Scattering of a CO(2) laser beam at 10.6 microm by bare soils: experimental study of the polarized bidirectional scattering coefficient; model and comparison with directional emissivity measurements.

The bistatic polarized scattering by bare soil samples of a CO(2) laser beam at 10.6 microm has been experimentally studied. Large differences between HH and VV curves are usually observed, particularly in the forward plane. A simple phenomenological parameterization is proposed, based on the assumption of totally incoherent scattering by a rough medium. The normalized function F(theta)/F(0) accounting for slope distribution and shadowing is found from angular backscatter to be of the form cos(m)(theta), with m = 5.24 for all samples. This result is generalized to account for the bistatic case. The index of refraction of the medium is obtained from the ratio of HH and VV curves in the forward plane. Good agreement is found between experimental and calculated curves in the case of sand. The directional reflectivity and emissivity are calculated and compare well with experimental data. The calculated emissivity at nadir, for lambda = 10.6 microm, is within 0.5% of the value directly measured from emitted radiation. The backscattered peak has not yet been addressed in detail, therefore preventing relating in a semiquantitative manner the intensity of the backscattered light and the emissivity.