ISSN:0920-5071    

DOI:10.1163/156939391X00518

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The effective propagation constants and attenuation rates in dense nontenuous media with coated dielectric particles governed by size distributions are studied. A coated particle consists of two layers with different materials. In a dense medium, particles do not scatter independently and the effects of correlated scattering have to be included. The correlated scattering can be taken into account by using the quasicrystalline approximation (QCA) or the quasicrystalline approximation with coherent potential (QCA-CP). The Lippmann-Schwinger equation for coated particles is solved analytically to calculate the T-matrix for both QCA and QCA-CP. We also consider the dense medium with particle sizes governed by size distribution. The pair distribution functions for particles of different sizes are computed by using the Percus-Yevick approximation. For small particles, closed form analytic expressions are obtained for the complex effective propagation constants. For moderate size particles, the complex effective propagation constants are calculated numerically by searching for complex roots of a determinantal equation. We illustrate the numerical results of the complex effective propagation constants using the modified gamma size distribution. The case of ice particles coated with a thin water layer studied shows a strong attenuation due to absorption. We also show that a medium consisting of a broad size distribution of particles tends to follow independent scattering more than a medium with a narrow size distribution.

ISSN:0920-5071    

DOI:10.1163/156939391X00527

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Scalar wave propagation in an isotropic randomly fluctuating medium is analyzed in terms of the multiple-scattering expansion, i.e., expansion of the scattering coefficient in powers of the fluctuation strength. It is shown that, in the limit of small absorption and sufficiently weak fluctuations, the leading terms in the multiple-scattering series can be summed exactly, and represented in terms of the radiative transfer equation (or its modification, taking into account back-scattering enhancement). On the other hand, the full series, including non-leading terms, is only asymptotic (non-convergent). For a weakly absorbing medium, and a typical shape of the correlation function, the first, the next-to-leading, correction to the cross-section is of the order π δ (2,πα/λ0)3, δ being the variance of two-point correlations, α the correlation length, and λ0the wavelength. The main effect of this correction is to modify the interaction kernel of the radiative-transfer equation in the way that favors individual back-scattering events in the multiple-scattering process.

ISSN:0920-5071    

DOI:10.1163/156939391X00536

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

This paper compares the first- and second-order moments computed by applying two distinctly different methods to the one-dimensional wave equation. The transmission-line form of the one-dimensional wave equation is a special case of a spectral-domain formulation that we developed earlier, but the principal results derived in this paper can be generalized to three dimensions. An essentially complete statistical description of the scattered wave field can be obtained from the method of invariant imbedding, but that method is difficult apply to the three-dimensional problem. The more restrictive Markov approximation, which is used to characterize forward scattering in continuous random media, can be applied in general treatments that accommodate backscatter. We show first that the closure hypothesis obtained by using the Novikov-Furutsu theorem provides a closed system for computing field moments that is correct to terms that are second-order in the perturbation to the constitutive parameter. The Markov approximation is then used to simplify these equations, whereby simple solutions are obtained. We compare these results to the more general results obtained from invariant imbedding. The effective wavenumbers for the coherent wave fields obtained by the two methods differ only by a phase factor that can be neglected under the Markov approximation. For the second-order moments, we develop some general constraints that apply to all solutions, and then show that the Markov approximation leads to the equations of radiative transfer ; however, the radiative transfer equations agree with the more general solution only if an optical-depth parameter is less than unity. The ramifications of these results are discussed.

ISSN:0920-5071    

DOI:10.1163/156939391X00545

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

A model representing vegetation as an ensemble of randomly oriented dielectric disks overlaying an infinite half space with a rough boundary has been implemented to compute in a unified approach both the microwave backscattering coefficient and the emissivity. A parametric analysis has been carried out at X-band to assess the effects of the single soil and vegetation parameters on the responses of active and passive sensors. To validate the model, its numerical results have also been compared with experimental data collected on the field by radar and radiometric systems at L-, X- and Ku- bands.

ISSN:0920-5071    

DOI:10.1163/156939391X00554

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

It is shown that the standard surface integral equations for a dielectric surface naturally lead to an iterative solution which does not reduce properly when the surface becomes perfectly conducting. This means that an inconsistency is contained in such a solution which is expected to be poor for surfaces with moderate or large dielectric values. Many studies have shown that the Kirchhoff fields provide excellent initial estimates. Hence, a reformulation of the surface integral equations is carried out to permit the use of the Kirchhoff fields as the initial estimates. It is shown that the resulting iterative solution of the tangential fields reduces correctly when the surface becomes perfectly conducting. This formulation allows an analogous development of surface scattering models for dielectric surfaces as was done for perfectly conducting surfaces.

ISSN:0920-5071    

DOI:10.1163/156939391X00563

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The standard Kirchhoff surface scattering model is extended to include the third order surface statistics which account for the skewness in surface geometry. The bistatic scattering coefficient is derived in terms of the surface correlation function and the surface skewness function. Numerical illustrations are carried out for the backscattering coefficient and the effect of the skewness function is also illustrated. It is found that major changes in the backscattering coefficient due to a change in direction appears through the directional surface correlation function, while a change in the sense of direction is reflected through the skewness function of the surface.

ISSN:0920-5071    

DOI:10.1163/156939391X00572

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

ISSN:0920-5071    

DOI:10.1163/156939391X00581

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor