Corner Reflector Research Articles

Low-profile and compact retroreflector enabled by a wide-angle and high-efficiency metalens

In this paper, a low-profile metasurface retroreflector based on an ultra-thin wide-angle and high-efficiency metalens is proposed and investigated. Owing to its stable high transmission magnitude and the insensitive phase shift of the proposed meta-atoms within a continuous wide-angle range, the designed metalens can efficiently focus both normal and various oblique incident angle waves to a pre-designed curved trajectory continuously. By placing a concave metal mirror which is exactly coincide with the determined focused spots, the cascaded structure thus can realize backscattering enhancement working at a continuous wide-angle range. A prototype is also fabricated, measured and characterized and the experimental results verify that the proposed retroreflector can perform an excellent backscattering enhancement within a stable 3 dB monostatic RCS for almost ±30° angle range. Moreover, the relationship between the focusing performances of wide-angle metalens (e.g., focused spot power intensity, focusing efficiency and positions) and the backscattering enhancement of cascaded wide-angle retroreflector is established and highlighted. Besides, its excellent backscattering enhancement (the absolute RCS level and angular range) can be compared to some traditional widely-used radar retroreflectors like trihedral corner reflector or Luneburg lens reflector for the similar physical size. The proposed metalens retroreflector also posses of a noticeable working bandwidth of about 0.2 GHz within X band. The presented retroreflective device is promising to develop into a low-profile, light-weight and cost-effective wide-angle retroreflector thus enabling various applications for modern microwave engineering.

Monitoring dynamiki ruchów powierzchni osuwisk w warunkach zmiennego poziomu wód gruntowych z wykorzystaniem reflektorów radarowych

The main objective of the presented analysis was to investigate to what extent even minor changes in the hydrological and hydrogeological environment affect the changes in thedynamicsoflandslidesurfacedisplacement.Theresearchwascarriedoutforselectedmonitored landslides, which, in addition to the in-depth monitoring devices, were equipped with corner reflectors suitable for satellite radar interferometry measurements. The high temporal resolution of the interferometric data allowed demonstrating the existence of a relationship between the speed of surface movements and slight changes in the depth to the groundwater table. The analyses were performed for five landslides, which also showed a high dependence of these compounds on the geological structure of the landslide and its substrate.

Graphical acoustic computing method incorporated with the shooting and bouncing ray: application to target strength prediction of concave objects with second-order reflection effects

The acoustic backscattering prediction of underwater targets has been drawing concerns from underwater vehicle designers. A graphical acoustic computing method combined with the shooting bouncing ray method (GRACO-SBR) is introduced to solve the acoustic target strength of complex underwater target with concave surfaces. The graphical acoustic computing (GRACO) is an asymptotic method based on physical acoustics. Aided with OpenGL 3D graphical rendering pipeline, this method can process the acoustic integration parallelly and efficiently. Since the traditional GRACO only considers the first-order scattering, the shooting and bouncing ray (SBR) is compounded with the GRACO to cope with the higher-order scattering of concave structures. The target strength of an impedance sphere is solved with several different methods including the GRACO. The results show that the GRACO has an outstanding advantage of the efficiency over the finite element method (FEM) and the traditional Kirchhoff approximation at minor cost of the accuracy in high frequency range. Besides, the prediction of the backscattering of a dihedral corner reflector tells that the GRACO-SBR which considers the second-order scattering can acquire much more accurate solutions than the traditional GRACO method. Further, a stern with cross rudders is considered by both GRACO and GRACO-SBR, and the solutions show that the second-order scattering cannot be overlook when predicting the backscattering of concave structures.

3-D Printed Annular Linear-to-Circular Dielectric Polarizer and Its Applications to Omnidirectional and Multibeam Antennas

This article presents a 3-D printed dielectric polarizer. It is constituted by a number of annually arranged dielectric slabs, and each slab is 45° rotated with respect to the azimuth plane. It is able to transform the polarization state of the waves in azimuth plane from linear to circular. Based on the polarizer, two circularly polarized (CP) antenna designs are proposed. One of them is an omnidirectional CP antenna, and the other is a multibeam CP antenna with capability of full azimuthal scanning. They are fed by a dielectric filled biconical structure and six corner reflectors, respectively. Their polarizers are individually designed with different geometric parameters. Both antennas are one-piece configurations and can be directly 3-D printed without assemblies in later stage. Partial surfaces of the antennas are coated with copper films by an electroplating technique to act as the necessary metal components. To validate the design ideas, two prototypes are demonstrated in X-band. Experimental results show that the omnidirectional antenna can achieve a wide 3 dB axial ratio (AR) bandwidth of 28.3% from 8.5 to 11.3 GHz and a peak gain of 3.3 dBic. Meanwhile, the multibeam antenna is with a wide 3 dB AR bandwidth of 22.4% from 8.7 to 10.9 GHz and a peak gain of 9.2 dBic. Within the AR bandwidth, the AR values in full azimuth plane are lower than 3 dB.

High-frequency backscattering properties of quasi-omnidirectional corner reflector: The great-icosahedral-like reflector

The backscattering of perfectly conducting great-icosahedral-like reflectors is studied in the high-frequency domain. This particular faceted polyhedron, composed of 60 trihedral corner reflectors, is introduced to obtain closer omnidirectional backscattering. Due to the high cost of traditional methods, an estimation method for the full-polarized radar cross section is proposed, which is modified from the geometrical optics approximation method. The validity of the improved method is discussed, and its velocity is determined. The estimated results of the reflectors are studied, which lead to a conclusion that this complex structure has high-frequency properties of quasi-omnidirectivity and depolarization.

Polarization Analysis of Trihedral Corner Reflector With High-Frequency Approximation

Due to that the polarization scattering matrices (PSM) of single reflection (SR), double reflection (DR), and single diffraction (SD) at different angles are ignored, the PSM of trihedral corner reflector (TCR) is usually assumed to be [1 0; 0 1], which results in inaccuracy by utilizing its polarization to antijamming. Therefore, this article comprehensively analyzes the PSM of TCR based on high-frequency approximation. First, we separately analyze various reflection components based on the physical optics (PO)-shooting bouncing ray (SBR) model, so as to cure the deficiency that the classification discussion methods are susceptible to changes in target size and shape. Then, the PSMs of SR, DR, triple reflection, and SD are rigorously deduced and demonstrated by combining amplitude/phase information. In this process, we successfully integrate the six kinds of DR by the transforming of polarization base. Subsequently, the PSMs in the 2-D space are extended into the 3-D space, which are more suitable for the polarization characteristics of TCR. Finally, we compare the model in this article with the method of moment (MoM) of Feko software and obviously accelerate the model in this article through compute unified device architecture (CUDA). The conclusion of this article is significant for the polarization analysis of TCR to antijamming.

The analysis of impact factors in different application scenes of corner reflectors

The corner reflectors are a sort of electromagnetic structure equipment, with a number of important applications in different fields. The applications such as radar calibration and passive influence in military are summarized. Also, theoretical analysis and experimental simulation verification on practical production and application process are conducted. The machining edge length error, machining angle error and the extent of reflective surface deformation of the corner reflector and other errors that are easily introduced in the actual working environment are specifically listed and analyzed, which provides guidance on how to avoid and cope with such errors, both in the design of corner reflectors and in practical applications.

A low-cost active reflector and a passive corner reflector network for assisting landslide monitoring using multi-temporal InSAR

ABSTRACT A C-band Low-cost Active Reflector (AR) has been tested in a real experimental campaign aimed at monitoring through multi-temporal InSAR an area threatened by a landslide that occurred in 2019. To monitor and characterize the movement, a network of eight Passive Corner Reflectors and one Active Reflector were installed along a forested slope. A set of 285 interferograms obtained combining 60 Sentinel-1 SAR images were processed to evaluate the stability of the area. The AR, installed in a stable location close to the landslide, was used to provide a reference point in this low coherence area. Despite the high sensitivity of the phase response of such devices to temperature changes, the device operates with a stability of ±2 mm in deformation retrieval, a value acceptable for monitoring purposes, with a moderate range of temperature values.

Precise motion compensation method in terahertz ISAR imaging based on sharpness maximization

According to the characteristics of the manoeuvring targets and the superiorities of Terahertz Inverse Synthetic Aperture Radar (THz-ISAR), a precise motion compensation method based on image sharpness maximization for manoeuvring targets in the terahertz radar was proposed in this letter. Computational efficiency and compensation accuracy are improved by modified Newton's method. Then a 330-GHz imaging radar and an experiment on a corner reflector were introduced. The validity of the method was verified, by the experimental results, to be promising in THz-ISAR imaging of manoeuvring targets.

Uni-modal retroreflection in multi-modal elastic wave fields

The phenomenon of retroreflection of light characterized by a single transversely polarized wave has been extensively explored. However, the phenomenon of retroreflection of a longitudinally polarized elastic wave in a solid medium is far behind the current theory because an incident longitudinal wave can be diffusively reflected into multiple wave modes, both longitudinal and transverse. Here, our experiments show that a uniquely engineered meta-corner reflector can perfectly retroreflect an incident longitudinal wave onto the same longitudinal wave without generating a transverse wave. The meta-corner reflector consisted of a novel sequential stack of a full longitudinal-to-transverse mode-converting layer and an angle-tuned corner reflector. We present a theoretical foundation for retroreflection and its critical application in measuring the thickness of a thin film coated on a substrate that is usually difficult to measure.

A Ship Discrimination Method Based on High-Frequency Electromagnetic Theory

Ship target detection using radar has important applications in the military and civilian fields. As a decoy, the corner reflector (CR) can successfully deceive a radar by its strong radar cross-section (RCS) to protect a ship. In order to discriminate between a CR and ship, this paper proposes a discrimination method based on three-dimensional characteristics. First, we deduce the basic scattering of CR by the high-frequency electromagnetic theory, and propose a CR decomposition which can solve the problem that the Krogager decomposition has terrible errors in clutter. Then, we introduce the definition of the main scattering polarization and give the multi-dimensional characteristic of CR. Subsequently, we analyze the spatial-time characteristic of radar based on the three-dimensional proportional guidance. With the CR mean square error (MSE), a CR discrimination method is proposed based on the time-spatial-polarization (TSP) joint domains. Finally, the proposed method is analyzed and compared using the fully polarimetric data of Feko software, which can achieve 95% discrimination probability and 4.1% false alarm probability.

Intelligent Target Design Based on Complex Target Simulation

The emergence and popularization of various fifth-generation fighter jets with supersonic cruise, super maneuverability, and stealth functionalities have raised higher and more comprehensive challenges for the tactical performance and operational indicators of air defense weapon systems. The training of air defense systems requires simulated targets; however, the traditional targets cannot simulate the radar cross-section (RCS) distribution characteristics of fifth-generation fighter aircrafts. In addition, the existing target aircrafts are expensive and cannot be mass-produced. Therefore, in this paper, a corner reflector and a Luneburg ball reflector with RCS distribution characteristics of a fifth-generation fighter in a certain spatial area are designed for target simulation. Several corner reflectors and Luneburg balls are used to form an array to realize the simulations. The RCS value and distribution characteristics of the target can be combined with fuzzy clustering and a single-chip microcomputer to design an intelligent switching system, which improves the practicability of the intelligent target design proposed in this paper.

Development of a magnetic cardan suspension coupled with a single actuated axis for gravity assisted pointing of directional devices: A case study of Moon to Earth alignment for a laser ranging reflector

A new class of Lunar Laser Ranging (LLR) experiments has been recently investigated by the international Space Agencies, in order to deploy a set of Next Generation Lunar Reflector (NGLR) on the Moon surface, funded under NASA's Commercial Lunar Payload Services (CLPS) program (Currie et al., 2020). In principle the Cube Corner Reflector (CCR) arrays deployed by Apollo and Lunokhod missions in the 70’s will be flanked (at different locations) by single bigger reflectors, thus removing the systematic errors in the Time of Flight (TOF) measurement induced by the Moon librations (Turyshev et al., 2012).In this paper we describe a mechanism formed by a passive cardan magnetic suspension, dedicated to the alignment of the CCR toward the mean Earth direction. The suspension is able to compensate for the misalignment due to the slope of the landing site that can be different at each site (Zupp, 2013). If proper control of azimuth angle of the spacecraft at touchdown cannot be guaranteed, an additional motor to rotate the magnetic suspension must be provided by the lander, for the alignment toward the mean Earth direction.Once the landing site is defined, the geometry of the retroreflector housing, together with that of the cardan suspension, will autonomously (without human intervention) and passively (without provision of electrical power by the lander) provide the right elevation angle just by gravity, no matter what the morphology of the site is.The mechanical performances of the magnetic suspension have been preliminarily investigated in (Ancillai et al., 2020). The solution presented can be applied to any directional device, requiring pointing capability, like antennas, and can be adopted with low friction rolling bearings as well.

An 1.7–2.7 GHz all-weather broadband High-gain antenna with W-shape reflector for maritime communications

An all-weather broadband high-gain antenna for maritime communication applications is developed in this paper. The proposed antenna, which is devised from the perspective of hydrodynamics, hydrostatics, and electromagnetic performance, is based on combining a wideband high-efficiency electromagnetic radiation structure (WHEMS) and a W reflector to achieve both superior mechanical and electrical performances. The proposed W reflector has a fractional 3-dB gain bandwidth of 150 % for a three-dipole array radiator, which is 53 % and 90 % larger than the gain bandwidth of the classic planar reflectors and the corner reflectors, respectively. To better adapt to the harsh marine environment and its potential IoT demands, the proposed antenna is devised to be light weight, small wind loading, high mechanical strength, small size, low cost, lightning-strike resistant, and consistent coefficient of thermal expansion, without the need of any radome. Two W reflector prototypes are ideated and manufactured from sheets and metal rods, denoted as Antenna I and Antenna II, respectively. Both the antennas realize 10–13 dBi gains in a 45 % bandwidth (1.7–2.7 GHz) with the sidelobe level lower than 12 dB, and a front-to-back ratio larger than 18 dB. The anti-deformation property of the proposed Antenna II is demonstrated to be 3.8 times higher than that of the classic Yagi antenna to be used in an open-ocean environment with a typhoon of force 1.

Calibration of D3R Weather Radar Using UAV-Hosted Target

The accuracy of weather radar-measured products, such as reflectivity, plays a crucial factor in obtaining good quality, derived remote sensing products, such as hydrometeor classification. A slight mismatch of a few decibels in reflectivity may cause the hydrometeor classification to deviate from the actual truth. In order to obtain accurate remote-sensing measurements, calibration of weather radars should be carried out at regular intervals of time. The dual-frequency, dual-polarization, Doppler radar (D3R) is a well-established tool for measuring light rain and snow. There are various methods which are used for the calibration of weather radars, such as suspending a metallic sphere from a weather balloon or using corner reflectors on top of a tower or structure. In this work, we have shown the potential of using a UAV to suspend a calibration sphere, which is then used for calibrating the D3R radar. In this work, the advantages along with the practical aspects to be considered for calibration using UAV are discussed in detail. From the calibration results, it was observed that an offset of 2.2 dB was present in the Ku H-Pol reflectivity, Ku V-Pol was well calibrated and offsets within 2 dB were observed in the Ka H-Pol and V-Pol reflectivities.

Nationwide, Operational Sentinel-1 Based InSAR Monitoring System in the Cloud for Strategic Water Facilities in Hungary

The intensive development of both interferometric technology and sensors in recent years allows Interferometric Synthetic Aperture Radar (InSAR)-based applications to be accessible to a growing number of users. InSAR-based services now cover entire countries and soon even the whole of Europe. These InSAR systems require massive amounts of computer processing power and significant time to generate a final product. Most, if not all, of these projects have a limited “monitoring component”, aimed at historical analysis but are rarely, if ever, updated. Consequently, the results do not necessarily meet every purpose or specific user requirement. It is now clear that the increasing computing capacity and big data provided by the sensors have initiated the development of new InSAR services. However, these systems are only useful when linked to specific real-world operational problems. Continuous monitoring of a country’s ageing water management infrastructure has become an increasingly critical issue in recent years, in addition to the threats posed by climate change. Our article provides a comprehensive overview of a nationwide, dedicated, operational InSAR application, which was developed to support the operational work of the Hungarian Disaster Management Service (HDMS). The objective was to provide monthly monitoring of 63 water facilities, including 83 individual objects, distributed throughout Hungary, in combination with the development of a near real-time warning system. Our work involved the compilation of a completely new InSAR System as a Service (SaaS) which incorporates user requirements, preparatory work, the compilation of the Sentinel-1 automatic processing pipeline, the installation of corner reflectors, a special early warning system, and a dedicated user interface. The developed system can automatically start to evaluate the S1 measurements within 24 h of downloading the data into the system storage forward the results toward the warning system before the next image arrives. Users are provided with detailed information on the stability of 70% of the 83 water facility objects monitored through the dedicated user interface. The additional early warning system currently operates as a preliminary “spatial decision support system”, but the HDMS is willing to make it fully operational over the next few years.

Fast Detection of Small Targets in High-Resolution Maritime Radars by Feature Normalization and Fusion

Joint exploitation of multiple features is a recognized way to implement the effective detection of small targets on the sea in high-resolution maritime radars at the dwelling mode. The tri-feature-based and feature-compression-based detectors use the time-consuming convex hull learning and decision. In this article, a fast feature-fusion-based detector using seven salient features is proposed, where the convex hull learning and decision are replaced by simple threshold determination and decision. It is found that the seven features of sea clutter can be modeled well by the Burr type XII and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</i> -distributions. From the fitted distribution, each feature is normalized to approximate the standard Gaussian distribution by a nonlinear transformation. In the seven-dimensional normalized feature space, an analytical method is given to calculate the optimal weights of feature fusion by the maximization of the interclass distance of the two-class samples. Owing to the normalization of the seven features, the feature fusion loss is significantly reduced comparing with the direct fusion of seven features. The fast feature-fusion-based detector was evaluated on the open and recognized radar databases and offshore experimental data using an unmanned aerial vehicle with a corner reflector. The results show that the fast detector attains competitive performance with the existing best feature-based detector.

Resistance of a corner cube reflector with a dielectric antireflection coating to the space environment

Subject of study. The negative effect of corpuscular radiation of outer space on the optical properties of a corner cube reflector with a multilayer dielectric coating is studied. Aim. The feasibility of applying a bilayer antireflection dielectric coating for radiation with a wavelength of 532 nm on the input face of a corner cube reflector operated in a high-orbit spacecraft is investigated. Method. Electron–proton irradiation simulating the effect of hot magnetospheric plasma on corner cube reflectors with and without antireflection coatings was carried out using a simulation stand. Because the corner cube reflector is made of KU-1 radiation-resistant quartz glass, special attention was devoted to the formation of electrostatic discharges that accompany the irradiation of the samples. Main results. The induced optical density of the coating of the input face of corner reflectors caused by magnetospheric plasma particles increased with the distance from the geometric center of the aperture of the input face to the metal frame. This can be explained by the fact that discharges with the maximum intensity were observed in the vicinity of the frame, resulting in the destruction of the near-surface layers of the irradiated samples. It has been experimentally established that the antireflection effect of a bilayer coating, which comprises HfO2 (thickness of approximately 25 nm) and SiO2 (thickness of approximately 100 nm), leveled with increasing irradiation dose. Practical significance. The results demonstrate that the use of the investigated bilayer antireflection coating is advisable in optical systems of a high-orbit spacecraft with an active lifetime not exceeding 6 years or under conditions of lower dose load.

Reflection Mechanism of Dielectric Corner Reflectors: The Role of the Diffraction of Evanescent Waves and the Goos-Hänchen Shift.

Nano- and microstructures have been developed for asymmetric light transmission (ALT) filters operating in a wide wavelength range. One of the most straightforward structures with ALT properties is a dielectric corner reflector (DCR) comprising a one-dimensional grating of a triangular shape on one surface. The DCR possesses strong reflection only for one-way light illumination due to multiple total internal reflections (TIRs) inside the triangular grating. For triangular structures being much larger than the wavelength of light, the reflection properties are expected to be fully described by geometrical optics. However, geometrical optics do not account for the Goos–Hänchen (GH) shift, which is caused by the evanescent wave of the TIR. In this work, the reflection mechanism of DCRs is elucidated using the finite element method and a quantitative model built by considering the GH shift. The reduction in reflection of the DCR is dominated by diffraction of the evanescent wave at the corner of the triangular structure. Our model is based on simple mathematics and can optimize the DCR geometry for applications addressing a wide wavelength range such as radiative cooling.

Characteristics of an Eight-Quadrant Corner Reflector Involving a Reconfigurable Active Metasurface.

The traditional corner reflector is a type of classical passive jamming equipment but with several shortcomings, such as fixed electromagnetic characteristics and a poor response to radar polarization. In this paper, an eight-quadrant corner reflector equipped with an electronically controlled miniaturized active frequency-selective surface (MAFSS) for X band is proposed to obtain better radar characteristics controllability and polarization adaptability. The scattering characteristics of the new eight-quadrant corner reflector for different switchable scattering states (penetration/reflection), frequency and polarization are simulated and analyzed. Results show that the RCS modulation depth, which is jointly affected by the electromagnetic wave frequency and incident directions, can be maintained above 10 dB in the majority of directions, and even larger than 30 dB at the resonant frequency. Moreover, the RCS adjustable bandwidth can be as wide as 1 GHz in different incident directions.