Polarimetric backscatter characterization of road surface faults at millimeter-wave frequencies

To design a reliable obstacle detection radar sensor for automated highway system applications, thorough knowledge of the backscatter response of various road surface conditions is required. In this paper, the polarimetric backscatter behavior of typical road surface faults, such as surface cracks and potholes, is considered. Although cracks do not pose any safety threat, the study of their backscatter response is important as far as the false alarm rate caused by cracks is concerned. Two theoretical models are developed to predict the backscatter response of cracks perpendicular to the antenna boresight. The first model simulates a crack by two impedance wedges next to each other. The method developed by Maliuzhinets is used to calculate the total field in the presence of an impedance wedge at normal incidence condition. A non-uniform expression for impedance wedge diffraction is chosen to represent the diffraction coefficient. The diffracted fields from two adjacent wedges are then added coherently. At MM wave frequencies the radii of curvature of asphalt or concrete crack edges become comparable with the wavelength and the wedge model may become invalid. For this situation a second model for curved edges based on scattering from impedance cylinders is developed. The validity of the theoretical models is verified by the experimental results. The simulation results of the second model exhibit better agreement with the measured data at 94 GHz than do those of the first model. Also, the experimental backscatter behavior of potholes with different depths over a wide range of incidence angles is presented.