Physical optics models for the backscatter response of road-surface faults and roadside pebbles at millimeter-wave frequencies

Theoretical models, based on the physical optics (PO) approximation, are presented to predict the backscatter response of road-surface faults and roadside pebbles. Two types of surface faults are considered, cracks and potholes. By applying the PO model, the backscattering coefficients of a road-surface crack are approximated by the radar cross section (RCS) per unit length of a lossy dielectric cylinder. For a road-surface pothole with a simple geometry, its RCS is estimated by the coherent sum of the backscattered fields from the pothole edges with significant backscatterers. The backscatter response of roadside pebbles is a combination of the surface scattering from surface roughness and the volume scattering from a layer of rock particles. The surface scattering is depicted by the integral equation method, a simplified second-order iterative physical optics approximation. The hybrid scattering model based on the theory of vector radiative transfer is employed to predict the volume scattering. The validity of the theoretical models is examined by comparing the simulation results with experimental data from the University of Michigan, where backscatter measurements at W-band frequencies were conducted on roadsurface faults and roadside pebbles at near grazing incidence angles (74(degree)-88(degree)).