TRANSFORMATION OF THE ANGULAR SPECTRUM OF SCATTERED RADIATION OF THE POINT SOURCE PASSING ABSORPTIVE RANDOMLY INHOMOGENEOUS LAYER

Transmission of radiation from a point source through a plane layer of an absorptive medium with randomly smooth inhomogeneities of dielectric permittivity is considered. The influence of the distance between both the emitter and the receiver and the layer boundaries is analysed numerically for power-law spectrum of dielectric permittivity fluctuations using Monte-Carlo method. Angular power spectrum can have two or three maxima. A relative disposition of the emitter and the receiver with respect layer determines their amplitude, position and width. INTRODUCTION It is well known nowadays that absorption of electromagnetic waves in random inhomogeneous media exerts significant influence on statistical characteristics of the angular power spectrum (APS) of scattered radiation [1–3]. The case of random inhomogeneous layer between the source and the receiver has the great practical importance. Remote sensing of ocean water or dense clouds by translucent method is a good example of such situation. In this case APS at the observation point substantially depends on the layer properties as well as the mutual disposition of the source, the receiver and the layer. This problem has been solved in papers [1–3] using the ray-optics approximation [4] in the case of small-angle scattering. In this paper we research the APS in random absorptive medium with power-law spectrum of permittivity fluctuations by the statistical simulation (Monte-Carlo) method. This approach gives more informative results than the ray-optics solution and is free from small-angle scattering restriction. FORMULATION OF THE PROBLEM Let a plane layer of random inhomogeneous absorptive medium be irradiated by a spherical electromagnetic wave. This wave is emitted from a point source located in homogeneous non-absorbing medium with dielectric permittivity 0 at a distance 1 L above the layer. The layer has thickness Z and it’s dielectric permittivity equals ) ( 1 0 r i ( 0 1 ), where (r) 1 is a random function of the spatial coordinates, describing permittivity fluctuations. The receiver is located in a homogeneous non-absorbing medium 0 on a distance of 2 L below the layer. The straight-line between the source and the receiver makes the angle with the normal direction to the boundary. The expression of the correlation function of the complex field at the observation points spaced apart at a very small distance has been obtained in [1–3] using the ray-optics approximation [4] in the case of small-angle scattering. It has been shown that in the most interesting case of strong fluctuations of the phase, the APS (which is the Fourier transformation from the correlation function) has a Gaussian form. In these papers the expressions for the first and second order statistical momenta have been obtained. These moments (the displacement of the APS maximum and APS widths in the incidence and transversal planes) are expressed through spatial derivatives of the correlation function of the complex field. Unfortunately, this solution has some serious restrictions for it’s practical usability. The most important restriction is the limited depth of the layer, where we can use the small-angle approximation. With immersion depth increasing, the APS loses it’s Gaussian form and transforms into more complicated thing. Due to a general character of statistical simulation method, we are able to calculate the APS at any reasonable depth in the layer. The serious disadvantage of this approach is the enormous time, needed for APS calculation. We could not compute the APS using the standard C programming language and only deep program optimization at machine-code level allowed us to get reliable results.