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Orfeo, Daniel Jerome

Mechanical Engineering

2021

Ph. D.

There is an urgent and longstanding need for non-contact subsurface imaging for the detection and identification of buried objects. This dissertation investigates the use of specialized electromagnetic (EM) remote sensing techniques for the detection of subsurface objects with little or no metal content, such as utility pipes, landmines, and improvised explosive devices. Ground penetrating radar (GPR) is a well-established remote sensing technology with widespread use [58]; despite this, significant performance limitations remain. First, a high degree of sensitivity is necessary to detect and locate nonmetallic targets. Second, target geometries and congested target configurations are difficult to identify and resolve. Third, radio frequency (RF) spectral congestion is such that commercial penetrating radars are power limited by law to prevent harmful interference with other devices. In this dissertation, controls on field shape and field structure are shown to be effective methods to address these three challenges (sensitivity, target geometry, and spectral efficiency). A related inquiry involving low-frequency magnetic field shaping is also presented. The first objective is to use antenna orientation configuration to shape the microwave EM sensing field to improve radar sensitivity. A high standoff distance between antennas and the ground surface enables greater area coverage and easier system movement over uneven or treacherous terrain; however, a greater standoff distance also results in larger signal loss during transmission. Studies on the effects of antenna height and antenna angle are reported here in order to improve the sensitivity of a bistatic air launched GPR intended for a low-flying unmanned aerial vehicle application. The second research objective of this dissertation is to have a functional, high performance GPR capable of addressing the challenges of high sensitivity measurements and good spectral efficiency. Notably the custom system designed here is easily sourced and reproduced since it uses only commercially available hardware components. This modular, multistatic, time domain radar system is shown to provide consistent, high quality data for GPR, through-wall, and field-testing applications. A narrow pulse time domain radar also results in low integrated RF emissions, an important consideration for alleviating RF spectral congestion. Compared to other GPR systems, it has swappable modular componentry, customizable MATLAB software, compatibility with air launched horn antennas, superior triggering stability for clear imaging, high power output, and high-speed high-resolution sampling. The third research objective is to leverage waveform structure, specifically orbital angular momentum (OAM), to address GPR challenges relating to target geometry and spectral efficiency. Shaped EM waves with properties dependent on spatial distribution (independent of polarization) are said to be "structured". Control of OAM in microwave systems is a novel example of wave structure that exploits EM degrees of freedom that most conventional systems do not use. OAM is characterized by an integer OAM mode where zero represents the case of a plane wave, and nonzero OAM modes propagate with a helical wavefront. In this study a circular phased array is used to transmit and receive microwaves with OAM characteristics. Control of OAM is shown improve radar sensitivity to certain chiral targets. OAM is also implemented in a dual function RF sensing and communication system, which may achieve improved spectral efficiency, data rates, and streamlined hardware requirements. In summary, these investigations leverage numerical calculations, EM modeling, and laboratory testing for improved GPR performance relating to sensitivity, target geometry, and spectral efficiency. Key results include antenna configuration, a unique GPR system design, OAM phase front characterization, the discrimination of OAM modes, and configuration of a network analyzer ultra-wideband (UWB) radar with synthetic OAM mode-control via signal post-processing. The unique chirality-detection capability of OAM radar is demonstrated, as well as an OAM-based information transmission scheme.