Reconstruction techniques for sparse multistatic linear array microwave imaging

Abstract

Sequentially-switched linear arrays are an enabling technology for a number of near-field microwave imaging applications. Electronically sequencing along the array axis followed by mechanical scanning along an orthogonal axis allows dense sampling of a two-dimensional aperture in near real-time. The Pacific Northwest National Laboratory (PNNL) has developed this technology for several applications including concealed weapon detection, groundpenetrating radar, and non-destructive inspection and evaluation. These techniques form three-dimensional images by scanning a diverging beam swept frequency transceiver over a two-dimensional aperture and mathematically focusing or reconstructing the data into three-dimensional images. Recently, a sparse multi-static array technology has been developed that reduces the number of antennas required to densely sample the linear array axis of the spatial aperture. This allows a significant reduction in cost and complexity of the linear-array-based imaging system. The sparse array has been specifically designed to be compatible with Fourier-Transform-based image reconstruction techniques; however, there are limitations to the use of these techniques, especially for extreme near-field operation. In the extreme near-field of the array, back-projection techniques have been developed that account for the exact location of each transmitter and receiver in the linear array and the 3-D image location. In this paper, the sparse array technique will be described along with associated Fourier-Transform-based and back-projection-based image reconstruction algorithms. Simulated imaging results are presented that show the effectiveness of the sparse array technique along with the merits and weaknesses of each image reconstruction approach.

Cylindrical Three-Dimensional Millimeter-Wave Imaging via Compressive Sensing

“…Millimeter-wave (MMW) imaging techniques have been applied for the detection of concealed weapons and contraband carried by personnel at airports and other secure locations [1][2][3][4][5]. Millimeter waves can penetrate common clothing barriers to form an image of a person as well as any concealed items with different reflectivity or emissivity.…”

Efficient millimetre‐wave imaging structure for detecting axially rotated objects

“…In multi-static structures, a transmitter antenna illuminates the target and the reflected wave is measured by a number of receivers uniformly, or even randomly, distributed throughout the array (see Fig. 3a) [17,18,20]. Therefore, in the case of an axially rotated target, unlike the mono-static structure, where a noticeable amount of reflected signal is lost by the receiver due to the specular reflection, the receiver array in a multi-static system captures nearly the entire reflected signals as they are spread in all locations along the array (see Fig.…”

“…Second, the image reconstruction relation is based on SAR or generalised-synthetic-aperture-focusing-technique (GSAFT) methods [18], which are computationally too expensive compared to the FFT-based mono-static image reconstruction techniques [4]. Although previous efforts have been performed for FFT-based reconstruction of multi-static array [17], they all suffer from a low image-quality due to the multi-dimensional interpolation, or spherical-wave approximation based on multiple plane-waves and k-space partitioning.…”

“…Another dimension of challenge in using MMWI systems relates to the development of efficient image reconstruction algorithms to detect axially rotated concealed objects via employing multi-static array and image processing techniques [12][13][14][15][16]. In fact, when a transmitter antenna illuminates the target in a mono-static array, due to its rotation, the reflected wave reverts in a direction that cannot be completely measured by the receiver antenna [17]. Mono-static structures and corresponding postprocessing algorithms, therefore, fail to detect the rotated targets in a plenary manner.…”

Thorough approach toward cylindrical MMW image reconstruction using sparse antenna array

“…Moreover, in this paper, in order to improve the performance of the cylindrical MMW imaging system in terms of the total cost and image quality, a sparse multi-static 1D antenna array has been designed and implemented. There have been some previous efforts for implementing sparse MMW imaging systems [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]. In a group of works, compressive sensing theory has been utilised in order to reduce the number of required samples for obtaining an acceptable number of antennas [12][13][14][15][16].…”