A Wiener Filter-Based Synthetic Aperture Radar Algorithm for Microwave Imaging of Targets in Layered Media

In recent years, single-frequency and wideband synthetic aperture imaging algorithms have been used in radar imaging for microwave and millimeter wave nondestructive testing (NDT) of a wide range of materials and structures. Synthetic aperture radar (SAR) techniques are robust and therefore offer great utility when incorporated with NDT applications. Unlike most radar applications, however, NDT applications are often concerned with layered composite structures in which an anomalous indication may exist. Therefore, any such SAR algorithm must be able to account for the influences of each layer according to its dielectric properties and thickness (that is, account for the influence of reflected and transmitted waves at each boundary). In such applications where small thicknesses are of interest, if the layered nature of the structure is not properly taken into account, then the image of a target may correspond to a wrong location within the structure, or may be masked or otherwise significantly altered. In this paper, an effective one-way SAR algorithm is presented that is capable of correctly imaging and determining the location of a target in an arbitrary multilayered structure. This algorithm employs spectral-domain Green's function for a multilayered structure, which accounts for all reflected and transmitted signals at each boundary. Subsequently, a Wiener filter-based deconvolution process was employed to reconstruct an image. Reconstructed images produced in this way are devoid of unfocused and/or misplaced targets, which usually happens if standard SAR imaging techniques developed for free-space applications with homogeneous backgrounds are used for imaging a multilayered structure. This paper presents the results of simulations, as well as a representative measurement, to show the efficacy of the proposed technique.