ISAR Imaging With 3-D Large Angles Using Nonuniform Polar Format and FrFT Segmentation Compensation

The application of large-angle inverse synthetic aperture radar (ISAR) imaging significantly enhances resolution and facilitates the capture of more intricate target details. However, 3-D larger angles can amplify previously negligible motion errors, complicating the imaging process. This article introduces a nonuniform polar format and fractional Fourier transform segmentation compensation (NUPF-FRFTSC) ISAR imaging algorithm, based on a high-precision large-angle signal model. The algorithm efficiently decouples motion errors into distinct 2-D and 3-D components, sequentially correcting each. To address the complex high-order 2-D spatially varying (SV) range cell migration (RCM) and phase error (PE), the NUPF algorithm (NUPFA) is proposed. This approach mitigates 2-D errors by mapping echo data acquired during nonuniform rotation into a uniform wavenumber domain. The necessary rotation parameters for this mapping are estimated through a combination with image entropy evaluation and particle swarm optimization (PSO). Furthermore, to correct 3-D SV PE, which are independent of the range-Doppler (RD) scale in ISAR images, the FrFTSC method is developed. This technique compensates signals within each wavenumber domain cell regionally, utilizing the similarity of second-order phase coefficients of neighboring scattering points. Electromagnetic computational data and measured experiment demonstrate that the proposed NUPF-FRFTSC algorithm successfully achieves large-angle focused imaging of targets, yielding results superior to traditional ISAR imaging methods.