Asymptotic methods allowing considering a large set of surface roughness scales have been developed. Three parameters are used to characterize the surface:
 The roughness depends on the ratio from the surface RMS to the wavelength,
 The slope is defined as the ratio of the roughness to the autocorrelation distance of the irregularities,
 The autocorrelation function
Three kinds of models have been developed allowing considering a large number of cases:
 Small roughness : the small slope approximation, augmented by the non local small slope approximation
 Average and high roughness : the algorithm is based on the Kirchhoff approximation
 Very strong roughness : the algorithm uses the second order Kirchhoff approximation
Scattering function
Kirchhoff approximation1^{st} and 2^{nd} ordre

Scattering function
Small Slope Approximation technique (SSA)
compared to the Kirchhoff approximation

References
(full texts available in the publications section)
Y. Beniguel, G. Berginc, "Une revue des techniques utilisees pour le calcul de la diffraction par les surfaces rugueuses aleatoires", Journees d'etudes sur la propagation electromagnetique dans l'atmosphere : du decametrique a l'angstrom, Rennes, 2002.
G. Berginc, Y. Beniguel, B. Chevalier, "Small slope approximation method : Higher order contributions for scattering from conducting 3D surfaces", SPIE99 Proceedings, Denver, 1999