Modeling and measurements of scattering from road surfaces at millimeter-wave frequencies

Millimeter-wave radar-based sensors are being considered for a number of automotive applications including obstacle detection and collision warning, true-speed, and road-surface recognition. The interaction of electromagnetic waves with asphalt road surfaces, possibly covered with ice or water, at millimeter-wave frequencies is studied. First, an experimental procedure for determining the effective dielectric constant of bituminous mixtures used in road-surface constructions is developed. In this procedure, the effective dielectric constant is derived using a simple inverse-scattering algorithm to the measured radar cross sections of cylindrical specimen of a standard asphalt mixture. Then the vector radiative transfer equation is used to formulate the scattering from a multilayer medium representing an ice- or water-covered asphalt surface. The University of Michigan polarimetric 94-GHz radar system was deployed for characterizing the polarimetric backscatter responses of asphalt surfaces under many physical conditions near grazing incidence angles (70°-88°). The measured backscatter coefficients and parameters of copolarized phase difference statistics of a dry asphalt surface with smooth interface at one incidence angle were used to derive the phase and extinction matrices of the asphalt medium. The experimentally determined phase and extinction matrices are substituted in the radiative transfer formulation to predict the scattering from asphalt surfaces under all conditions. Excellent agreement between theoretical predictions and measured quantities is obtained