High Resolution Through-the-Wall and Subsurface Imaging Using Small Number of Transceivers on Moving Robotic Platforms

Abstract

Imaging of visually obscured objects within buildings or buried under ground using low frequency EM waves have many military, security, civilian, and industrial applications. In imaging systems, range and cross-range resolution are important parameters that determine the ability of the system to resolve closely spaced objects. Range resolution generally depends on the signal bandwidth while the cross-range resolution depends on the antenna aperture size. In certain application such as through-the-wall imaging and detection of buried land mines and pipelines where the objects are closely spaced and/or located at far distance from the transmitter and receiver, the imaging system must have high cross-range resolution. Typically, this is achieved by using a large array of directive antennas with limited field of view and low mobility. In this research, new imaging methods based on synthetic aperture processing are presented to address low cross-range resolution, limited field of view, and low mobility of currently available imaging systems. For through-the-wall imaging applications, the large array of directive antennas used in current systems is replaced by a small number of omnidirectional transceivers mounted on moving robots. One or more receivers move and sample the scattered signal at different locations within the imaging area. By applying back-projection technique to the obtained samples, a large 2-D array is synthesized for high resolution imaging. Using omnidirectional antennas, images with a 360° field of view are generated in a short period of time. To suppress the effect of direct coupling between the transmitter and receiver and multiple reflections in the image, orthogonal circular polarizations are used for the transmitting and receiving antennas. In this research, a novel wideband circularly polarized omnidirectional antenna is presented for through-the-wall imaging applications. The antenna operates based on excitation of orthogonal field distributions similar to a circular waveguide TE21 mode. A bi-static FMCW radar system realizing the presented through-the-wall imaging method is designed and fabricated. The system is configured to account and completely compensate for the delays inherent to the circuits and errors in transmitter/receiver synchronization completely. Performance of the method is evaluated in different real-world scenarios using the fabricated radar system and the associated algorithms. Two new methods for image enhancement in through-the-wall imaging are developed. The first method enhances the range resolution and reduces the background noise in the image by detecting the locations of the reflections and forming the image only at those points. The second method discriminates wall surfaces from small size objects and detects the location and orientation of all wall surfaces within the imaging area. The second part of the dissertation describes a new method for subsurface imaging using the general idea developed for the through-the-wall imaging. The method utilizes bi-static ground looking transmitters and receivers mounted on moving robots to sample scattered fields at different locations. The samples are processed coherently to form a large 2-D synthetic array which provides high cross-range resolution 3-D images of the subsurface in a short period of time. This cannot be achieved using conventional ground penetrating radars. A new low profile wideband antenna is designed for the imaging of deeply buried targets or targets buried in soil with high losses. Performance of the designed antenna and the proposed imaging method have been tested through field measurements. The imaging results show high resolution 3-D imaging capability of the method.