Synthetic aperture radar (SAR) is a well-established approach for retrieving images with high resolution. How- ever, common hardware used for SAR systems is usually complex and costly, and can suffer from lengthy signal acquisition. In near-field imaging, such as through-wall-sensing and security screening, simpler and faster hardware can be found in the form of dynamic metasurface antennas (DMAs). These antennas consist of a waveguide-fed array of tunable metamaterial elements whose overall radiation patterns can be altered by DC signals. By sweeping through a set of tuning states, near-field imaging can be accomplished by multiplexing scene information into a collection of measurements, which are post-processed to retrieve scene information. While DMAs simplify hardware, the post-processing can become cumbersome, especially when DMAs are moving in a fashion similar to SAR. In this presentation, we address this problem by modifying the range migration algorithm (RMA) to be compatible with DMAs. To accommodate complex patterns generated by DMAs in the RMA, a pre-processing step is introduced to transform the measurements into an equivalent set corresponding to an effective multistatic configuration, for which specific forms of the algorithm have been derived. As we are operating in the near field of the antennas, some approximations made in the classical formulation of RMA may not be valid. In this paper, we examine the effect of one such approximation: the discarding of amplitude terms in the signal-target Fourier relationship. We demonstrate the adaptation of the RMA to near field imaging using a DMA as central hardware of a SAR system, and discuss the effects of this approximation on the resulting image quality.
Microwave imaging systems have become increasingly prevalent owing to their ability to obtain 3D images while penetrating optically-opaque materials. These capabilities have motivated the development of various microwave imaging systems for applications ranging from security screening to biomedical imaging. Recent demonstrations have evidenced the idea that metasurface apertures can improve the hardware characteristics of microwave imaging systems due to their lightweight, low-cost, and planar nature. While metasurfaces can improve the antenna hardware, the large spectral bandwidth required for microwave imaging still incurs complex, costly, and performance-limiting RF components. To address the drawbacks inherent to using a large bandwidth, recent works have suggested that near-field microwave imaging can be performed at a single frequency point. In this work, monochromatic imaging is demonstrated by deploying two metasurface apertures to form a near-field microwave imaging system. By leveraging the unique radiation patterns emitted by metasurfaces, a pair of metasurface antennas, one acting as a transmitter and the other as a receiver, can acquire range and cross range information with measurements taken at a single frequency. We will show that this operation can then be supplemented by introducing aperture synthesis in the height direction to obtain fully 3D images. To account for the unusual illumination strategy, a reconstruction algorithm based on the range migration algorithm is formulated and implemented to enable efficient reconstruction of 3D images. Ultimately, the metasurface hardware, aperture synthesis, and monochromatic operation are combined to form an imaging system with high performance capabilities, without requiring complex and costly hardware.
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