be predicted from that at another; this has application in the design of earth‐to‐satellite communications. Also, some inferences on the drop size distribution of rain can be drawn from simultaneous attenuation measurements at two frequencies. The theory of rain attenuation is reviewed, and it is noted that the attenuation is a complex function of drop size, shape, orientation, index of refraction, and rain intensity along the path. Attenuation ratios are computed on the basis of both drop size and rain rate for frequencies at which there are measured data. Results obtained by other investigators are reviewed, and it is found that frequently, attenuation ratios that would not have been predicted on the basis of a surface rain model such as a Laws and Parsons model have been reported; indications are that attenuation is often produced from a widely dispersed distribution of large raindrops. During eight rainy days in 1975–1976, more than 10,000 simultaneous measurements of 15‐ and 35‐GHz attenuations were recorded in the Boston area. On the basis of these results it is concluded that it is not possible to represent the drop size distribution of rain along a slant path in the Boston area by a Laws and Parsons model, since the ratios of the 35‐GHz attenuations to the 15‐GHz attenuations are significantly below those that would have been predicted by that model. The ability to predict slant path rain attenuation at one frequency on the basis of that at another is shown to be a function of frequency separation, climatology, and the type of statistics whic

ISSN:0048-6604    

DOI:10.1029/RS015i004p00781

年代:1980

数据来源: WILEY

摘要:

Approximate expressions for high‐frequencyDregion refractive indices at mid‐latitudes are developed from the exact Sen‐Wyller formula. The proposed approximations, suitable for use with partial reflection and wave interaction experiments, are significantly better than the use of the strictly longitudinal Sen‐Wyller approximations at mid‐latitude. It is suggested that this approximation be incorporated intoDregion partial reflection and wave interaction ex

ISSN:0048-6604    

DOI:10.1029/RS015i004p00797

年代:1980

数据来源: WILEY

摘要:

In this paper we examine the severity of radio wave amplitude scintillation measured at two stations near the equator but far separated in longitude: Kwajalein, Marshall Islands (167°E), and Ancon, Peru (−77°E). The data used are long‐term observations of the Defense Nuclear Agency (DNA) Wideband satellite signal intensity at VHF, UHF, andLband frequencies. The seasonal behavior of the scintillation at the two stations is similar; each shows a broad 8‐ to 9‐month disturbed season centered about local summer. There is short‐term variability in the scintillation occurrence statistics but no clear equinoctial maxima. Little difference is observed in the occurrence or severity ofLband scintillation at the two stations, although a systematic difference in the frequency dependence of the scintillation produces significantly stronger VHF and UHF scintillation at Ancon. The VHF and UHF latitudinal distributions of scintillation are asymmetric about the geomagnetic equator at bo

ISSN:0048-6604    

DOI:10.1029/RS015i004p00801

年代:1980

数据来源: WILEY