Abstract
With the strengthening of global anti-terrorist measures, it is increasingly important to conduct security checks in public places to detect concealed objects carried by the human body. ...Research in recent years has shown that deep learning is helpful for detecting concealed objects in passive terahertz images. However, previous studies have failed to achieve superior accuracy and performance for real-time labeling. Our research aims to propose a novel method for accurate and real-time detection of concealed objects in terahertz images. To reach this goal we trained and tested a promising detector based on deep residual networks using human image data collected by passive terahertz devices. Specifically, we replaced the backbone network of the SSD (Single Shot MultiBox Detector) algorithm with a more representative residual network to reduce the difficulty of network training. Aiming at the problems of repeated detection and missed detection of small targets, a feature fusion-based terahertz image target detection algorithm was proposed. Furthermore, we introduced a hybrid attention mechanism in SSD to improve the algorithm’s ability to acquire object details and location information. Finally, the Focal Loss function was introduced to improve the robustness of the model. Experimental results show that the accuracy of the SSD algorithm is improved from 95.04 to 99.92%. Compared with other current mainstream models, such as Faster RCNN, YOLO, and RetinaNet, the proposed method can maintain high detection accuracy at a faster speed. This proposed method based on SSD achieves a mean average precision of 99.92%, an F1 score of 0.98, and a prediction speed of 17 FPS on the validation subset. This proposed method based on SSD-ResNet-50 can provide a technical reference for the application and development of deep learning technology in terahertz smart security systems. In the future, it can be widely used in some public scenarios with real-time security inspection requirements.
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2.
Volcanic history of the Imbrium basin Zhang, Jinhai; Yang, Wei; Hu, Sen ...
Proceedings of the National Academy of Sciences - PNAS,
04/2015, Volume:
112, Issue:
17
Journal Article
Peer reviewed
Open access
We report the surface exploration by the lunar rover Yutu that landed on the young lava flow in the northeastern part of the Mare Imbrium, which is the largest basin on the nearside of the Moon and ...is filled with several basalt units estimated to date from 3.5 to 2.0 Ga. The onboard lunar penetrating radar conducted a 114-m-long profile, which measured a thickness of ∼5 m of the lunar regolith layer and detected three underlying basalt units at depths of 195, 215, and 345 m. The radar measurements suggest underestimation of the global lunar regolith thickness by other methods and reveal a vast volume of the last volcano eruption. The in situ spectral reflectance and elemental analysis of the lunar soil at the landing site suggest that the young basalt could be derived from an ilmenite- rich mantle reservoir and then assimilated by 10–20% of the last residual melt of the lunar magma ocean.
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To achieve high-resolution 2-D images, through-wall imaging (TWI) radar with ultra-wideband and long antenna arrays faces considerable technical challenges such as a prolonged data collection time, a ...huge amount of data, and a high hardware complexity. This paper presents a novel data acquisition scheme and an imaging algorithm for TWI radar based on compressive sensing (CS), which states that a signal having a sparse representation can be reconstructed from a small number of nonadaptive randomized projections by solving a tractable convex program. Instead of measuring all spatial-frequency data, a few samples, by employing an overcomplete dictionary, are sufficient to obtain reliable target space images even at high noise levels. Preliminary simulated and experimental results show that the proposed algorithm outperforms the conventional delay-and-sum beamforming method even though many fewer CS measurements are used.
Multiple-input-multiple-output synthetic aperture radar (MIMO-SAR) is being studied and applied in more and more scenarios. However, there is still a certain distance away from real-time imaging ...using advanced algorithms. The traditional backpropagation algorithm (BPA) multi-accumulation integration is unsuitable for dealing with large-size scanning data, and the wavenumber domain algorithm requires the array to satisfy Nyquist sampling law in azimuth to avoid aliasing in imaging reconstruction. Based on these issues, a novel 3D imaging method is proposed for MIMO-SAR. An appropriate transformation and inverse Fourier transform (FT) is carried out for the frequency domain; thus, accumulation in the wavenumber domain is not required, which is easy to implement. The computational complexity of the algorithm is much lower than BPA and has better generalizability than the wavenumber domain algorithm. Coherence factor (CF) is also introduced to achieve sidelobe suppression. Proof-of-principle experiments were also carried out in the 92.5 GHz band based on the MIMO-SAR prototype system. Both simulation and experimental results of different distributed targets show good performance of imaging and do not lose the quality of image reconstruction.
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China’s first Mars exploration mission (HuoXing-1) has been named as ‘Tianwen-1’ meaning Heaven Inquiry. Tianwen-1 was launched on July 23, 2020. In this paper, the scientific objectives of earlier ...and current Mars exploration missions worldwide are reviewed, and the scientific objectives, payloads and preliminary scientific investigation plan of China’s first Mars exploration mission are introduced, and expected scientific achievements are analyzed.
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DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
With the development of millimeter-wave radar systems, substrate-integrated waveguide (SIW) slot antennas have been widely used. This paper presents a computational approach for calculating the ...equivalent normalized admittance of slot in low sidelobe SIW longitudinal slot antennas. In contrast to the existing methods, this method takes into account the impact of the N-slots coupling on the admittance. Through this method, fitting functions for resonant length and conductance are obtained, which describe the relationship between the resonant length, resonant conductance, and offset of slots. Moreover, based on the two fitting functions, a low sidelobe SIW longitudinal slot linear antenna is designed for millimeter-wave radar applications at 77 GHz. This antenna consists of a transition from the standard WR-12 waveguide to the SIW and a set of radiating slots. The simulation results indicate an impedance bandwidth of 4.8% and a gain of 13.2 dBi. The sidelobe level in the H-plane is below −28 dB, which meets the desired specifications and confirms the accuracy of the presented method.
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The dielectric permittivity is one of the most significant properties of lunar material. For the past decades, researchers have assumed or used an indirect density versus depth relation to constrain ...the subsurface permittivity on the Moon. On 3 January 2019, Chang'E‐4 (CE‐4) rover‐borne lunar penetrating radar (LPR) successfully landed on the Von Kármán crater on the Moon's farside. CE‐4 LPR is the first surface radar on the Moon's farside for exploring regolith thickness and geological structure. Numerous parabolic‐shaped diffractions (PSD), which indicate subsurface objects, are identified in LPR data. We estimate the permittivity of the first ~50 m at the landing region using these PSDs based on a hyperbolic fitting method. A best‐fit permittivity/density versus depth relation is obtained, and this relation could be used at different areas on the Moon.
Plain Language Summary
Currently, rover‐deployed/spaceborne radar systems are frequently employed in lunar exploration. The depth and thickness of lunar subsurface structure observed from radar data are completely influenced by the dielectric properties of subsurface materials. The dielectric properties also provide vital clues on compositions and origin of subsurface materials. However, except the results measured with returned sample from lunar surface, we have no more information about dielectric properties of global and deeper lunar subsurface materials. In January, China's CE‐4 spacecraft softly landed on the farside of the Moon for the first time and released the lunar penetrating radar (LPR). In this study, we estimate the dielectric permittivity of the first ~50 m at the landing region with LPR data and propose a best‐fit permittivity/density versus depth relation which could be used at different areas on the Moon.
Key Points
The permittivity of lunar subsurface materials at CE‐4 landing site is estimated
A best‐fit permittivity/density versus depth relation is obtained
The obtained best‐fit relation can be used at different areas on the Moon
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The thickness estimation of the top surface layer and surface layer, as well as the detection of road defects, are of great importance to the quality conditions of asphalt pavement. Although ground ...penetrating radar (GPR) methods have been widely used in non-destructive detection of pavements, the thickness estimation of the thin top surface layer is still a difficult problem due to the limitations of GPR resolution and the similar permittivity of asphalt sub-layers. Besides, the detection of some road defects, including inadequate compaction and delamination at interfaces, require further practical study. In this paper, a newly-developed vehicle-mounted GPR detection system is introduced. We used a horizontal high-pass filter and a modified layer localization method to extract the underground layers. Besides, according to lab experiments and simulation analysis, we proposed theoretical methods for detecting the degree of compaction and delamination at the interface, respectively. Moreover, a field test was carried out and the estimated results showed a satisfactory accuracy of the system and methods.
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A novel active terahertz imaging scheme based on the combination of fan-beam scanning and aperture synthesized reconstruction techniques is presented. In the horizontal direction, high resolution is ...obtained by the narrow side of the fan-beam based on real aperture focusing with special pillbox-like quasi-optics. In the vertical direction, high resolution is achieved by the aperture synthesized focusing techniques based on the broad side of the fan-beam. Appropriate focusing algorithms are proposed both for the single-frequency 2-D case and wideband 3-D imaging. With a 0.2-THz heterodyne transceiver, a prototype imager is developed and proof-of-concept experiments are performed. The experimental results on the resolution, specular reflection, and concealed threat objects detection are given in detail, which demonstrate the performance of the proposed imaging scheme and its potential application for fast personnel screening.
In this paper, in order to avoid the phenomenon that the diffraction waves cover the incident wave in the reflective frequency-scanning antenna system, which causes insufficient utilization of the ...beam scanning angular scope, a transmissive planar binary periodical structure metasurface was chosen to achieve frequency-controlled beam scanning, and an approach to realize beam steering with high efficiency was proposed. Specifically, a coherence-enhancing method based on two subcells was proposed to increase conversion efficiency from the incident wave to the diffraction wave. According to the proposed concepts, the frequency scanning antenna based on the transmissive metasurface was designed, fabricated, and measured in the frequency range of 0.25 THz to 0.3 THz. The measured results demonstrate that the designed transmissive metasurface can realize scanning angle 12.5° with comparatively high transferring efficiency. Besides, the proposed transmissive frequency-scanning device also has a lot of unique advantages, such as low cost, simple design and fabrication, which have potential application values in terahertz high frame rate imaging, target detections, communications, etc.
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