a 'stochastic' component resulting in instantaneous real- tered energy in a region near the direction of the specular isations of the scattering process, not only its mean values. direction, and a minimum of the cross-polar components Similar to the Kirchhoff formulations, the approach is in the specular f-direction. The comparison of coherent based on a tangential plane approximation, i.e. it is appli- scattering from less rough surfaces shows a good agree- cable to surfaces with gentle undulations, whose horizontal ment between the Kirchhoff model and the stochastic dimensions are large compared with the incident wave- scattering approach as well. By applying this stochastic length. However, in contrast to the Kirchhoff models, scattering approach, the inclusion of random (but di- which are only valid for either slightly rough or very rough rective) surface scattering into ray-optical propagation surfaces, the proposed stochastic scattering approach in- modelling becomes possible. The purely deterministic GO cludes both the coherent and incoherent components at modelling is expanded by a 'stochastic' component, the same time. Using ray launching, each locally plane resulting in varying prediction results; thus allowing for the wavefront is actually represented by multiple discrete rays first time to account for non-deterministic scattering in instead of only one ray. Instead of reflecting all these dis- ray-optical wave propagation modelling. crete rays at the same boundary plane, the orientation of In [DEGDA04] and [DEESGK02a] an approach for the plane (i.e. its normal vector) and its position (i.e. its the integration of the diffuse scattering contribution height) are varied statistically for each discrete ray and for from both surface and volume inhomogeneities into RT each reflection. In that sense each discrete ray is seen as models is proposed. The so-called Effective Roughness a representative of an elementary wave for the locally plane (ER) approach first proposed in [DeBe99] and [Degl01] incident wavef