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A Low Earth Orbit (LEO) constellation augmenting satellite navigation is important in the future development of Global Navigation Satellite System (GNSS). GNSS augmented by LEO constellations can improve not only the accuracy of Positioning, Navigation, and Timing (PNT), but also the consistency and reliability of secure PNT system. This paper mainly analyzes the diverse demands of different PNT users for LEO augmented GNSS, including the precision demand in real-time, the availability demand in special areas, the navigation signal enhancement demand in complex electromagnetic environments, and the integrity demand with high security. Correspondingly, the possible contributions of LEO constellations to PNT performance are analyzed from multiple aspects. A particular attention is paid to the special PNT user requirements that cannot be fulfilled with existing GNSS, such as the PNT service demand in the polar regions and the onboard GNSS orbit determination demand of some LEO satellites. The key technologies to be considered in the constellation design, function realization, and payload development of the LEO-augmented navigation system are summarized. [ABSTRACT FROM AUTHOR]

SARS-Cov-2 infection, which has caused the COVID-19 global pandemic, triggers cellular senescence. In this study, we investigate the role of the SARS-COV-2 spike protein (S-protein) in regulating the senescence of RPE cells. The results showed that administration or overexpression of S-protein in ARPE-19 decreased cell proliferation with cell cycle arrest at the G1 phase. S-protein increased SA-β-Gal positive ARPE-19 cells with high expression of P53 and P21, senescence-associated inflammatory factors (e.g., IL-1β, IL-6, IL-8, ICAM, and VEGF), and ROS. Elimination of ROS by N-acetyl cysteine (NAC) or knocking down p21 by siRNA diminished S-protein-induced ARPE cell senescence. Both administrated and overexpressed S-protein colocalize with the ER and upregulate ER-stress-associated BIP, CHOP, ATF3, and ATF6 expression. S-protein induced P65 protein nuclear translocation. Inhibition of NF-κB by bay-11-7082 reduced S-protein-mediated expression of senescence-associated factors. Moreover, the intravitreal injection of S-protein upregulates senescence-associated inflammatory factors in the zebrafish retina. In conclusions, the S-protein of SARS-Cov-2 induces cellular senescence of ARPE-19 cells in vitro and the expression of senescence-associated cytokines in zebrafish retina in vivo likely by activating ER stress, ROS, and NF-κb. These results may uncover a potential association between SARS-cov-2 infection and development of AMD. [ABSTRACT FROM AUTHOR]

Soil environmental quality assessment traditionally focuses on a single objective. Therefore, related results only reflect the pollution of the assessment objective and ignore the interaction and relationships between soil, ecology, and human health. In this study, we collected and filtered data about heavy metal content (Cr, Pb, Cd, Hg, As, Cu, Zn, and Ni) in city soils from more than 1000 published papers. Spatial multi-criteria decision-making theory was used to establish the combined evaluation model of soil environmental quality using the geochemical accumulation, ecological risk, non-carcinogenic risk, and carcinogenic risk indexes. Our results showed that the pollution degree and elements causing most pollution were significantly different when these different methods were used for assessment. The proposed framework was able to unify the overall pollution trend in regional soil environmental quality assessment and could output more robust and reasonable assessment results of soil, ecology, and human health. The random forest model was used to analyze the potential sources for excessive accumulation of heavy metal elements in soils, and the results showed that the main potential sources were the proportion of secondary industry and urbanization rate, each with an average contribution ratio of 20%. This study provides comprehensive and specific information for the prevention and control of soil heavy metal pollution in China. [ABSTRACT FROM AUTHOR]

Accurate soil organic carbon (SOC) data are very important for management of agricultural production and climate change mitigation. Visible near-infrared diffuse reflectance spectroscopy is an inexpensive, non-destructive, efficient, and reliable technique for monitoring soil properties. Soil spectral libraries can contain large sets of diverse soil data for empirical calibration. In this study, we focused on improving the prediction accuracy of the SOC content at the local field scale in Tibet using field-wet, intact spectra and different spectral libraries. The direct standardization algorithm and piecewise direct standardization algorithm were used to remove the influence of environmental factors from the in situ vis-NIR spectra. These algorithms effectively removed the influence of environment factors from the field-wet, intact spectra. The ratio of performance to deviation values for prediction of the SOC content using the field and laboratory spectra with the local spectral library were 1.57 and 1.98, respectively. The local spectral library models outperformed spiked national spectral library models and had higher ratio of performance to deviation values for shrub meadows, forests, and the total dataset. [ABSTRACT FROM AUTHOR]

Satellite navigation systems are vulnerable. To guarantee the positioning, navigation and timing (PNT) safety of core infrastructure, it is necessary to establish a secure PNT system with hybrid physical principles. In this paper, the augmentations of the BeiDou satellite system (BDS) itself are analysed, namely augmentations through the BDS inter-satellite link, BDS geostationary orbit (GEO) and inclined geostationary orbit (IGSO) satellites, and BDS PNT services supported by low earth orbit (LEO) satellites. Then, taking BDS as the core component, the comprehensive PNT infrastructure seamlessly covering deep space and deep ocean is described, consisting of the deep space PNT constellation, the sea-floor PNT sonar beacon network, and the ground-based low frequency and very low frequency (VLF) long wave radio stations. Moreover, the key technologies of resilient PNT application matching comprehensive PNT and various autonomous perception PNT information are discussed, such as resilient PNT sensor integration, the resilient PNT functional model and the resilient stochastic model. As a future development direction, the key factors of intelligent PNT services are analysed, including the intelligent perception of PNT application scenes, the intelligent optimization of PNT functional and stochastic models and the intelligent fusion of multisource PNT information. [ABSTRACT FROM AUTHOR]

With the gradual promotion and the application of difficult-to-machine materials such as titanium matrix composites in the aerospace field, high-quality hole-making technology has become a major demand in aviation manufacturing. In order to improve the hole-making quality of TiBw/TC4 composites, asynchronous mixed frequency vibration-assisted hole-making (AMFVAHM) method is proposed. The process consists of two steps: ultrasonic vibration-assisted drilling (UVAD) base hole and low-frequency torsional vibration-assisted helical milling (LFTVAHM) target hole. Based on this process, the cutting trajectory modeling is established, and the hole-making experiment on TiBw/TC4 composites is conducted. The experimental data show that during the hole expansion stage, the maximum XY-plane average milling force decreases by 30.96% and the maximum axial average milling force decreases by 24.49% compared with conventional helical milling (HM) when the torsional vibration frequency and the milling frequency are the same in LFTVAHM. The hole-making experiment shows that AMFVAHM can reduce the chip size, tool wear, and some other defects such as entrance/exit burrs, scratches, and fractures of the hole wall. Comparing with HM and UVAD, the verticality of hole wall increases by 71.43% and 86.21%, the inlet damage decreases by 27.98% and 31.60%, the outlet damage decreases by 2.80% and 14.47%, the hole wall roughness (Ra) decreases by 36.29% and 63.43%, and the maximum white layer thickness decreases by 19.99% and 67.66%. Meanwhile, AMFVAHM process not only reduces the cutting force and cutting temperature but also improves the hole-making quality due to the fretting friction effect of LFTVAHM in secondary hole expansion, which meets the need for high-quality hole-making of difficult-to-machine materials in practical engineering applications. [ABSTRACT FROM AUTHOR]

Carbon fiber–reinforced plastics (CFRP)/titanium alloy (Ti) stacks are extensively applied in the aerospace industry due to their excellent mechanical properties. However, their poor machinability poses great challenges. In this study, longitudinal-torsional ultrasonic vibration drilling (LT-UVD) is innovatively introduced to improve the quality of CFRP/Ti drilling. First, the separation mode of LT-UVD is analyzed by kinematic equations. Then, an experimental platform is built based on the LT-UVD vibration actuator to perform CFRP/Ti drilling experiments. The thrust force, interface temperature, hole wall quality, hole defects, Ti chip morphologies, and tool wear in LT-UVD are experimentally compared against conventional drilling (CD) and longitudinal ultrasonic vibration drilling (L-UVD). The experimental results show that, compared with CD and L-UVD, the thrust force of CFRP in LT-UVD decreases by 20.36–40.55% and 2.04–14.61%, while the thrust force of Ti decreases by 19.08–24.83% and 1.95–9.34%. Moreover, a relatively low maximum interface temperature is achieved in LT-UVD. In addition, the hole size accuracy, surface roughness for the hole's inner surface, and delamination factor are improved in LT-UVD. Fiber pullout defects, chip-breaking performance, and tool wear of CFRP are improved due to torsional vibration in LT-UVD. Finally, according to the high-speed camera, damage forms of the interface area are different when drilling CFRP/Ti stacks with various drilling sequences. [ABSTRACT FROM AUTHOR]

Due to the popular trend of combining online and offline interactions among users in event-based social networks (EBSNs), event recommendation which helps users discover their interesting events has become progressively urgent. Different from classic item recommendation, candidate events usually have short life cycle and occur in the future, resulting in severe challenges of data sparsity and cold-start. However, these problems are not well studied by previous works. In this article, we propose a Meta-Graph-based Attention Recommendation (MGAR) model to tackle aforementioned challenges by fully exploring complex semantic information based on meta-graphs extracted from EBSNs. First, we model the interactions between different entities as a heterogeneous information network and construct multiple meta-graphs to characterize the latent semantic preferences of users. Subsequently, we utilize convolutional neural networks and attention mechanisms to learn user and event latent factors by extracting semantic features of meta-graphs. Furthermore, the fused latent features are utilized to predict the ratings of a user to events and the events with top-k scores are recommended to the user. We collect several real-world datasets from a popular EBSN platform and conduct extensive experiments on the datasets. Our proposed model attains superior recommendation performance over several state-of-the-art approaches. Moreover, the results demonstrate that meta-graphs can reveal the semantic properties between users and events and improve event recommendation performance. [ABSTRACT FROM AUTHOR]

As a type of novel surface engineering technology, the application of cold spraying in the manufacturing and remanufacturing of high-end precision parts is very suitable. However, the disadvantage of low interface bonding strength can easily induce coating peeling at the coating/substrate interface under the effect of applied loads. Therefore, finding a suitable nondestructive testing method is significant to evaluate the cold-sprayed coating peeling process and provide prediction results. To achieve this aim, magnetic Barkhausen noise (MBN) testing was used in this paper because of its high sensitivity to microstructure variation. The Ni-Zn-Al2O3 ferromagnetic coating deposited on the ductile cast iron substrate was prepared by cold spraying. Based on the wavelet coherence analysis, the frequency of MBN signals induced by magnetic domain movement was mainly concentrated in the range of 1–4 kHz. The spectrum power between 1 kHz and 4 kHz was extracted as the characteristic value, and the linear fitting relationship between the characteristic value and applied load was established. When the power reaches the threshold value range of 3.106×10−8–3.557×10−8 W, the probability that the interfacial stress exceeds the coating/substrate bonding strength and that Ni-Zn-Al2O3 coating peeling occurs is 95%. Combined with the microstructure and morphology analysis, the mechanism of cold-sprayed coating peeling was revealed, and the underlying reason for MBN signal variation was explained. This research can accurately evaluate the ferromagnetic cold-sprayed coating peeling process and the interface bonding strength. [ABSTRACT FROM AUTHOR]