Estimating the soil dielectric constant via scattering measurements along the specular direction

We propose a soil dielectric constant retrieval scheme based on the use of the ratio of power densities scattered at HH and VV polarizations along the specular direction for different incidence angles and/or frequencies. The method relies on the minimum squares technique and is based on the observation that at variance with the backscattering case, in the specular case the small perturbation method (SPM) and the Kirchhoff approach both lead to the same expression of the copolarized ratio. Accordingly, it is expected that its evaluation is rather robust, to hold under a wide range of surface roughnesses. We present method-of-moments (MoM) simulations that confirm this expectation, and we test the validity of the overall retrieval scheme, as well as its sensitivity to measurement errors (calibration errors and fading or speckle noise), by applying the algorithm to data simulated by using the MoM. Theoretical and practical problems related to the implementation of measurements along the specular direction are also discussed. The proposed method is amenable to the new generation of bistatic remote sensing instruments.