Multiple scattering from randomly rough surfaces

In wave scattering from a randomly rough surface, the scattering characteristics are determined by the surface geometric and dielectric properties. Because of the complexity of the problem, satisfactory solutions are restricted to certain ranges of the rough surface parameters, dielectric values and frequency values. In the 1960s modeling of wave scattering from a randomly rough surface was carried out in the low- and high-frequency regions. In the 1970s many attempts were made to unite the above low- and high-frequency ranges to achieve a better agreement with the measurements.; Beginning in the 1980s many endeavors were made to obtain models with a wider range of validity without restricting the surface to consist of two scales. In most cases the results were generally too complex to provide a practically useful form. Among the many approaches the original IEM surface scattering model has the simplest form in single scattering. Among the assumptions leading to the approximation is the use of a simplified expression for the Green's function in spectral form. Mathematically, there is no reason to make this assumption. In this study we want to remove this assumption by keeping this phase term and recompute statistical averages and finally the scattering coefficient. It is found that in all cases considered the existing IEM model gives satisfactory predictions in single scattering. However, in multiple scattering calculations it does not generate correct results except for small slope surfaces.; The IEM approach is an equivalent current approach. For single scattering only a first-order estimate is needed and shadowing effects can be included separately as a multiplication factor. In multiple scattering calculation the equivalent current itself is generated from multiple surface scattering. For this reason it is necessary to separate the scattered field into two portions: one propagating upward and the other downward so that an appropriate amount of shadowing can be incorporated with each portion. The phase term with the absolute sign in both the Green's function and the gradient of the Green's function is the term to help make this separation. This is why the original version of the IEM model cannot provide the correct calculation in multiple scattering.; In the late 1980s the observations of backscatter enhancement and strong depolarization of waves scattered from randomly rough Gaussian surfaces with large slopes were reported in measurements and have stimulated critical discussions. The backscatter enhancement takes place when the surface rms slope is of the order of unity and is due to multiple surface scattering. The MIEM model shows that the backscatter enhancement becomes evident when the surface rms slope is larger than 0.5 and surface parameters are larger than an incident wavelength. In comparison of MIEM with experimental data excellent agreement was obtained.; To further validate the MIEM model comparisons are shown with the small perturbation model (SPM) and measurements from known randomly rough surfaces. In the latter case a wide range of angle and frequency data were available for comparison. The excellent agreement obtained support the MIEM model in the low- and middle frequency range. Finally, comparisons were also made of the MIEM with data collected from randomly rough surfaces with large scale roughness over a wide range of frequency and angle. Here again excellent agreement was obtained.