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Passive multispectral imaging and active bathymetric lidar make a great achievement for bathymetry in optically shallow waters. However, due to the attenuation of the water column precludes deep penetration of the light, accurately obtaining the underwater topography in turbid waters through remote sensing techniques, both passive and active, is still a challenging task. Airborne lidars can obtain water surface topography with high accuracy and resolution, which can further be used to derive the water depth based on wave theory. In this study, an “indirect” method to determine water depth is proposed using airborne lidar measured water surface points. As the wavelength and wave direction can be accurately tracked from the water surface topography, a $20\times20$ m underwater topography near Ganquan Island, South China Sea, is generated with a root mean square error (RMSE) of 0.91 m and a mean absolute percent error (MAPE) of 7.1%. The basic theory of deriving water depths is totally different from airborne lidar bathymetry (ALB), i.e., this method is independent of water clarity and can be used in turbid waters or even works with near infrared airborne lidar that can only obtain water surface points. [ABSTRACT FROM AUTHOR]

The new generation photon-counting lidar on Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2) can obtain the subsurface optical properties of sea waters. Recent studies highlight the applications of deriving the lidar attenuation coefficient $K_{\mathrm {lidar}}$ and then substituting $K_{\mathrm {lidar}}$ into the bio-optical model to obtain more information of sea waters. As commonly used bio-optical models are built for the diffuse attenuation coefficient $K_{d}$ that are traditionally derived by passive ocean color sensors, whether $K_{\mathrm {lidar}}$ derived from ICESat-2 can be directly used as $K_{d}$ is a fundamental question. Given that $K_{d}$ is an apparent optical property (AOP) in the water column rather than an inherent optical property (IOP), $K_{d}$ is closely related to the zenith angle of the incident light. The zenith angle of the incident light of the sunlight is normally tens of degrees for ocean color sensors, while the maximum laser off-nadir angle is ~1.5° for the ICESat-2 lidar. To demonstrate this issue, we select hundreds of ground tracks of ICESat-2 in both open ocean and coastal sea waters and compare the derived $K_{\mathrm {lidar}}$ with their corresponding Moderate Resolution Imaging Spectroradiometer (MODIS)-derived $K_{d}$. The results indicate that the corrected results of $1.2\times K_{\mathrm {lidar}}$ , instead of the direct results of $K_{\mathrm {lidar}}$ , are more consistent with MODIS $K_{d}$. This study is of great significance to the better fusion of active lidar data and passive optical data in ocean observations. [ABSTRACT FROM AUTHOR]

A new method for the design of multilayer printed partially reflective surface (PRS) is proposed to achieve broadband, high efficiency, and miniaturization of Fabry–Pérot resonant antenna (FPRA). In this letter, every two-layer structure is regarded as a resonator to obtain a narrowband positive reflection phase gradient and then cascaded to obtain a broadband. Finally, a three-layer PRS is designed and integrated to form a C-band compact FPRA with overall dimensions of 0.65 λ0×0.65 λ0×0.52 λ0 (λ0 is the free-space wavelength at the center frequency). The antenna was simulated, fabricated, and validated. The results show it has a high aperture efficiency due to the small aperture and a wide impedance bandwidth well covering 4.0–9.57 GHz. [ABSTRACT FROM AUTHOR]

This work proposes an efficient hybrid method consisting of the efficient iterative method of moments–physical optics (EI-MoM-PO) and the equivalent dipole moment (EDM) with the high-order impedance boundary condition (HOIBC) to explore the electromagnetic scattering from the object with an isotropic or anisotropic surface coating. By introducing the HOIBC into the EDM method, the relationship between the equivalent electric and magnetic dipole moments is established. Therefore, the EDM method is extended to general cases. For the composite scattering from electrically large objects including MoM and PO regions, the EDM method is implemented to fill the MoM impedance matrix and to accelerate the evaluation of the coupling between the MoM and PO regions, which considerably improves the computing efficiency. The numerical results indicate that the proposed hybrid method allows for a substantial decrease in computation time. Also, they also agree well with the traditional finite element method, MoM, or MoM-PO method, which proves the validity of the proposed method. [ABSTRACT FROM AUTHOR]

To take the advantages of a convolutional neural network (CNN), U-net, and a complex-valued CNN (complex-CNN), a new complex-valued U-net (CU-net) is proposed for deep learning (DL)-based methods to solve inverse scattering problem (ISP). With the proposed CU-net, the complex scattered data carrying rich information of object can be directly used for inversion without any preprocessing, which is very helpful for the accuracy improvement of the final result. To validate the performance of proposed method, a microstructure, consisting of a finite periodic set of circular cylindrical dielectric rods, is considered and detected for textural abnormalities, which contains the missing, flaw, and displacement of the rods. The distances between rods and diameters of rods are both subwavelength, well beyond the Rayleigh criterion, which causes this ISP extremely ill-posed. For comparison, both the conventional iterative method and DL-based method are used to solve this nonlinear problem. Numerical simulations demonstrate that the well-trained DL-based methods can successfully produce excellent results almost in real time and can greatly outperform the conventional iterative methods in terms of quality and efficiency. [ABSTRACT FROM AUTHOR]

A method to realize the flat-topped radiation pattern of Fabry–Pérot cavity antenna is proposed based on the ray-tracing model. In the design, a nonuniform partially reflective surface of dielectric substrate is proposed to reduce the sidelobe level and get a fast roll-off in the operating frequency band. Besides, a dumb source array is added around the dual-polarized feed array to minimize the disparity of beamwidth between E- and H-plane. The antenna was calculated with the proposed method, simulated by the full-wave simulation, and validated by the experiment. Compared with the traditional array synthesis method, there is no need to design a complex feed network because each coaxial port is fed equally in phase and amplitude. In addition, based on the proposed method, the aperture size is small and the flat-topped beamwidth is flexible. [ABSTRACT FROM AUTHOR]

Detailed information on lake bathymetry is essential for both hydrology-related studies and water resource management. Conventionally, lake bathymetry was mapped using high-cost approaches (e.g., ship/boat-based multibeam echosounders or airborne bathymetric lidars). With only satellite remotely sensed data sets, a method for deriving high-resolution bathymetry for dynamic areas was proposed by combining the new Ice, Cloud, and land Elevation Satellite-2 (ICESat-2) lidar data and the Landsat-based Global Surface Water Data Set (GSWD). First, ICESat-2 can provide accurate along-track topographic points after the point cloud processing and bathymetric error correction, and the GSWD can supply water occurrence information within the lake dynamic area between 1984 and 2018. Second, using the derived relationship between the elevation and water occurrence, the bathymetric map of Lake Mead, USA, was produced with the dynamic area exceeding 235 km2, elevation ranging nearly 37 m, and a resolution of 30 m. The local reference data (i.e., the airborne topographic lidar data and ship/boat-based bathymetric data) in six areas around Lake Mead were used for the validations. In general, the produced lake bathymetry achieved an accuracy of approximately 2 m in elevation with R2 of 0.97. The proposed method is promising to obtain global bathymetry for inland water bodies (e.g., the lake and reservoir) and coastal areas (e.g., the tidal zone) where water level fluctuations are strong and the water clarity is sufficient. [ABSTRACT FROM AUTHOR]

A low-profile, wide-scan antenna array operating in the 6–12 GHz band with planar and conformal variants is reported in this letter. The design comprises capacitively coupled dipoles, integrated with an artificial magnetic conductor layer to achieve a low profile (< 0.2λh). A patch with feed via is used as a conventional feed balun. In addition, a via fence is employed for resonance mitigation and improved impedance matching at the lower end of the frequency band. The array exhibits a wide-angle scan capability (±60°) in E-, H-, and D-plane for active voltage standing wave ratio (VSWR) < 3. A 16 × 3 conformal array, axially mounted onto a cylindrical surface with a radius of 250 mm (10λh) and 8 × 8 planar array configurations, is simulated and analyzed. The measurement results of the conformal array prototype show good agreement with the simulations in terms of VSWR, radiation pattern, and realized gain. Therefore, the proposed design is an attractive candidate for planar and conformal wireless devices at C, X-band frequencies. [ABSTRACT FROM AUTHOR]

Surface-water-level changes reflect Earth’s water resource variations (e.g., trends and fluctuations) and they are helpful to understand potential drivers (e.g., climate change and human activities). Currently, Australia is facing serious water crisis owing to rainfall shortage and climate change, and national-scale data set on surface-water-level changes is required for supporting sustainable water resource management. Here, we used all Landsat Thematic Mapper (TM)/Enhanced Thematic Mapper (ETM)+/Operational Land Imager (OLI) data available on Google Earth Engine to obtain annual surface water during 2003–2019 and produced 1506 boundaries of water bodies with areas greater than 1 km2 across Australia. The produced surface water map in Australia is more accurate than the existing global lake databases (e.g., the Global Lakes and Wetlands Database), which can be downloaded for free. Then, 52 water bodies (lakes and reservoirs) with areas larger than 1 km2 and available Ice, Cloud, and land Elevation Satellite (ICESat/ICESat-2) data for more than 5 years were combined to estimate trends in surface water levels in Australia. Across Australia, from 2003 to 2019, the area-weighted mean of water level change rates is −0.046 m/year with 17 lakes (32.7%) with increasing water levels and 35 lakes (67.3%) decreasing with water levels. In detail, the largest lakes (>100 km2) dominate the total change trend and most of the largest lakes underwent decreasing trend (−0.046 m/year), whereas the mean water levels of small lakes (< 10 km2) increased in the past 17 years. In situ water levels of three typical lakes/reservoirs were used to validate our estimation results, which exhibited a very good agreement (R2 =0.98). [ABSTRACT FROM AUTHOR]

The sensing performance of antimonene towards SF6 decomposition gases was investigated by first principles calculations in this paper. By analyzing the adsorption energy, charge transfer, work function variation, and density of states (DOS), we concluded that antimonene is sensitive and selective to the SF6 decomposition gas product SO2. Unlike previous studies, the effect of Stone-Wales (SW) defects and biaxial strain on the sensing performance of antimonene was also explored, indicating that SW defects can expand the range of available antimonene sensor structures and that antimonene is highly stable to biaxial strain. As stability is one of the main challenges that hinders the practical application of sensing materials, we concluded that anitmonene would be a promising candidate for the development of SF6 decomposition gas sensors with high sensitivity and stability that could replace other sensitive but unstable two dimensional materials such as phosphorene. [ABSTRACT FROM AUTHOR]

This contribution identifies the uncertainty sources influencing the component uncertainties for an airborne LiDAR bathymetry (ALB) measurement and presents the models for various component uncertainties (arising from the device, environment, and target for ALBs). Since various instrumental and environmental factors introduce vertical and horizontal uncertainties in ALB data, these uncertainties should be first analyzed and then precisely modeled to ensure the accuracy of the measurements. For this purpose, ten different effects that influence the accuracy of ALB data are systematically analyzed and modeled for four aspects in this article: the device aspect (laser pointing deflection, trajectory uncertainty, and boresight/lever arm offset), environmental aspect (atmospheric limitation, refraction on the sea surface, refraction in water, scattering in water, and water level fluctuation), target aspect (irregular bottom), and other aspect (accuracy of coordinate transformation model). In addition, the effect of the laser spot size is also discussed. To verify the presented uncertainty models, an ALB survey was operated around Yuanzhi Island in the South China Sea. For a water depth of 10 m, the theoretical overall root-sum-squares (RSS) for ten different effects of an ALB system (approximately 34 cm calculated using the total vertical uncertainty (TVU) models) is generally in accordance with the actual performance of the ALB data (approximately 39 cm) performance. The difference is mainly attributed to the limited accuracy of the ground truth data, and the difference between the water depth and laser ranging is reasonable. In this process, the topography data in the same region captured by a shipborne multibeam echo sounder (MBES) were used as the ground truth. The results indicated that for the typical ALB system, the laser pointing uncertainty and refraction uncertainty on the sea surface are primary uncertainty sources and should be corrected in a higher priority to meet the seafloor topographic accuracy demand of the International Hydrographic Organization (IHO) Standards for Hydrographic Surveys (S-44). The proposed uncertainty models can be used not only to guide the actual measurement of an ALB system but also to provide the uncertainty correction reference for ALB data postprocessing. [ABSTRACT FROM AUTHOR]

As an essential indicator of climatic and environmental change, water levels of lakes are sensitive to various natural factors and anthropogenic activities from local to global scales. Understanding the global lake water level change can help to uncover the earth’s water resource change and its response to the potential drivers. To date, most studies on lake water level changes were performed at local and regional scales. However, comprehensive information on the magnitude of lake water level changes at a global scale remains poorly understood. Here, with the help of the lake polygons from the global lakes and wetlands database (GLWD) at a global scale, we analyzed all available Ice, Cloud, and Land Elevation Satellite (ICESat) data to estimate the change in water levels of 14 981 lakes with areas greater than 0.1 km2 and reservoirs with storage capacities greater than 0.5 km3 around the world. We found global lakes exhibiting a significant spatial heterogeneity with 58.68% increase and 41.32% decrease in water levels during 2003 and 2009. The average change rate of global lake levels is 0.013 m/yr. We discovered an obvious and broad lake level increase in North America and Siberia region, Tibetan Plateau, and the Amazon basin. Our results provide detailed satellite-based evidence of the global increase in lake levels. In the future, more works should be conducted to estimate the lake level change at a longer temporal scale using multi-source satellite data and investigate the potential drivers under the climate warming and growing human footprint. [ABSTRACT FROM AUTHOR]

A new efficient aggregate basis function method is proposed for the electromagnetic scattering from composite structures consisting of finite periodic array and other object. The aggregate basis functions composed of Rao–Wilton–Glisson basis functions are defined, which leads to a good decrease in the number of unknowns, but with the enough accuracy. To obtain the aggregate basis functions, the approximate solution is implemented. By taking advantage of the nature of periodic array, the filling number of the impedance matrix blocks is substantially decreased, and the procedure of getting the approximate solutions is accelerated. The accuracy and efficiency of the proposed method are demonstrated in a typical scattering example. [ABSTRACT FROM AUTHOR]

In this letter, we took a first principles calculation of five SF6 decomposition gas molecules (SO2, H2 S, SOF2, SO2F2, and SF6) adsorption on monolayer MoTe2. By calculating adsorption energy, charge transfer, and work function combined with differential charge density analysis, we predict that MoTe2 is sensitive and selective to the SO2 molecule. Furthermore, the total density of states analysis and projected density of states analysis demonstrate that the orbital hybridization is the main reason of the intense charge transfer between the SO2 molecule and monolayer MoTe2. In summary, it can be concluded that MoTe2 is a promising candidate for SF6 decomposition gas sensors with high sensitivity and selectivity. [ABSTRACT FROM PUBLISHER]

Phosphorene is a promising candidate for gas sensing materials. This letter describes our study of the adsorption of SF6 and SF6 decomposition gases (SO2 and H2 S) on phosphorene. We used first principles calculations to explore phosphorene’s potential applications as gas sensor to diagnose the state of online gas insulated switchgear (GIS). The calculation results showed that only the adsorption of SO2 induced a moderate adsorption energy and apparent charge transfer. We further investigated the current–voltage ( I – V ) relationships before and after gas absorption through the non-equilibrium Green’s function method. It was found that only SO2 induced a dramatic change in the I – V relationships. Therefore, phosphorene appears to be a promising candidate for highly sensitive and selective SF6 decomposition gas sensors for online GIS diagnosis. [ABSTRACT FROM AUTHOR]

Far-field focusing with superresolution aided by a proposed metallic plate is investigated in this paper. The proposed near-field structure can be employed to distinguish point-like sources with subwavelength distance and enhance the field strength based on the spoof surface plasmon polaritons theory, and by the simulations of near- and far-field spectrums, it can be found that some near-field evanescent waves carried the superresolution information can be converted to propagation waves and propagate to the far-field. Therefore, with the help of time reversal imaging process, the far-field superresolution focusing is validated by the experiments in laboratory environment. A resolution of $\sim$ $\lambda$/13 is achieved from the last results, which is well beyond the diffraction limit. [ABSTRACT FROM PUBLISHER]

A time-reversal imaging system for far-field super-resolution imaging of scatterers is proposed. First, a grating plate, similar to the optical far-field super lens, is designed to convert near-field evanescent waves to propagative waves with wavevector modulation. Then, the scattered signals are processed by a time-reversal (TR) technique and retransmitted back. Finally, the images of the targets are reconstructed by doing demodulation operations in the k-domain to the TR field at the refocusing time. Imaging results show that two scatterers separated by 0.1 λ can be distinguished clearly. [ABSTRACT FROM PUBLISHER]

Planar resonant lenses (PRLs) consisting of periodically distributed strip resonators with different lengths are proposed for far-field super-resolution electromagnetic imaging. With the help of PRLs, evanescent waves can be converted into propagation waves. So, based on time-reversal (TR) technique, time-reversal mirror in the far-field can receive the electromagnetic signals carrying the subwavelength information. Experiments are performed to validate the proposed method in laboratory environment at microwave band. Far-field super-resolution imaging results with resolutions of \lambda /20 and \lambda /10, respectively, are obtained, well beyond the diffraction limit. The compact size and multifrequency property of the proposed PRLs will make them easily be integrated to new super-resolution imaging systems. [ABSTRACT FROM PUBLISHER]