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Passive millimeter-wave (PMMW) imaging systems are widely used in concealed weapon detection due to their passive nature. Improving imaging resolution according to target distance is an essential technique in these systems, and it plays a vital role in detecting concealed weapons. Hence, a variable focal length optical configuration has been applied to a PMMW imaging system to enhance imaging resolution depending on the target distance. In this way, it is possible to set the system for variable target distances. The system parameters, aside from the effective focal length and the target distances, were kept constant throughout the tests. Experiments were carried out taking into account the detection of concealed weapons up to 45 m in indoor environments, and the results were presented. The experiments validate that it is possible to improve the resolution of PMMW images using the optical setup with a variable focal length. As a consequence, this research will contribute to the development of PMMW imaging systems against suicide bombers in the near future to detect concealed weapons at long target ranges.

Passive millimeter-wave imaging systems (PMWIS) are employed for detecting concealed objects by mapping millimeter waves emitted from materials or living tissues. The emitted waves are measured by a receiver or radiometers without employing external wave sources. In this paper, a new super-heterodyne receiver front-end of a PMWIS at 35 GHz with an even order band pass filter is simulated and implemented. The receiver has a suitable temperature resolution for the use of hidden object imaging, is integrated and lightweight assembled on one-layer board. It has a bandwidth of 1.5 GHz, a noise figure (NF) of 2.2, and a temperature resolution of 0.126 K. The even order filter is implemented based on the substrate-integrated waveguide (SIW) technology, with Chebyshev response. The filters are designed at the central frequency of 35 GHz with the bandwidth of 1.5 GHz and one of them has controllable transmission zeroes. The filters are made by printed circuit board technology, employing SIW as resonators, having a high-quality factor of 23.33. Additionally, a triple-stage radio frequency (RF) low noise amplifier has been implemented having the specification of: RF 34.25–35.75 GHz; bandwidth 1.5 GHz; gain >60 dB; NF <2.3 dB, which are better indexes compared to some other works.

This paper presents a Ka band eight-channel integrated packaged phased array receiver front-end for a passive millimeter-wave imaging system. Since multiple receiving channels are integrated in a given package, the mutual coupling issue affecting the channel will deteriorate imaging quality. Therefore, in this study, the influence of channel mutual coupling on the system array pattern and amplitude phase error is analyzed, and the design requirements are proposed according to the results. During the design implementation, the coupling paths are discussed, and passive circuits in the path are modeled and designed to reduce the level of channel mutual coupling and spatial radiation. Finally, an accurate coupling measurement method for a multi-channel integrated phased array receiver is proposed. The receiver front-end achieves a 28~31 dB single channel gain, a 3.6 dB noise figure, less than −47 dB of channel mutual coupling. Furthermore, the array pattern of the two-dimensional 1024 channel system composed of the front end of the receiver is consistent with the simulation, and the receiver’s performance is verified by a human-body-imaging experiment. The proposed coupling analysis, design, and measurement methods are also applicable to other multi-channel integrated packaged devices.

What are the main findings? * Ambient noise seismic interferometry successfully retrieves empirical Green’s functions containing the vertical component of Rayleigh waves and P-wave reflection energy, enabling passive seismic imaging of Antarctic drilling sites. * The method provides stable and repeatable constraints on firn structure, ice thickness, and the ice–bedrock interface. Ambient noise seismic interferometry successfully retrieves empirical Green’s functions containing the vertical component of Rayleigh waves and P-wave reflection energy, enabling passive seismic imaging of Antarctic drilling sites. The method provides stable and repeatable constraints on firn structure, ice thickness, and the ice–bedrock interface. What are the implications of the main findings? * Passive seismic imaging provides a fast and cost-effective tool for Antarctic drilling site selection and subsurface characterization. * More generally, it can serve as a complement to active-source seismic methods, helping extend subsurface investigation to areas where conventional exploration is limited by logistical, environmental, or operational constraints. Passive seismic imaging provides a fast and cost-effective tool for Antarctic drilling site selection and subsurface characterization. More generally, it can serve as a complement to active-source seismic methods, helping extend subsurface investigation to areas where conventional exploration is limited by logistical, environmental, or operational constraints. Antarctic drilling projects provide critical information for investigating ice-sheet stability, reconstructing paleoclimate evolution, and characterizing subglacial geological structures through ice-core and bedrock recovery. Drilling site selection currently relies on high-resolution geophysical methods such as radio echo sounding and active-source seismic methods; however, radar imaging near the ice–bedrock interface is limited by electromagnetic attenuation, while active-source seismic methods in polar regions are constrained by logistical complexity and high cost. To address these limitations, this study proposes a passive integrated imaging approach that integrates P-wave responses and vertical-component Rayleigh-wave information retrieved from continuous ambient noise recordings near drilling sites using seismic interferometry. Based on their distinct propagation characteristics, signal selection and processing workflows are developed to jointly image near-surface firn structure, ice-sheet thickness, and subglacial bedrock structure. Application to the Princess Elizabeth Land drilling project in East Antarctica demonstrates that high- signal-to-noise-ratio P-wave responses and vertical-component Rayleigh-wave signals can be retrieved from as little as 24 h of ambient noise data, while stacking the full 20-day record further suppresses incoherent noise and yields more reliable imaging of the ice–bedrock interface. These results indicate that passive seismic imaging provides a rapid, cost-effective, and environmentally friendly complement for drilling site selection and operational support.

In this paper, the design, fabrication, and measurement of a compact broadband (4–8 GHz) analog complex correlator for a passive millimeter-wave imaging system are presented. To achieve high sensitivity and high integration of the imaging system, the wideband and miniaturization of the correlator are required. The correlator achieves wide bandwidth by using the add-and-square method, which is composed of a six-port circuit and a detection circuit. In order to realize the miniaturization of the correlator, the six-port circuit is realized on the chip base on the 0.15-μm gallium arsenide (GaAs) process. The influence of mismatch of the detection circuit that employs zero-bias Schottky diodes on the correlator is also analyzed to guide the design of the correlator. The measurement results of the designed chips and detector are consistent with the simulation result. Finally, a Sweep-frequency test is applied to the designed correlator, and the measurement results show that, within the frequency range of 4–8 GHz, the correlation amplitude fluctuation is less than 1.9 dB and the correlation efficiency is larger than 99%, which reveal that the correlator is suited for interferometric passive millimeter-wave imaging applications.

Post-processing techniques for polarimetric passive millimeter wave (MMW) imagery are proposed to display imaging information comprehensively. Initially an image fusion method based on two-scale decomposition is proposed to realize polarimetric passive imagery fusion. The fusion rules are separately designed for base layer and detail layer to reconstruct weight maps. Then an improved technique for displaying polarization information through color is proposed to present polarization features simultaneously with unpolarized imagery. Experimental results demonstrate that the proposed post-processing techniques are capable of presenting more informative imagery.

Passive millimeter-wave (MMW) and TeraHertz (THz) imaging systems have become increasingly popular in recent years due to their cost-effectiveness and non-invasive characteristics compared to active systems, prompting a surge in research interest. Evaluating the quality of reconstructed images used in these systems is essential for revealing the fine details. General image quality metrics such as the structural similarity index (SSIM) and the peak signal-to-noise ratio (PSNR) require a reference image in order to compare the reconstructed image. However, there is a notable gap in the literature regarding the evaluation of reconstruction or deconvolution algorithms with a reference image in the passive MMW/THz bands. This study proposes a reference image generation technique for passive MMW/THz imaging systems using an infrared imaging system that shares a similar physical background. Then, passive MMW/THz images were evaluated using the reference images at varying target distances and spatial resolutions. Besides these, the assessment of passive MMW/THz images with the SSIM and PSNR metrics after the reconstruction algorithms were performed. The metrics SSIM and PSNR, are inadequate in the evaluation of reconstruction algorithms alone in terms of concealed object (CO) detection. Because of this reason, the contrast level (CL) method was proposed to address the application-based shortcomings of PSNR and SSIM metrics. Hence, the image quality metric, CL, indicates that the Richardson–Lucy (RL) algorithm yielded superior results in variable optical configurations and target distances with the aid of CL metric. Finally, contrast enhancement techniques were developed in order to increase the contrast level of the CO. As a result, the introduction of these novel methods—the reference image generation technique using an infrared imaging system in passive MMW/THz bands, the evaluation of the reconstructed images with the application-based CL metric, and contrast enhancement techniques for single-band or multi-band imaging methods—holds the potential for the development of innovative techniques. These advancements may contribute to the creation of new applications within the passive MMW/THz bands, particularly focusing on the improvement of detection methods in the future.

The passive millimeter wave (PMMW) imaging sensor can generate images using the passive detection of the natural millimeter wave radiation from a scene. Despite the advantages of PMMW images in detecting concealed objects under clothing, they have lower resolution and fewer details than visible images. This paper proposes a new method to fuse PMMW and visible images to highlight concealed objects on the human body while preserving the details of the visible images. In this method, the PMMW image is initially segmented into three binary images, target, foreground, and background, utilizing an innovative segmentation algorithm that incorporates histogram-based thresholding and the generation of a saliency map image. Subsequently, the visible and PMMW images are individually decomposed into base and detail subbands using the new Gradient Transform (GT). Then, by individually fusing the base and detail subbands of the PMMW and visible images using innovative L2-norm weighting criteria, the fused image’s base and detail subbands are produced. Based on these criteria, between the two corresponding subbands of the input images, the subband with more detail contributes more to the final fused subband. Finally, the fused image is generated by applying the inverse GT to the newly generated fused subbands. Experimental results demonstrate a notable enhancement in terms of evaluation criteria like Q A B / F and MI , surpassing the most recent algorithms in this field.

In this paper, a high-performance detection algorithm of concealed forbidden objects on human body is presented based on deep neural networks (DNN) and complementary advantages of passive millimeter wave imagery (PMMWI) and visible imagery (VI). With well capacity of penetrability, PMMWI can effectively reveal suspected forbidden objects concealed on human body without harm of ionizing radiation compared with conventional X-ray methods. However, due to its current limited imaging capability, the resolution of PMMWI is still unsatisfactory and easy to result in false alarms. Therefore, by complementarity of superiorities, VI is employed to overcome the deficiency of confusions. In this way, massive image samples of PMMWI and VI are simultaneously acquired and manually annotated as necessary training datasets to carry out deep learning on DNN models so as to achieve high-performance human body profile segmentation on both PMMWI and VI. Then, high-precision region registration of human body profiles is implemented between PMMWI and VI to localize and confirm high confident suspected targets and remove false alarm regions as well. According to the principle of synthetic integration and global optimization, a high performance detection algorithm system is constructed, analyzed, and assessed. A series of comprehensive experiment results demonstrate the outstanding performance of our proposed detection algorithm.

Passive radiometry is a health-safe alternative to other imaging techniques conceived to be used on human beings for security screening or medical imaging applications. In this work we address a study on the feasibility of W-band passive radiometers in nano-scale CMOS technology for imaging applications. In particular, this study takes into account some of the most relevant aspects regarding the system-on-a-chip implementation, from system to circuit and technology limitations, with emphasis on the impact of the non-idealities of the detector on the overall system performances. Excluding the hardware differences between the investigated receiver architectures and their substantial implications, especially in case of array systems, the investigation shows how there is no a clear evidence to identify a priori the best performance receiver architecture, but it depends on the circuits, as well as operating frequency and technology.