The [[SBR Method|SBR method]] assumes that electric fields propagate as spherical waves with the general form:
<math> \mathbf{A}(\mathbf{r)} ) \frac{e^{-jkR}}{R} </math>
where R = |<strong>r-r'</strong>| is the distance between the source and observation points, <strong>r</strong> and <strong>r'</strong> are the position vectors of the observation and source points, respectively, k = âε<sub>r</sub>k<sub>0</sub>, ε<sub>r</sub> is the relative permittivity of the propagation medium, k<sub>0</sub> = 2Ïf/c is the free-space propagation constant, f is the operational frequency and c is the speed of light in the free space. <strong>A</strong>(<strong>r</strong>) is complex vector function that defines the polarization and strength of the field at the observation point. Propagating spherical waves are modeled as ray tubes or beams that emanate from a field source point (transmitter), travel in the free space, bounce from obstacles and scatterers and are collected at a field observation point (receiver). As a ray tube travels in space, its footprint expands due to the beam's spatial divergence. Every scatterer in the scene is discretized into a group of conjoined triangular facets. When a propagating ray is intercepted by a facet, one of the following events takes place: