Study on THz Imaging System for Concealed Threats Detection.

PhD ThesisMany research groups have conducted studies on Terahertz technology for various applications in the last decades. THz imaging for personnel screening is one prospective application due in part to its superior performance compared with imaging microwave bands. Because of the demand for the accurate detection, it is desirable to devise a high-performance THz imaging system for concealed threats detection. Therefore, this thesis presents my research on the low-cost THz imaging system for security detection. The key contributions of this research lie in investigating the linear sparse periodic array (SPA) THz imaging system for concealed threats detection, improving the traditional reconstruction algorithm of Generalized Synthetic Aperture Focusing Technique (GSAFT) to suppress the ghost images and applying the compressive sensing technique into the proposed SPA-THz imaging system to reduce the sampling data but maintain the image quality. The first part of the work is to investigate the linear sparse periodic array (SPA) and its configuration with large element spacing in simulation, deriving the design guideline for such a SPA THz imaging system. Meanwhile, the improved GSAFT reconstruction algorithm and multi-pass interferometric synthetic aperture imaging technique have been proposed to suppress the ghost image and improve the image quality, respectively. Secondly, the compressive sensing technique has been investigated to reduce the sampling data. Therefore, we have proposed the corresponding discrete CS SPA-THz reconstruction model and verified it in simulation. Finally, we have devised a simplified experimental set-up to assess the practical imaging performance, verifying the proposed SPA-THz imaging system. The set-up only uses 1 Tx and 1 Rx scanning on two separate tracks to effectively realize the proposed imaging system. The reconstructed images by the GSAFT and CS approaches with the measured data have both shown good consistency with the simulated results, respectively. And the multi-pass interferometric synthetic aperture imaging has been experimentally proved effective in improving image SNR and contras

Millimeter-wave and terahertz imaging techniques

This thesis presents the development and assessment of imaging techniques in the millimeterwave (mmW) and terahertz frequency bands. In the first part of the thesis, the development of a 94 GHz passive screener based on a total-power radiometer (TPR) with mechanical beamscanning is presented. Several images have been acquired with the TPR screener demonstrator, either in indoor and outdoor environments, serving as a testbed to acquire the know-how required to perform the research presented in the following parts of the thesis. In the second part of the thesis, a theoretical research on the performance of near-field passive screeners is described. This part stands out the tradeoff between spatial and radiometric resolutions taking into account the image distortion produced by placing the scenario in the near-field range of the radiometer array. In addition, the impact of the decorrelation effect in the image has been also studied simulating the reconstruction technique of a synthetic aperture radiometer. Guidelines to choose the proper radiometer depending on the application, the scenario, the acquisition speed and the tolerated image distortion are given in this part. In the third part of the thesis, the development of a correlation technique with optical processing applicable to millimeter-wave interferometric radiometers is described. The technique is capable of correlating wide-bandwidth signals in the optical domain with no loss of radiometric sensitivity. The theoretical development of the method as well as measurements validating the suitability to correlate radiometric signals are presented in this part. In the final part of the thesis, the frequency band of the imaging problem is increased to frequencies beyond 100 GHz, covering the THz band. In this case the research is centered in tomographic techniques that include spectral information of the samples in the reconstructed images. The tomographic algorithm can provide detection and identification of chemical compounds that present a certain spectral footprint in the THz frequency band.Postprint (published version

Advanced Ground-Based Real and Synthetic Aperture Radar

Ground-based/terrestrial radar interferometry (GBRI) is a scientific topic of increasing interest in recent years. The GBRI is used in several field as remote sensing technique for monitoring natural environment (landslides, glacier, and mines) or infrastructures (bridges, towers). These sensors provide the displacement of targets by measuring the phase difference between sending and receiving radar signal. If the acquisition rate is enough the GBRI can provide the natural frequency, e.g. by calculating the Fourier transform of displacement. The research activity, presented in this work, concerns design and development of some advanced GBRI systems. These systems are related to the following issue: detection of displacement vector, Multiple Input Multiple Output (MIMO) and radars with 3D capability

Experimental low-THz imaging radar for automotive applications

This thesis reports initial experimental results that provide the foundation for low-THz radar imagery for outdoor scenarios as expected in automotive sensing. The requirements for a low-THz single imaging radar sensor are outlined. The imaging capability of frequency-modulated continuous-wave (FMCW) radar operating at 150 GHz is discussed. A comparison of experimental images of on-road and off- road scenarios made by a 150 GHz FMCW radar and a reference 30 GHz stepped frequency radar is made, and their performance is analysed

Experimental low-THz imaging radar for automotive applications

This thesis reports initial experimental results that provide the foundation for low-THz radar imagery for outdoor scenarios as expected in automotive sensing. The requirements for a low-THz single imaging radar sensor are outlined. The imaging capability of frequency-modulated continuous-wave (FMCW) radar operating at 150 GHz is discussed. A comparison of experimental images of on-road and off- road scenarios made by a 150 GHz FMCW radar and a reference 30 GHz stepped frequency radar is made, and their performance is analysed

Millimetre wave imaging for concealed target detection

PhDConcealed weapon detection (CWD) has been a hot topic as the concern about pub- lic safety increases. A variety of approaches for the detection of concealed objects on the human body based on earth magnetic ¯eld distortion, inductive magnetic ¯eld, acoustic and ultrasonic, electromagnetic resonance, MMW (millimetre wave), THz, Infrared, x-ray technologies have been suggested and developed. Among all of them, MMW holographic imaging is considered as a promising approach due to the relatively high penetration and high resolution that it can o®er. Typical concealed target detection methods are classi¯ed into 2 categories, the ¯rst one is a resonance based target identi¯cation technique, and the second one is an imaging based system. For the former, the complex natural resonance (CNR) frequencies associated with a certain target are extracted and used for identi¯cation, but this technique has an issue of high false alarm rate. The microwave/millimetre wave imaging systems can be categorized into two types: passive systems and active sys- tems. For the active microwave/millimetre wave imaging systems, the microwave holographic imaging approach was adopted in this thesis. Such a system can oper- ate at either a single frequency or multiple frequencies (wide band). An active, coherent, single frequency operation millimetre wave imaging system based on the theory of microwave holography was developed. Based on literature surveys and ¯rst hand experimental results, this thesis aims to provide system level parame- ter determination to aid the development of a target detection imager. The goal is approached step by step in 7 chapters, with topics and issues addressed rang- ing from reviewing the past work, ¯nding out the best candidate technology, i.e. the MMW holographic imaging combined with the resonance based target recog- i nition technique, the construction of the 94 GHz MMW holographic prototype imager, experimental trade-o® investigation of system parameters, imager per- formance evaluation, low pro¯le components and image enhancement techniques, feasibility investigation of resonance based technique, to system implementation based on the parameters and results achieved. The task set forth in the beginning is completed by coming up with an entire system design in the end.