Target strength: Some recent theoretical developments

Some recent developments in the theory of diffraction of transient wavefields with high or moderate, but not excessively low, frequency content are reviewed here. Examples are cited to demonstrate that combinations of wavefronts and resonances, which organize the wave process according to progressing and oscillatory events, respectively, can furnish building blocks for interpretation of transient scattering data, and thereby for classification of target features. Conversely, these same spectral elements hold promise as a basis for physically sound parameterization of scattering data to reconstruct target characteristics. The discussion includes impenetrable and penetrable targets, interior-exterior coupling, real and complex spectra, and suggested directions for future research.