A Novel Imaging Method Using Fractional Fourier Transform for Near-Field Synthetic Aperture Radiometer Systems

In the domain of synthetic aperture radiometry, a Fourier transform (FT) relationship can be established between the brightness temperature of the target in the far-field region and the visibility function output by the system. However, when the target is in the near-field range of the imaging system, the existing far-field imaging methods cannot be used directly for the inversion of the near-field visibility function, because the target radiation signal cannot simply be regarded as a plane wave signal. In this letter, we propose a novel imaging method for synthetic aperture radiometer systems on the basis of the characteristics of the near-field target radiation signal. We introduce the near-field error term to reformulate the relationship between visibility function and brightness temperature in near field. Based on the reestablished signal model, we present an image reconstruction algorithm via fractional Fourier transformation, named near-field fractional FT (NF-FRFT), to estimate the near-field brightness temperature. Compared with the conventional near-field imaging method, the proposed NF-FRFT method can achieve better image reconstruction quality without extra hardware consumption. Furthermore, another advantage of this approach is that no additional array layout design is required. The validity of this method is demonstrated by a series of simulations and experiments.