Permittivity Extraction From Synthetic Aperture Radar (SAR) Images of Multilayered Media

A method to extract the complex permittivity from synthetic aperture radar (SAR) images of multilayered structures is proposed. Specifically, a three-term one-port calibration method is implemented to compensate for issues such as wave attenuation, antenna pattern, and multiple reflection effect between the antenna and the material under test (MUT) at each measurement frequency within a wideband operating frequency range. These issues are usually not considered by the
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SAR algorithm used here. The ability of SAR imaging to focus the beam at any location within the MUT, representing a plane-wave at that location, so that the calibrated SAR image can then be used in conjunction with a plane-wave reflection coefficient model for a multilayered structure to calculate the complex permittivity of a certain layer in the MUT through an inverse optimization procedure. Measurement results conducted on a number of multilayered dielectric sheets with finite thicknesses and different complex permittivities confirm the validity of the proposed method. The results also indicate that using a wide operating frequency range can help mitigate measurement variations, leading to more accurate results. Finally, the possible limitations of this method are also discussed, and a sensitivity analysis is conducted to show the accuracy of this proposed method for materials with different loss factors. All these together indicate the potential of the proposed method to accurately extract the complex permittivity distribution of a multilayer structure for nondestructive evaluation (NDE) purposes.