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.
Synthetic aperture radar (SAR) systems conventionally rely on mechanically-actuated reflector dishes or large phased arrays for generating steerable directive beams. While these systems have yielded high-resolution images, the hardware suffers from considerable weight, high cost, substantial power consumption, and moving parts. Since these disadvantages are particularly relevant in airborne and spaceborne systems, a flat, lightweight, and low-cost solution is a sought-after goal. Dynamic metasurface antennas have emerged as a recent technology for generating waveforms with desired characteristics. Metasurface antennas consist of an electrically-large waveguide loaded with numerous subwavelength radiators which selectively leak energy from a guided wave into free space to form various radiation patterns. By tuning each radiating element, we can modulate the aperture’s overall radiation pattern to generate steered directive beams, without moving parts or phase shifters. Furthermore, by using established manufacturing methods, these apertures can be made to be lightweight, low-cost, and planar, while maintaining high performance. In addition to their hardware benefits, dynamic metasurfaces can leverage their dexterity and high switching speeds to enable alternative SAR modalities for improved performance. In this work, we briefly discuss how dynamic metasurfaces can conduct existing SAR modalities with similar performance as conventional systems from a significantly simpler hardware platform. We will also describe two additional modalities which may achieve improved performance as compared to traditional modalities. These modalities, enhanced resolution stripmap and diverse pattern stripmap, offer the ability to circumvent the trade-off between resolution and region-of-interest size that exists within stripmap and spotlight. Imaging results with a simulated dynamic metasurface verify the benefits of these modalities and a discussion of implementation considerations and noise effects is also included. Ultimately, the hardware gains coupled with the additional modalities well-suited to dynamic metasurface antennas has poised them to propel the SAR field forward and open the door to exciting opportunities.
Microwave imaging systems have seen growing interest in recent decades for applications ranging from security screening to space/earth observation. However, hardware architectures commonly used for this purpose have not seen drastic changes. With the advent of metamaterials a wealth of opportunities have emerged for honing metasurface apertures for microwave imaging systems. Recent thrusts have introduced dynamic reconfigurability directly into the aperture layer, providing powerful capabilities from a physical layer with considerable simplicity. The waveforms generated from such dynamic metasurfaces make them suitable for application in synthetic aperture radar (SAR) and, more generally, computational imaging. In this paper, we investigate a dynamic metasurface aperture capable of performing microwave imaging in the K-band (17.5–26.5 GHz). The proposed aperture is planar and promises an inexpensive fabrication process via printed circuit board techniques. These traits are further augmented by the tunability of dynamic metasurfaces, which provides the dexterity necessary to generate field patterns ranging from a sequence of steered beams to a series of uncorrelated radiation patterns. Imaging is experimentally demonstrated with a voltage-tunable metasurface aperture. We also demonstrate the aperture’s utility in real-time measurements and perform volumetric SAR imaging. The capabilities of a prototype are detailed and the future prospects of general dynamic metasurface apertures are discussed.
Dynamic metasurface antennas are planar structures that exhibit remarkable capabilities in controlling electromagnetic wave-fronts, advantages which are particularly attractive for microwave imaging. These antennas exhibit strong frequency dispersion and produce diverse radiation patterns. Such behavior presents unique challenges for integration with conventional imaging algorithms. We analyze an adapted version of the range migration algorithm (RMA) for use with dynamic metasurfaces in image reconstruction. Focusing on the the proposed pre-processing step, that ultimately allows a fast processing of the backscattered signal in the spatial frequency domain from which the fast Fourier transform can efficiently reconstruct the scene. Numerical studies illustrate imaging performance using both conventional methods and the adapted RMA, demonstrating that the RMA can reconstruct images with comparable quality in a fraction of the time. In this paper, we demonstrate the capabilities of the algorithm as a fast reconstruction tool, and we analyze the limitations of the presented technique in terms of image quality.