| Numerical finite-difference time-domain (FDTD) techniques offer a new approach to studying radiowave scattering from geophysical surfaces, for which theoretical models are difficult to construct and of limited interpretive use. FDTD simulation yields the near-scattered fields in the vicinity of discrete, wavelength-scale surface and subsurface objects having complex shape and material inhomogeneities; Huygens's principle allows extension to the far field. Accuracies sufficient for comparison with radar remote sensing measurements are readily achievable.; Two- and three-dimensional case studies provide quantitative assessment of scattering cross section dependence on parameters such as object shape and burial depth. Buried objects and randomly oriented buried dipoles scatter in a manner consistent with a cosine power law, while surface objects exhibit local peaks in backscatter at mid- to high-incidence angles. These results suggest a subsurface origin to diffuse scatter associated with Fresnel transmission ‘filtering’ effects of the surface. Single scattering from surface rocks and ellipsoids produces both disk-integrated and limb values of circular polarization ratio in agreement with experimental measurements, while single scattering from buried objects does not.; Tabulated surface rock populations from the Viking Lander (VL) sites integrated with computed FDTD cross sections of rock-like scatterers yield depolarized specific cross sections σ0 of 0.014 at VL1 and 0.023 at VL2, as compared with measured ranges of 0.019–0.032 and 0.012–0.018, respectively. Spherical scatterers provide a poor match to the data. At both sites surface rocks contribute more to the calculated σ0 than do buried rocks. When loss in the rocks is considered, it is unlikely that single scattering alone can account for the σ0 measured at either site.; FDTD modeling of buried craters and refractive lenses, possible sources of bizarre radar echoes from icy Galilean satellites, permits exploration of scattering characteristics at scales smaller than previously examined via analytic techniques. Craters spanning up to 3 wavelengths λ0 do not exhibit any exotic backscatter behavior. Nonspherical refraction scatterers can produce circular polarization ratios μC > 1 and linear polarization ratios μL = 0.5–0.8 at diameters as small as ∼λ0. Refractive structures cannot be ruled out as contributors to the observed echoes even at scales approaching the wavelength. |
