Time-frequency analysis of backscattering from inlet cavities embedded in complex targets

Electromagnetic backscattering from inlet cavities embedded in complex targets is investigated. First, a numerical technique for the efficient analysis of long cavities is implemented. This technique combines the moment method with a cascading algorithm, using triangular patch basis functions. Data simulated with this tool are then analyzed, with time-frequency processing tools (short-time Fourier transform), to identify and understand the dominant scattering mechanisms. Based on this analysis, two other techniques are developed to analyze more efficiently that class of structures. The first technique is a super-resolution algorithm that allows the parameterization of waveguide-like dispersive mechanisms. This implementation uses the ESPRIT (Estimation of Signal Parameters via Rotational Invariance Techniques) algorithm applied to both the time and frequency domain to extract the parameters used in the expansion. The application of this algorithm to backscattered data from an open-ended circular waveguide shows all the excited modes with very good accuracy. The second technique combines the ISAR (Inverse Synthetic Aperture Radar) processing with joint time-frequency representation as a means of extracting the non-point scattering features from the standard ISAR image. This algorithm uses Gaussian basis functions to adaptively parameterize the data and as a consequence, the point scattering mechanisms and resonance phenomena can be readily separated. The joint time-frequency ISAR algorithm is tested using data numerically simulated for simple structures and chamber measurement data from a scaled model airplane. The results show that non-point scattering mechanisms can be completely removed from the original ISAR image, leading to a cleaned image with only scattering centers. The non-point scattering mechanisms, when displayed in the frequency-aspect plane, can be used to identify target resonances and cut-off phenomena.