A Polarimetric Scattering Model for the 2-Layer Problem
In this paper, polarimetric signatures from two layers are examined. In the two layer problem a low-loss dielectric layer sits on top of a shallow subsurface layer of scatterers. Volume scattering within the top surface layer is not considered. We show that for polarimetric backscatter data acquired from just one incidence angle, no parameter is proportional to the depth of the subsurface layer. We also show that separation of the two components and inversion of the polarimetric backscatter measurements is not possible, in the latter case due to the mismatch between the number of variables that determine the backscatter behavior, versus the degrees of freedom in the measured values. The interesting results obtained at a sand dune site in SW France courtesy of the group led by Dr. Phillippe Paillou are examined through the prism of this model. It is shown that the model agrees well with results obtained for a subsurface layer at 3 m depth, where the subsurface returns are significant, and that surface scattering alone can account for observations made over a subsurface layer at a depth of 5.8 m, provided the surface slope is adjusted by approx. 10 degrees. The implications for exploration of the near-surface of Mars, a significant proportion of which is obscured by a dust mantle, are discussed.