Effect of ground terrain on millimeter wave propagation

In the active and passive microwave remote sensing of earth terrain using radars and radiometers, the scattering effects due to medium inhomogeneities and surface roughness play a dominant role in the determination of radar back-scattering cross-sections and the brightness temperatures. Two theoretical models were developed to characterize terrain media: (1) a random medium model where scattering effects can be accounted for by introducing a randomly fluctuating part in the permittivities; and (2) the discrete scatterer model where discrete scatterers are imbedded in a homogeneous background medium. The earth terrain is then modelled as layers of such scattering media bounded by rough surface with air above and homogeneous half space below. In matching the theoretical results with experimental data collected from vegetation and snow fields, the following findings are summarized: (1) for radar observations near nadir, rough surface effects are important; and (2) for snow fields the horizontal correlation length is greater than the vertical correlation length whereas for vegetation fields their relative sizes depend on the types of vegetation.