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Ground penetrating radar (GPR) has become one of the key technologies in subsurface sensing and, in general, in non-destructive testing (NDT), since it is able to detect both metallic and nonmetallic targets. GPR for NDT has been successfully introduced in a wide range of sectors, such as mining and geology, glaciology, civil engineering and civil works, archaeology, and security and defense. In recent decades, improvements in georeferencing and positioning systems have enabled the introduction of synthetic aperture radar (SAR) techniques in GPR systems, yielding GPR–SAR systems capable of providing high-resolution microwave images. In parallel, the radiofrequency front-end of GPR systems has been optimized in terms of compactness (e.g., smaller Tx/Rx antennas) and cost. These advances, combined with improvements in autonomous platforms, such as unmanned terrestrial and aerial vehicles, have fostered new fields of application for GPR, where fast and reliable detection capabilities are demanded. In addition, processing techniques have been improved, taking advantage of the research conducted in related fields like inverse scattering and imaging. As a result, novel and robust algorithms have been developed for clutter reduction, automatic target recognition, and efficient processing of large sets of measurements to enable real-time imaging, among others. This Special Issue provides an overview of the state of the art in GPR imaging, focusing on the latest advances from both hardware and software perspectives.

Technology: general issues --- Ground Penetrating Radar (GPR) --- Unmanned Aerial Vehicles (UAVs) --- Synthetic Aperture Radar (SAR) --- Real Time Kinematic (RTK) --- Ultra-Wide-Band (UWB) --- landmine and IED detection --- non-destructive testing --- GPR --- coherence --- semblance --- attribute analysis --- imaging --- GPR trace --- high-resolution data --- large-scale survey --- archaeological prospection --- Ground-Penetrating Radar --- velocity analysis --- coherency functionals --- GPR data processing --- GPR data migration --- spatial-variant convolution neural network (SV-CNN) --- spatial-variant convolution kernel (SV-CK) --- radar image enhancing --- MIMO radar --- neural networks --- imaging radar --- ground penetrating radar --- wavelet scattering network --- machine learning --- support vector machine --- pipeline identification --- snow --- snow water equivalent (SWE) --- stepped-frequency continuous wave radar (SFCW) --- software defined radio (SDR) --- snowpack multilayer reflectance --- Ground Penetrating Radar --- Synthetic Aperture Radar --- landmine --- Improvised Explosive Device --- radar --- noise attenuation --- Gaussian spike impulse noise --- deep convolutional denoising autoencoders (CDAEs) --- deep convolutional denoising autoencoders with network structure optimization (CDAEsNSO) --- applied geophysics --- digital signal processing --- enhancement of 3D-GPR datasets --- clutter noise removal --- spectral filtering --- ground-penetrating radar --- nondestructive testing --- pipelines detection --- modeling --- signal processing --- Ground Penetrating Radar (GPR) --- Unmanned Aerial Vehicles (UAVs) --- Synthetic Aperture Radar (SAR) --- Real Time Kinematic (RTK) --- Ultra-Wide-Band (UWB) --- landmine and IED detection --- non-destructive testing --- GPR --- coherence --- semblance --- attribute analysis --- imaging --- GPR trace --- high-resolution data --- large-scale survey --- archaeological prospection --- Ground-Penetrating Radar --- velocity analysis --- coherency functionals --- GPR data processing --- GPR data migration --- spatial-variant convolution neural network (SV-CNN) --- spatial-variant convolution kernel (SV-CK) --- radar image enhancing --- MIMO radar --- neural networks --- imaging radar --- ground penetrating radar --- wavelet scattering network --- machine learning --- support vector machine --- pipeline identification --- snow --- snow water equivalent (SWE) --- stepped-frequency continuous wave radar (SFCW) --- software defined radio (SDR) --- snowpack multilayer reflectance --- Ground Penetrating Radar --- Synthetic Aperture Radar --- landmine --- Improvised Explosive Device --- radar --- noise attenuation --- Gaussian spike impulse noise --- deep convolutional denoising autoencoders (CDAEs) --- deep convolutional denoising autoencoders with network structure optimization (CDAEsNSO) --- applied geophysics --- digital signal processing --- enhancement of 3D-GPR datasets --- clutter noise removal --- spectral filtering --- ground-penetrating radar --- nondestructive testing --- pipelines detection --- modeling --- signal processing

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Ground penetrating radar (GPR) has become one of the key technologies in subsurface sensing and, in general, in non-destructive testing (NDT), since it is able to detect both metallic and nonmetallic targets. GPR for NDT has been successfully introduced in a wide range of sectors, such as mining and geology, glaciology, civil engineering and civil works, archaeology, and security and defense. In recent decades, improvements in georeferencing and positioning systems have enabled the introduction of synthetic aperture radar (SAR) techniques in GPR systems, yielding GPR–SAR systems capable of providing high-resolution microwave images. In parallel, the radiofrequency front-end of GPR systems has been optimized in terms of compactness (e.g., smaller Tx/Rx antennas) and cost. These advances, combined with improvements in autonomous platforms, such as unmanned terrestrial and aerial vehicles, have fostered new fields of application for GPR, where fast and reliable detection capabilities are demanded. In addition, processing techniques have been improved, taking advantage of the research conducted in related fields like inverse scattering and imaging. As a result, novel and robust algorithms have been developed for clutter reduction, automatic target recognition, and efficient processing of large sets of measurements to enable real-time imaging, among others. This Special Issue provides an overview of the state of the art in GPR imaging, focusing on the latest advances from both hardware and software perspectives.

Ground Penetrating Radar (GPR) --- Unmanned Aerial Vehicles (UAVs) --- Synthetic Aperture Radar (SAR) --- Real Time Kinematic (RTK) --- Ultra-Wide-Band (UWB) --- landmine and IED detection --- non-destructive testing --- GPR --- coherence --- semblance --- attribute analysis --- imaging --- GPR trace --- high-resolution data --- large-scale survey --- archaeological prospection --- Ground-Penetrating Radar --- velocity analysis --- coherency functionals --- GPR data processing --- GPR data migration --- spatial-variant convolution neural network (SV-CNN) --- spatial-variant convolution kernel (SV-CK) --- radar image enhancing --- MIMO radar --- neural networks --- imaging radar --- ground penetrating radar --- wavelet scattering network --- machine learning --- support vector machine --- pipeline identification --- snow --- snow water equivalent (SWE) --- stepped-frequency continuous wave radar (SFCW) --- software defined radio (SDR) --- snowpack multilayer reflectance --- Ground Penetrating Radar --- Synthetic Aperture Radar --- landmine --- Improvised Explosive Device --- radar --- noise attenuation --- Gaussian spike impulse noise --- deep convolutional denoising autoencoders (CDAEs) --- deep convolutional denoising autoencoders with network structure optimization (CDAEsNSO) --- applied geophysics --- digital signal processing --- enhancement of 3D-GPR datasets --- clutter noise removal --- spectral filtering --- ground-penetrating radar --- nondestructive testing --- pipelines detection --- modeling --- signal processing --- n/a

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Emerging wide bandgap (WBG) semiconductors hold the potential to advance the global industry in the same way that, more than 50 years ago, the invention of the silicon (Si) chip enabled the modern computer era. SiC- and GaN-based devices are starting to become more commercially available. Smaller, faster, and more efficient than their counterpart Si-based components, these WBG devices also offer greater expected reliability in tougher operating conditions. Furthermore, in this frame, a new class of microelectronic-grade semiconducting materials that have an even larger bandgap than the previously established wide bandgap semiconductors, such as GaN and SiC, have been created, and are thus referred to as “ultra-wide bandgap” materials. These materials, which include AlGaN, AlN, diamond, Ga2O3, and BN, offer theoretically superior properties, including a higher critical breakdown field, higher temperature operation, and potentially higher radiation tolerance. These attributes, in turn, make it possible to use revolutionary new devices for extreme environments, such as high-efficiency power transistors, because of the improved Baliga figure of merit, ultra-high voltage pulsed power switches, high-efficiency UV-LEDs, and electronics. This Special Issue aims to collect high quality research papers, short communications, and review articles that focus on wide bandgap device design, fabrication, and advanced characterization. The Special Issue will also publish selected papers from the 43rd Workshop on Compound Semiconductor Devices and Integrated Circuits, held in France (WOCSDICE 2019), which brings together scientists and engineers working in the area of III–V, and other compound semiconductor devices and integrated circuits.

Technology: general issues --- GaN --- high-electron-mobility transistor (HEMT) --- ultra-wide band gap --- GaN-based vertical-cavity surface-emitting laser (VCSEL) --- composition-graded AlxGa1−xN electron blocking layer (EBL) --- electron leakage --- GaN laser diode --- distributed feedback (DFB) --- surface gratings --- sidewall gratings --- AlGaN/GaN --- proton irradiation --- time-dependent dielectric breakdown (TDDB) --- reliability --- normally off --- power cycle test --- SiC micro-heater chip --- direct bonded copper (DBC) substrate --- Ag sinter paste --- wide band-gap (WBG) --- thermal resistance --- amorphous InGaZnO --- thin-film transistor --- nitrogen-doping --- buried-channel --- stability --- 4H-SiC --- turn-off loss --- ON-state voltage --- breakdown voltage (BV) --- IGBT --- wide-bandgap semiconductor --- high electron mobility transistors --- vertical gate structure --- normally-off operation --- gallium nitride --- asymmetric multiple quantum wells --- barrier thickness --- InGaN laser diodes --- optical absorption loss --- electron leakage current --- wide band gap semiconductors --- numerical simulation --- terahertz Gunn diode --- grooved-anode diode --- Gallium nitride (GaN) high-electron-mobility transistors (HEMTs) --- vertical breakdown voltage --- buffer trapping effect --- gallium nitride (GaN) --- power switching device --- active power filter (APF) --- power quality (PQ) --- metal-insulator-semiconductor high-electron-mobility transistor (MIS-HEMT) --- recessed gate --- double barrier --- high-electron-mobility transistors --- copper metallization --- millimeter wave --- wide bandgap semiconductors --- flexible devices --- silver nanoring --- silver nanowire --- polyol method --- cosolvent --- tungsten trioxide film --- spin coating --- optical band gap --- morphology --- electrochromism --- self-align --- hierarchical nanostructures --- ZnO nanorod/NiO nanosheet --- photon extraction efficiency --- photonic emitter --- wideband --- HEMT --- power amplifier --- jammer system --- GaN 5G --- high electron mobility transistors (HEMT) --- new radio --- RF front-end --- AESA radars --- transmittance --- distortions --- optimization --- GaN-on-GaN --- schottky barrier diodes --- high-energy α-particle detection --- low voltage --- thick depletion width detectors --- GaN --- high-electron-mobility transistor (HEMT) --- ultra-wide band gap --- GaN-based vertical-cavity surface-emitting laser (VCSEL) --- composition-graded AlxGa1−xN electron blocking layer (EBL) --- electron leakage --- GaN laser diode --- distributed feedback (DFB) --- surface gratings --- sidewall gratings --- AlGaN/GaN --- proton irradiation --- time-dependent dielectric breakdown (TDDB) --- reliability --- normally off --- power cycle test --- SiC micro-heater chip --- direct bonded copper (DBC) substrate --- Ag sinter paste --- wide band-gap (WBG) --- thermal resistance --- amorphous InGaZnO --- thin-film transistor --- nitrogen-doping --- buried-channel --- stability --- 4H-SiC --- turn-off loss --- ON-state voltage --- breakdown voltage (BV) --- IGBT --- wide-bandgap semiconductor --- high electron mobility transistors --- vertical gate structure --- normally-off operation --- gallium nitride --- asymmetric multiple quantum wells --- barrier thickness --- InGaN laser diodes --- optical absorption loss --- electron leakage current --- wide band gap semiconductors --- numerical simulation --- terahertz Gunn diode --- grooved-anode diode --- Gallium nitride (GaN) high-electron-mobility transistors (HEMTs) --- vertical breakdown voltage --- buffer trapping effect --- gallium nitride (GaN) --- power switching device --- active power filter (APF) --- power quality (PQ) --- metal-insulator-semiconductor high-electron-mobility transistor (MIS-HEMT) --- recessed gate --- double barrier --- high-electron-mobility transistors --- copper metallization --- millimeter wave --- wide bandgap semiconductors --- flexible devices --- silver nanoring --- silver nanowire --- polyol method --- cosolvent --- tungsten trioxide film --- spin coating --- optical band gap --- morphology --- electrochromism --- self-align --- hierarchical nanostructures --- ZnO nanorod/NiO nanosheet --- photon extraction efficiency --- photonic emitter --- wideband --- HEMT --- power amplifier --- jammer system --- GaN 5G --- high electron mobility transistors (HEMT) --- new radio --- RF front-end --- AESA radars --- transmittance --- distortions --- optimization --- GaN-on-GaN --- schottky barrier diodes --- high-energy α-particle detection --- low voltage --- thick depletion width detectors

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This Special Issue gathers topics of utmost interest in the field of antennas and propagation, such as: new directions and challenges in antenna design and propagation; innovative antenna technologies for space applications; metamaterial, metasurface and other periodic structures; antennas for 5G; electromagnetic field measurements and remote sensing applications.

Technology: general issues --- Energy industries & utilities --- 5G mobile communication --- Sub-6-GHz --- compact antenna --- channel selection --- channel filters --- metamaterials --- antenna radiation measurements --- common mode current --- distance averaging --- multipath site --- small antenna --- loop probe --- log-periodic dipole array --- wideband antenna --- VHF --- cross polarized --- array element --- radio detection --- cosmic rays --- ultra-wide band antennas --- energy-based antenna descriptors --- pulsed sources --- direction finding --- dual-band dipole --- CRLH antenna --- dual-band balun --- CRLH balun --- wireless communication --- textile antenna --- wearable antenna --- SAR --- flexible antenna --- low-profile antenna --- sensor network --- active test --- UWB --- 802.15.4z --- timestamp detection --- ranging --- multipath --- frequency fading --- dye-sensitized solar cells --- integration --- antenna array --- solar antenna --- multibandoperation --- slotted antenna --- microwave --- millimeter-wave band --- WLAN --- 5G --- frequency multiplexed --- IoT --- millimeter-waves --- multi-band --- n-band antenna --- antenna as a sensor --- meander line antenna --- periodic structure --- millimeter-wave antenna --- frequency scanning antenna --- leaky-wave antenna --- planar sensor --- non-invasive --- Split Ring Resonator --- dielectrics measurements --- RF absorbing materials --- higher symmetries --- glide symmetry --- periodic structures --- mode matching --- dispersion analysis --- radar cross section (RCS) measurement --- near-field to far-field transformation (NFFFT) --- spherical wave expansion (SWE) --- anisotropy --- dielectric constant --- material characterization --- planar resonators --- substrate bending --- textile fabrics --- wearable radiators --- reflectarray antennas --- multibeam antennas --- dual band reflectarrays --- communication satellites --- Ka-band --- near-field antenna effect --- radar calibration --- MIMO radar --- turntable radar --- UWB radar --- radar system --- scattering imaging --- inverse scattering problem --- radar resolution --- photomixer --- terahertz source --- two dimensional photonic crystal --- frequency selective surface superstrate --- terahertz antenna --- dielectric resonator antenna --- biomedical devices --- wireless communication link --- near-field region --- impedance matching characteristics --- horizontal polarization --- UAV ground station --- Omni-directional --- wireless power transmission (WPT) --- energy harvesting --- rectenna --- wireless sensors --- n/a

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Emerging wide bandgap (WBG) semiconductors hold the potential to advance the global industry in the same way that, more than 50 years ago, the invention of the silicon (Si) chip enabled the modern computer era. SiC- and GaN-based devices are starting to become more commercially available. Smaller, faster, and more efficient than their counterpart Si-based components, these WBG devices also offer greater expected reliability in tougher operating conditions. Furthermore, in this frame, a new class of microelectronic-grade semiconducting materials that have an even larger bandgap than the previously established wide bandgap semiconductors, such as GaN and SiC, have been created, and are thus referred to as “ultra-wide bandgap” materials. These materials, which include AlGaN, AlN, diamond, Ga2O3, and BN, offer theoretically superior properties, including a higher critical breakdown field, higher temperature operation, and potentially higher radiation tolerance. These attributes, in turn, make it possible to use revolutionary new devices for extreme environments, such as high-efficiency power transistors, because of the improved Baliga figure of merit, ultra-high voltage pulsed power switches, high-efficiency UV-LEDs, and electronics. This Special Issue aims to collect high quality research papers, short communications, and review articles that focus on wide bandgap device design, fabrication, and advanced characterization. The Special Issue will also publish selected papers from the 43rd Workshop on Compound Semiconductor Devices and Integrated Circuits, held in France (WOCSDICE 2019), which brings together scientists and engineers working in the area of III–V, and other compound semiconductor devices and integrated circuits.

GaN --- high-electron-mobility transistor (HEMT) --- ultra-wide band gap --- GaN-based vertical-cavity surface-emitting laser (VCSEL) --- composition-graded AlxGa1−xN electron blocking layer (EBL) --- electron leakage --- GaN laser diode --- distributed feedback (DFB) --- surface gratings --- sidewall gratings --- AlGaN/GaN --- proton irradiation --- time-dependent dielectric breakdown (TDDB) --- reliability --- normally off --- power cycle test --- SiC micro-heater chip --- direct bonded copper (DBC) substrate --- Ag sinter paste --- wide band-gap (WBG) --- thermal resistance --- amorphous InGaZnO --- thin-film transistor --- nitrogen-doping --- buried-channel --- stability --- 4H-SiC --- turn-off loss --- ON-state voltage --- breakdown voltage (BV) --- IGBT --- wide-bandgap semiconductor --- high electron mobility transistors --- vertical gate structure --- normally-off operation --- gallium nitride --- asymmetric multiple quantum wells --- barrier thickness --- InGaN laser diodes --- optical absorption loss --- electron leakage current --- wide band gap semiconductors --- numerical simulation --- terahertz Gunn diode --- grooved-anode diode --- Gallium nitride (GaN) high-electron-mobility transistors (HEMTs) --- vertical breakdown voltage --- buffer trapping effect --- gallium nitride (GaN) --- power switching device --- active power filter (APF) --- power quality (PQ) --- metal-insulator-semiconductor high-electron-mobility transistor (MIS-HEMT) --- recessed gate --- double barrier --- high-electron-mobility transistors --- copper metallization --- millimeter wave --- wide bandgap semiconductors --- flexible devices --- silver nanoring --- silver nanowire --- polyol method --- cosolvent --- tungsten trioxide film --- spin coating --- optical band gap --- morphology --- electrochromism --- self-align --- hierarchical nanostructures --- ZnO nanorod/NiO nanosheet --- photon extraction efficiency --- photonic emitter --- wideband --- HEMT --- power amplifier --- jammer system --- GaN 5G --- high electron mobility transistors (HEMT) --- new radio --- RF front-end --- AESA radars --- transmittance --- distortions --- optimization --- GaN-on-GaN --- schottky barrier diodes --- high-energy α-particle detection --- low voltage --- thick depletion width detectors --- n/a