Interaction of electromagnetic waves with some natural surfaces

The problem of the interaction of electromagnetic radiation with nonuniform surfaces (terrain, roadways, etc.) is of interest for predicting the apparent temperature of radiometers or radio telescopes. In this paper, the interaction is described by the differential scattering coefficients of the surface, in terms of which one may express such parameters of the surface as the radar cross section, the absorption coefficient, the albedo, etc. By making use of the reciprocity properties of the differential scattering coefficients, Kirchhoff's radiation law is derived in its most general form, which takes account of both the angular dependence and the polarization properties of the emitted radiation. Thus, the emissivity of the surface can also be expressed in terms of the scattering coefficients. General formulas for apparent surface temperature are obtained and are used to calculate the apparent temperature of an asphalt roadway and a vegetation-covered surface. The predicted temperatures are found to be in reasonably good agreement with measurements of Britt, Tolbert and Straiton at 4.3 mm wavelength.