The Magellan altimeter system measured backscattered energy within about 15o of the nadir. These echoes were used to estimate the surface elevation, Fresnel reflectivity, and rms slope by fitting "templates" based on the Hagfors scattering function to each group of returned pulses [Ford and Pettengill, 1992]. This model assumes a gently undulating surface with no wavelength-scale roughness. Natural surfaces generally depart from this assumption, so a correction is made to the reflectivity for diffuse scattering by wavelength-scale rocks. The resulting reflectivity estimate is related to the dielectric constant by
For smooth areas, these results should be reasonably accurate. On rough terrain (tesserae, ridges, or rough lava flows), the Hagfors model is no longer applicable, and the reflectivity estimates should be treated with great caution even with the diffuse correction. As a comparison to values obtained from the emissivity, they should be discussed in the map analysis. Values for the dielectric constant versus Fresnel reflectivity are listed in table 4.