For oceans, a model was developed to simulate the emissivity vector using a two-scale roughness approximation (Yueh, 1997). The emission from large-scale waves is normally polarimetric and is modeled by the geometrical optics (GO) (Stogryn, 1967). In the GO model, the large-scale waves are modeled by tilting surface facets, and the scattering coefficients are proportional to the number of surface facets with a sloping angle satisfying the specular reflection condition. The slope distribution of the large-scale roughness is computed from an ocean surface spectrum (Cox and Munk, 1954; Durden and Vesecky, 1985). However, the GO scattering theory underestimates the directional signals in the first three components of the microwave emissivity vector and predicts no signals in the fourth component (Gasiewski and Kunkee, 1994). Bragg scattering from the small-scale waves was found to be useful in explaining the dependence of the emissivity on ocean wind direction and the existence of the fourth component in the emissivity vector (Yueh, 1997). The cutoff wavenumber for separating between the large and small-scale waves depends on frequency and can be optimally derived (Liu et al., 1998). With a two-scale model, the emissivity and brightness temperature vectors can be simulated and are found to be consistent with those obtained from a airborne microwave radiometer (St. Germain and Poe, 1998). A similar model theory is also applicable for infrared wavelength (Wu and Smith, 2000) and integrated into CRTM.