A Novel High-Resolution ISAR Imaging Method for Large-Size Space Target in the Terahertz Band Based on Defocused PSF

As the demand for precise detection of space targets and orbital maintenance increases, it is crucial to obtain high-resolution images of space targets. The terahertz (THz) radar with carrier frequency of 0.1–10 THz has the characteristics of larger bandwidth and shorter wavelength compared with microwave radar. Accordingly, under the same inverse synthetic aperture radar (ISAR) imaging conditions, it has advantages in both range resolution and azimuth resolution theoretically. However, for large-size space targets, the phenomenon of high-order range cell migration (RCM) and azimuth phase error are prone to occur during small-rotation-angle (3°–5°) ISAR imaging, which results in severe imagery defocusing. In this article, a high-resolution ISAR imaging method based on defocused point spread function (DPSF) is proposed. By means of third-order Taylor expansion performing on the range-Doppler (RD) imaging model, the impact of high-order terms on imaging is analyzed. Through revealing the relationship between high-order terms and inclination of defocused PSF measured with Hough transform (HT), the rotation velocity and range center deviation (RCD) are estimated accurately and independently in image domain. After parameters estimation, the first-order RCM is corrected by Keystone transform (KT). Utilizing the estimated rotation velocity, the second-order RCM is eliminated by fast Gaussian gridding nonuniform fast Fourier transform (FGG NUFFT). Then the spatial-variant phase error is compensated based on previous estimation. Finally, azimuth resolution is improved by FGG NUFFT due to nonuniformly sampled data. Numerous simulations and real measured experimental results demonstrate the effectiveness and outperformance of the proposed algorithm for large-size space target ISAR imaging in the THz band.