Critical Study of the Radiative Transfer Approach for the Remote Sensing of Layered Random Media.

摘要

The Radiative Transfer (RT) approach is widely used in applications involving scattering from layered random media with rough interfaces. Although this approach involves approximations, in most applications they are not explicitly stated or well understood. In order to better understand the RT approach to our problem we adopt a statistical wave approach and then transition to the RT equations. The geometry of our problem consists of a multi- layer discrete random medium with rough boundaries which are planar on the average. The random medium in each layer consists of a homogeneous background medium in which discrete scatterers are randomly distributed. The regions above and below the random medium stack are homogeneous. Using the Greens functions of the problem without the volumetric fluctuations we represent our problem as a system of integral equations. Employing the T-matrix description we first average with respect to volumetric fluctuations and then use the Twersky approximation to obtain a system of integral equations. We next average with respect to surface fluctuations, apply the weak surface correlation approximation and arrive at a closed system of integral equations for the first and second moments of the electric fields. We use the Wigner transforms to translate the coherence functions to radiant intensities, which are the fundamental quantities in the RT approach. On applying the quasi-static field approximation we hence arrive at a system of equations identical to those used in the RT approach. From this study we learn that there are more conditions involved in the RT approach than widely believed to be sufficient. This means that the applicability of the RT approach is much smaller than what is generally conceived by the remote sensing community.