1. A non-contact, far-field radar nondestructive testing (NDT) method capable of: detecting at least one of defects, damages, and reinforcement conditions in near-surface region of multi-layer systems using monostatic inverse synthetic aperture radar (ISAR) measurements and applicable to various types of structural elements, the method comprising the steps of conducting far-field monostatic ISAR measurements, executing an imaging algorithm, and executing a progressive image focusing algorithm.