Microwave imaging of multilayered structures using ultrawideband noise signals

A microwave ultrawideband (UWB) radar system is developed and used for non-destructive testing (NDT) of multilayered structures. The effectiveness of this approach lies in the choice of critical parameters such as waveform type, operational frequency range, and stand-off distance. Matched filtering of the received waveform with a replica of the transmitted signal is used to locate hidden anomalies or flaws within a multilayered structure, most of which are not apparent through common visual-inspection techniques. Due to the impulse-like autocorrelation properties of white noise, noise waveforms provide the ability to locate material discontinuities as well as, or better than, other common waveforms such as Gaussian-modulated sinusoidal pulses (GMSP) or linearly frequency modulated chirp signals. Experimental data are gathered using a specially developed microwave imaging system which operates in the X-band frequency range. Subsequently, two-dimensional (2-D) and three-dimensional (3-D) images are rendered following a raster scanning approach to reveal internal boundaries, hidden inclusions, and defects due to impact damages or corrosion in fiber-reinforced polymer (FRP) materials and other dielectric materials.