Approximate calculation of scattered field from three-dimensional narrow cracks

The calculation of electromagnetic scattering from electrically large targets without small features can be accomplished efficiently and accurately due to the development of asymptotic techniques and high-speed computers. Several software packages are now available to compute the radar cross section (RCS) almost in real time. However, the asymptotic techniques alone cannot simulate the effect of small features, such as cracks and bumps, often found on realistic targets. These small features can have significant contributions to RCS, which is particularly true for low observable targets. Such problems can be coped with using hybrid techniques. Although the hybrid techniques are quite accurate, they still require the numerical characterization of small features using a numerical method such as the finite element method (FEM) and the method of moments (MoM). In applications where a very high accuracy is unnecessary, one can develop an approximate method to greatly simplify the characterization of small features and reduce the computing time to a negligible amount. One successful example is the development of the incremental length diffraction coefficients (ILDC) for semi-cylindrical channel and bosses. We present an approximate and efficient formulation for calculating the scattered field from a 3-D narrow crack.