95-GHz scattering by terrain at near-grazing incidence

This study, consisting of three complimentary topics, examines the millimeter-wave backscattering behavior of terrain at incidence angles extending between 70 and 90/spl deg/, corresponding to grazing angles of 20/spl deg/ to 0/spl deg/. The first topic addresses the character of the statistical variability of the radar backscattering cross section per unit area /spl sigma//sub A/. Based on an evaluation of an extensive data set acquired at 95 GHz, it was determined that the Rayleigh fading model (which predicts that /spl sigma//sub A/ is exponentially distributed) provides an excellent fit to the measured data for various types of terrain covers, including bare surfaces, grasses, trees, dry snow, and wet snow. The second topic relates to the angular variability and dynamic range of the backscattering coefficient /spl sigma//sup 0/, particularly near grazing incidence. We provide a summary of data reported to date for each of several types of terrain covers. The last topic focuses on bare surfaces. A semi-empirical model for /spl sigma//sup 0/ is presented for vertical (VV), horizontal (HH), and cross (HV) polarizations. The model parameters include the incidence angle /spl theta/, the surface relative dielectric constant /spl epsiv/, and the surface roughness ks, where k=2/spl pi///spl lambda/ and s is the surface root mean square (RMS) height.