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Combining time-frequency transforms to create a sequence of instantaneous range-Doppler images in ISAR processing

In ISAR processing, ship classification is usually based on a range-Doppler (RD) image. The RD image is obtained by combining range alignment, phase compensation and short-time Fourier transform. Nevertheless, this leads to sophisticated approaches to weaken unwanted phenomena such as blurred images. In this paper, an alternative approach is proposed based on the time-frequency analysis. In that case, the Wigner-Ville transform provides geometrical structured events which correspond to the scatterers and unstructured ones which are associated to the artefacts and cross-terms. Taking advantage of these geometrical features, we first suggest computing a time-frequency local gradient analysis. This variational analysis provides a confidence map of the time-frequency distribution we can use to remove artefacts. Then, the practitioner can exploit the corresponding time-frequency images to choose the more appropriate instantaneous RD image. Simulation results confirm the effectiveness of this approach.