Inverse Synthetic Aperture Radar Imaging With MATLAB Algorithms, 2nd Edition

11Near‐Field ISAR Imaging

The conventional inverse synthetic aperture radar (ISAR) imaging algorithm relies on the assumption that the target is situated in the far‐field zone of the ISAR antenna as detailed explained in Chapter 4. The raw backscattered electromagnetic (EM) data are usually collected within a narrow frequency band‐width and small look‐aspects. Then, the final ISAR image is constructed by processing these two‐dimensional (2D) frequency‐aspect data to generate a projection onto 2D spatial coordinates of range and cross‐range. On the other hand, there are some applications at which the object to be imaged should be in the near field of the antenna. For these near‐field ISAR applications, the data need to be usually collected for a wider look‐angles to be able to focus the target along the cross‐range (Fortuny 1998; Yu et al. 2012). Concealed weapon detection to inspect hostile and terrorist activities at critical security sites (e.g. airport) can be taken as good examples for the near‐field ISAR imaging (Demirci et al. 2011, 2012). As another application, researchers have also utilized near‐field imaging to characterize the target's scattering features by means of near‐field radar cross‐section (RCS) measurements since it is very difficult to constitute the far‐field distance requirement of antennas at high frequencies in indoor measurement facilities such as anechoic chambers (Broquetas et al. 1992, 1998; Vertiy et al. 2011; Yu et al. 2012; Vaupel 2015).

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