Corner Reflector Research Articles

МОДЕЛЮВАННЯ ВИКОРИСТАННЯ ПАСИВНИХ ЗАСОБІВ ІМІТАЦІЇ ДЛЯ СПОТВОРЕННЯ РАДІОЛОКАЦІЙНОЇ ІНФОРМАТИВНОЇ ОБСТАНОВКИ

A 3-D model of corner reflectors of various types and sizes was built on the basis of the analysis of the results of mathematical modeling using the CST Studio automatic decision-making system. As a result of the simulation, the numerical values of the parameters of the effective scattering area were determined, which made it possible to clarify the dependence of the relative change of the effective scattering area of corner reflectors on the shape of the rib, to confirm the hypothesis regarding the quadratic dependence of the effective scattering area on the radiation frequencies. A comparison was made with the results of calculations based on known analytical formulas, which were determined empirically to calculate the maximum effective scattering area for corner reflectors of typical configurations. For a more in-depth analysis of the simulation results, a conventional cross-section of the scattering diagram and its expansion relative to different angles of rotation were performed. A comparative analysis of the modeling results allowed us to conclude that for different observation angles, the effective scattering area of a small-angle reflector corresponds on average to the effective scattering area of a Mi-8T helicopter. It was also established that the most appropriate for simulating fighter jets are large corner reflectors with a rib size of 120 cm. At the same time, the weight of the corner reflector needs to be significantly reduced, which becomes possible when using light materials, such as foam plastic, which is pasted over with foil of the appropriate thickness. Electrodynamics’ modeling was confirmed by an experiment in an anechoic chamber. Experiments on changing the parameters of the effective scattering area of corner reflectors depending on a number of controlled factors have been chosen as the direction of further research. Keywords: modeling, passive means of imitation, radio engineering means, distortion, radar information environment.

A Channel Imbalance Calibration Scheme with Distributed Targets for Circular Quad-Polarization SAR with Reciprocal Crosstalk

As polarimetric antennas can be isolated through excellent electronic frameworks in circular quad-polarization (CQP) synthetic aperture radar (SAR) systems, cross-polarization (x-pol) and co-polarization (co-pol) channel imbalances are more challenging and essential to calibrate than crosstalk in polarimetric calibration (PolCAL). In uncalibrated CQP SAR images without corner reflectors (CRs), the reciprocity and reflection symmetry assumptions of the distributed targets are commonly used to estimate the x-pol and co-pol channel imbalances, respectively. To suppress the influence of additive noise on determining channel imbalances through distributed targets, high signal-to-noise ratio (SNR) distributed targets should be obtained from the x-pol and co-pol channels of the CQP SAR images: namely, surface-dominated and volume-dominated targets. However, some reflection symmetry assumptions used in the existing calibration literature have poor applicability with volume-dominated targets, resulting in unsatisfactory estimation results for the co-pol channel imbalance phase. In this paper, we assess the priority of the reflection symmetry properties of volume-dominated targets used to calibrate the co-pol channel imbalance phase in CQP SAR data synthesized from linear quad-polarization data of ALOS, GF-3, and RADARSAT-2. In the theoretical part, high-priority reflection symmetry (termed semireflection symmetry) assumptions are confirmed as the most suitable for estimating the co-pol channel imbalance phase, and were selected to develop an algorithm for estimating the co-pol channel imbalance phase. Furthermore, based on the novel method for estimating the co-pol channel imbalance phase, a channel imbalance calibration scheme is proposed for CQP SAR systems with reciprocal crosstalk, including extracting surface-dominated and volume-dominated targets, and estimating and filtering channel imbalances. We demonstrate the effectiveness of our proposed scheme with CRs in simulated CQP SAR images. The experimental results show that the calibration scheme is an effective workflow for estimating channel imbalances in CQP SAR systems with reciprocal crosstalk.

Monitoring nonlinear and fast deformation caused by underground mining exploitation using multi-temporal Sentinel-1 radar interferometry and corner reflectors: application, validation and processing obstacles

ABSTRACT In this study, Differential Interferometric Synthetic Aperture Radar Interferometry (DInSAR) of artificial Corner Reflectors (CRs) were validated in the area of fast and nonlinear deformation gradient caused by active coal longwall exploitation. Three Sentinel-1 datasets were processed using conventional DInSAR, Persistent Scatterer Interferometry (PSI), and Small BAseline Subset methods implemented in ENVI SARscape™. For evaluation, leveling and Global Navigation Satellite System (GNSS) measurements were used. Considering the challenge of snow cover, the removal of all winter images was not a successful strategy due to the long temporal baseline and strong movement, which cause phase unwrapping problems and underestimate the real deformation. The results indicate that only conventional DInSAR and SBAS with low network redundancy allow us to capture maximal deformation gradient and the root mean square error calculated between the CRs and the ground truth is on the level of 2–3 cm for the vertical and easting deformation component, respectively. For the small deformation gradient represented by the permanent GNSS station (4 cm/year), all SBAS techniques appeared to be more accurate than DInSAR, which corresponds to higher redundancy and better removal of the atmospheric signal. In contrast, DInSAR results allowed to capture information about two subsidence basins, which was not possible with SBAS and PSI approaches.

An Influence of Snow Covers on the Radar Interferometry Observations of Industrial Infrastructure: Norilsk Thermal Power Plant Case

This manuscript presents the results of the study of snow covers’ influence on the interferometric measurements of the stability of industrial infrastructure in the vicinity of Norilsk city, Russia. Fuel tanks of the Norilsk thermal power plant (TPP) were selected as an object of study due to a well-known accident when about 20,000 tons of diesel fuel spilled from one of the tanks. Sentinel-1 synthetic aperture radar data acquired over the territory of Norilsk TPP were used in the DInSAR study of the possible displacements of the tanks that could be the cause of the tank’s damage. For twelve days, radar interferograms that were generated in the study covered the cold and warm seasons of 2018–2020, including the catastrophic event—the rupture of the tank with diesel fuel—in order to shed light on the possible impact of the area subsidence because of permafrost thaw under the tanks. As the tank walls and adjacent concrete base constituted the virtual dihedral corner reflector, the accumulation of snow on the surface near the tanks created a distorting effect on the results of monitoring the stability of the tank’s location. Three models of snow layer within the dihedral proposed could help explain the deviations in the signal amplitude and phase in the case of snowfalls occurring between radar observations. We propose three ways to minimize the influence of snow on interferometric measurements. One of them, the selection of the radar data acquired in proper observation conditions, made it possible to assess the stability of the mutual location of the tanks. Among the most important processing and analysis results in the paper is a conclusion about the high stability of the fuel tank’s location on the yearly time interval, including the troubleshooting tank.

INTERFEROMETRIC PROCESSING OF A DEVELOPED MIMO GBSAR FOR DISPLACEMENT MONITORING

Abstract. This study demonstrates the interferometric processing experiments of our developed multiple-input multiple-output ground-based synthetic aperture radar (MIMO GBSAR) system. GBSAR systems are known as precise noncontact instruments for monitoring earth dynamics. In recent years W band MIMO radars have shown interesting potential in this field due to their low cost, compact size, and high phase sensitivity. MIMO capability enables the angular discrimination of multiple targets in the same range as the radar sensor. In our previous works, we developed a high-resolution MIMO GBSAR system based on the combination of MIMO radar and mechanical rail. Accordingly, this study investigates the developed system’s displacement monitoring capability by presenting a controlled experiment, using fixed and moving corner reflectors and gathering 36 time series of data. We compare and discuss the results obtained from MIMO GBSAR and MIMO radar configurations. The results show that our developed system highly agrees with MIMO radar’s interferometric measurements while providing a better target discrimination capability and higher signal noise ratio.

Radar Cross-Section of Ceramic Corner Reflectors in the W-Band Fabricated With the LCM-Method

Corner reflectors (CR), especially when combined with information coding, are potential candidates for radar beacon applications, which could be used for the self-localization of autonomous vehicles. In order to further improve the radar cross-section (RCS) of such devices, additively manufactured filled corner reflectors (FCR) made of ceramics in comparison to classical metal based CRs in the W-Band, were evaluated. The ceramic CRs were implemented using Lithography-based Ceramic Manufacturing (LCM), which is a novel high quality, high-resolution technology gaining traction in the RF community.

3-D Stereo Geolocation of Radar Reflectors Using Multiaspect SAR Acquisitions

With well-controlled orbits and a more accurate timing system of the new-generation synthetic aperture radar (SAR) satellites, the ability of high-precision geolocation with multiple SAR acquisitions has been verified based on radar reflectors. In this letter, SAR absolute and differential geolocation methods are introduced and implemented to retrieve the coordinates of two different types of radar reflectors: corner reflectors (CRs) and multidirectional dihedral reflectors (MDRs). The experimental results demonstrate that decimeter- even centimeter-level positioning accuracy can be obtained by SAR stereo geolocation based on multiaspect SAR acquisitions from RadarSAT-2 (RS-2), COSMO-SkyMed (CSK), Sentinel-1 (S1), and TerraSAR-X (TSX), respectively.

Методика получения значений сигнал-шум на выходе синтезированной антенной решетки ультразвукового стенда при наличии различного типа активной помехи

A method of obtaining signal-to-noise values at the output of the synthesized antenna array of an ultrasonic stand in the presence of active interference is considered. This method uses an algorithm for implementing the experimental scenario, which has spatial correlations during processing radar information in an ultrasonic location stand. It is shown that when forming the measurement procedure, the spatial position of the transmitter and the angle reflector also contributes to the evaluation of the data obtained, as does the position of the interference source, which directly affects the formation of the result. Examples of the received information about the position of the corner reflector according to the ultrasonic location stand data with direct interference are given. The considered method makes it possible to conduct research to assess the effect of active interference and noise on the algorithms of operation and functioning processes of real radar stations based on an ultrasonic location stand.

Antenna With Corner Reflector Based on Solar Cells

The work explores the possibility of forming a reflector for an antenna system based on solar panels. The reflector design is formed on the basis of two solar panels, which form a corner reflector. Conducted studies show that the use of solar panels as a reflector makes it possible to ensure a high level of antenna directivity, provide a wide range of operating frequencies, and also provide electricity generation based on green (carbon-free) energy technologies. The results obtained are confirmed by the results of electrodynamic modeling, as well as experimental measurements of the antenna layout. The developed design, thanks to the use of a planar antenna as an irradiator, made it possible to ensure a high level of electricity generation, since a small overlap of solar panels is ensured. The results of measurements and simulations are illustrated with pictures of radiation patterns, as well as measurements of the characteristics of electrical energy that the solar panel produces.

An angular blinking jamming method based on electronically controlled corner reflectors

An angular blinking jamming method based on electronically controlled corner reflectors

A Trihedral Corner Reflector for Radar Altimeter Calibration

A trihedral corner reflector has been used to evaluate the capability of passive reflectors to calibrate radar altimeters such as the Poseidon-4 altimeter on board Sentinel-6A. The reflector location, placed on the top of a mountain ridge and about 4-km off-nadir of the Sentinel-6A subsatellite track, allows capturing echoed signals with a Signal-to-Clutter Ratio around 40 dB when processing the received data with a Fully-Focussed SAR algorithm. Results obtained show a range bias of 33.9 mm with 8.5 mm of standard deviation and datation bias of -2.3 μs with standard deviation of 1.8 μs for the measurement campaign between September 2021 and April 2022. Such values are comparable to what is currently achieved by means of active transponders and therefore it is demonstrated that passive reflectors may be of interest to support radar altimeter regular calibration.

Maden Sahalarında Köşe Reflektör (CR) Tesisi ve Sabit Saçıcı (PS) Kontrolü

Deformations in mining areas have been started to be monitored by satellite SAR-based methods, which have recently emerged as an alternative to classical methods (GNSS receivers, Geometric Leveling, Terrestrial Lidar, UAV Photogrammetry, etc.). The most important advantage of this method is that quite wide areas can be monitored remotely. However, the mentioned way also has some disadvantages. The first of these; deformations can only be determined in the direction of the Satellite Line Of Sight (LOS), and the second one is that there is no correlated reflective surface due to the continuous change of topography in the locations where mining activities are carried out. Within the framework of this study, it is aimed to monitor PS by placing corner reflectors (CR) in the locations where sufficient Permanent Scatterer (PS) cannot be produced in Kalburçayırı and Hamal Coal Fields of Kangal Thermal Reactor Elektric production A.Ş. By considering the area through Sentinel-1A/B satellites, new PSs constituted by employing the CR facility has been successfully observed in locations, where PS could not be produced before. The results show us that; placing corner reflectors in locations where sufficient PS cannot be made or not determined at all, increases the number and quality of PS positively.

Quick Quality Assessment and Radiometric Calibration of C-SAR/01 Satellite Using Flexible Automatic Corner Reflector

C-SAR/01, the successor of China’s Gaofen-3 Satellite, which launched on 23 November 2021, is the first synthetic aperture radar (SAR) satellite to be launched in China’s civil space infrastructure plan and has served as an invaluable data resource. Radiometric calibration and validation are prerequisites for the quantitative application of SAR data. In this study, the radiometric calibration experiments of C-SAR/01 data of the ultra-fine strip (UFS) and fine strip I (FSI) modes were conducted applying flexible automatic triangular trihedral corner reflectors deployed in Xilinhot SAR satellite calibration and validation site. Accordingly, the image quality and radiometric calibration accuracy were evaluated. The results show that the spatial resolution, peak sidelobe ratio, and integrated sidelobe ratio of UFS and FSI mode data of C-SAR/01 are better than those of the design indexes, and the calibration results from the integral method are more stable than those from the peak method. Furthermore, the standard deviation of the calibration constant for UFS mode data is 0.234 dB, with the relative and absolute calibration accuracies obtained as 0.233 and 0.532 dB, respectively, whereas the standard deviation calibration constant for FSI mode data is 0.198 dB, with its relative and absolute calibration accuracies evaluated as 0.199 and 0.333 dB, respectively.

ESMD-WSST High-Frequency De-Noising Method for Bridge Dynamic Deflection Using GB-SAR

Ground-based synthetic aperture radar (GB-SAR), as a new non-contact measurement technique, has been widely applied to obtain the dynamic deflection of various bridges without corner reflectors. However, it will cause some high-frequency noise in the obtained dynamic deflection with the low signal-to-noise ratio. To solve this problem, this paper proposes an innovative high-frequency de-noising method combining the wavelet synchro-squeezing transform (WSST) method with the extreme point symmetric mode decomposition (ESMD) method. First, the ESMD method is applied to decompose the observed dynamic deflection signal into a series of intrinsic mode functions (IMFs), and the frequency boundary of the original signal autocorrelation is filtered by the mutual information entropy (MIE) for each IMF pair. Second, the high-frequency IMF components are fused into a high-frequency sub-signal. WSST is performed to remove the influence of noise to reconstruct a new sub-signal. Finally, the de-noised bridge dynamic deflection is reconstructed by the new sub-signal, the remaining IMF components, and the residual curve R. For the simulated signal with 5 dB noise, the signal-to-noise ratio (SNR) after noise reduction is increased to 11.13 dB, and the root-mean-square error (RMSE) is reduced to 0.30 mm. For the on-site experiment for the Wanning Bridge, the noise rejection ratio (NRR) is 5.48 dB, and ratio of the variance root (RVR) is 0.05 mm. The results indicate that the proposed ESMD-WSST method can retain more valid information and has a better noise reduction ability than the ESMD, WSST, and EMD-WSST methods.

Using Drone Swarms as a Countermeasure of Radar Detection

The deployment of a drone swarm from carrier aircraft can support critical operations in which target aircraft should be protected by ground-based radar detection. To be successful as a countermeasure, the drones that are involved in the swarm must be equipped with proper payload systems and the geometry among the swarm units must be well designed. This paper describes the development of a scaled drone swarm geometry that can electromagnetically obscure a target from radar detection. Different drone swarm configurations were analyzed to select a feasible solution to develop ground and in-flight tests. During the tests, a compact software radar was used to detect a target drone. Several ground tests were executed to characterize the radar echo response of the target drone installed on a wooden tripod. Different geometries of corner reflectors were tested on tripods to select the swarm geometry that better obscured the target drone. Flight tests were executed involving decoy drones equipped with a corner reflector and a target drone to validate the proposed drone swarm geometry. The presented design of a drone swarm geometry allowed us to characterize the scaled distances among the swarm units and between the swarm and the target that must be protected against a ground-based radar detection.

Classification of Plates and Trihedral Corner Reflectors Based on Linear Wavefront Phase-Modulated Beam

Wavefront-modulated beams such as vortex beams have attracted much attention in the field of target recognition due to the introduced degrees of freedom. However, traditional wavefront-modulated beams are doughnut shaped, and are not suitable for radar detection or tracking. To solve this problem, a linear wavefront phase-modulated beam with a maximum radiation intensity in the center was proposed in a previous study. In this paper, we continue to study target characteristics under the linear wavefront phase-modulated beam. Through analysis of the target scattering based on the physical optics (PO) method, we find that a part of the monostatic or bistatic radar cross-section (RCS) of the target could be obtained by changing the phase gradient of the modulated beam. Taking this part of RCS for feature extraction, we recognize the plates and trihedral corner reflectors through the support vector machine (SVM) method. For data visualization, we use the t-distributed stochastic neighbor embedding (t-SNE) method for data dimensionality reduction. The results show that the recognition probability of the plates and trihedral corner reflectors can reach 91% with an antenna array having an aperture of 20 wavelengths when the signal-to-noise ratio (SNR) is 20 dB, while the traditional plane beam cannot classify these two targets directly.

Development of SMA Spring Linear Actuator for an Autonomous Lock and Release Mechanism: Application for the Gravity-Assisted Pointing System in Moon to Earth Alignment of Directional Devices

The next generation lunar reflector (NGLR) experiment is one of the experiments selected by NASA in the framework of the commercial lunar payload services (CLPS) initiative. The experiment, inspired by the lunar laser ranging (LLR) experiments of the Apollo era, is basically a single cube corner reflector (CCR) capable of reflecting a beam coming from a laser station on Earth that must be deployed on the Moon and pointed toward the mean Earth direction. In this work, a prototype of an actuator for the lock and release system of the reflector package was conceived, built, and tested in laboratory conditions. Since the entire pointing system must be passive, the actuator is designed to be operated by an SMA spring actuated by the thermal radiation of the Sun and regolith on the Moon. In lab conditions, the prototype, activated by a heat gun, showed the capability of the SMA spring to operate a lock and release pin, whose diameter is 4 mm, subjected to a preload of F = 7 N exerted by the releasing spring.

Distance-restrained atmospheric parameters correction (DR-APC) model for GB-SAR transmission power attenuation compensation in bridges dynamic deflection measurement

A distance-restrained atmospheric parameter correction (DR-APC) model is presented in this study to reduce the influences of atmospheric parameters and electromagnetic wave power attenuation on dynamic deflection measurement of bridges using ground-based synthetic aperture radar (GB-SAR). The proposed model considers electromagnetic wave transmission distance together with factors of bridges geometric configuration, instrument layout and atmospheric refractive index. First, the theoretical DR-APC model is derived with restrictions of the wave propagation distance, the height from radar unit to bridge lower surface and the elevation angle of radar unit. Second, the DR-APC model is determined based on continuous time-series displacement of 12 corner reflectors fixed on tripods in a stable environment area. Finally, the DR-APC model is applied to GB-SAR monitoring signals of three bridges to verify its effectiveness. The results show that the proposed model has an ability to improve the accuracy of dynamic deflection measurement using GB-SAR.

Ultrawideband Precision RCS Regulation for Trihedral Corner Reflectors by Loading Resistive Film Absorbers

In this paper, an absorber with a resistive film is proposed, which holds stable polarization absorption characteristics at large angles by exploring the principle of absorption structure. The absorber contains three layers: the top layer is a metal patch with a resistive film, and the middle layer is covered with a metal patch. The thickness of the absorber structure is only 0.5 mm, and the absorber can be perfectly attached to the trihedral corner reflector (TCR) without affecting the characteristics of the reflector. By laying the absorber on the TCR, the TCR is stabilized at 8–40 GHz in the range of the incident angle ±20°, and the radar cross-section (RCS) reduction value of the TCR fluctuates around 1 dBsm. Thus, the RCS values of the three frequency points of 10 GHz, 15 GHz, and 35 GHz are consistent to achieve the accurate regulation of RCS. The stability of the absorber at different frequencies can be achieved, which is essential for the precise RCS regulation of complex scatterer structures. The effectiveness of the method was verified in experiments using a TCR metamaterial absorbing unit with resistive film.

Analysis on the angular jamming effects of electronically controlled corner reflectors towing after a maneuver aircraft

Analysis on the angular jamming effects of electronically controlled corner reflectors towing after a maneuver aircraft