The relationship between the Kirchhoff approach and small perturbation analysis in rough surface scattering theory

Theoretical analyses of the scattering of the electromagnetic waves from rough surfaces have progressed using basically the Kirchhoff approach (KA) and the small perturbation method (SPM) of Rice. It is demonstrated that the latter method (SPM) is actually a special case of the former (KA). Reasons why SPM agrees with experimental data near grazing angle are given and arguments are made for the invalidity of the SPM second-order depolarized term.