Artificial Corner Reflectors Research Articles

Analysis of the Dihedral Corner Reflector’s RCS Features in Multi-Resource SAR

Artificial corner reflectors are widely used in the vegetated landslide for time series InSAR monitoring due to their permanent scattering features. This paper investigated the RCS features of a novel dihedral CR under multi-resource SAR datasets. An RCS reduction model for the novel dihedral corner reflector has been proposed to evaluate the energy loss caused by the deviation between the SAR incident angle and the CR’s axis. On the Huangtupo slope, Badong county, Hubei province, tens of dihedral CRs had been installed and the TSX–spotlight and Sentinel-TOPS data had been collected. Based on the observation results of CRs with more than ten deviation angles, the proposed reduction model was tested with preferable consistency under a real dataset, while 2 dBsm of systematic bias was verified in those datasets. The maximum incident angle deviation in the Sentinel data overlapping area is over 12°, which leads to a 2.4 dBsm RCS decrease for horizontally placed dihedral CRs estimated by the proposed model, which has also been testified by the observed results. The testing results from the Sentinel data show that in high, vegetation-covered mountain areas like the Huangtupo slope, the dihedral CRs with a 0.4 m slide length can be achieve 1 mm precision accuracy, while a side length of 0.2 m can achieve the same accuracy under TSX–spotlight data.

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.

MONITORING HANGING GLACIER DYNAMICS FROM SAR IMAGES USING CORNER REFLECTORS AND FIELD MEASUREMENTS IN THE MONT-BLANC MASSIF

Abstract. This paper focuses on the monitoring of a small hanging glacier with Synthetic Aperture Radar (SAR) images using a combination of artificial Corner Reflectors (CRs) and dGNSS field measurements. First, to test the performance of the CRs in high-resolution X-band and medium-resolution C-band images acquired by PAZ and Sentinel-1 satellites respectively, a series of tests was performed with two CRs installed in the Chamonix valley. After the confirmation of good visibility in the valley datasets from both satellites, four CRs were installed in summer 2021 on a steep (≈ 52° average slope) hanging glacier on the North face of Aiguille du Midi (3842 ma.s.l., Mont-Blanc massif). After the successful installation on the hanging glacier, all CRs were visible in both sets of images (PAZ and Sentinel-1). Using the CRs as a reference, glacier displacements were estimated in SAR images using complex cross-correlation, and validated by using ground measurements (dGNSS) and theoretical relationships between ice thickness and surface velocity. We observed average summer velocities of ≈ 0.05 m/day at the front of the glacier from field measurements and ≈ 0.04 m/day from SAR displacement estimates. The study shows the potential and limitations of using CRs as a tool for monitoring glaciers in complex topographies.

Validation of a corner reflector installation at Côte d’Azur multi-technique geodetic observatory

We present the procedure we followed to design an artificial corner reflector (CR) at the Calern site of Côte d’Azur Observatory (France). Although still few in number, such reflectors are an integral part of the Global Geodetic Observing System (GGOS) infrastructure. They can be used as a stable radar target in SAR images to connect local InSAR deformation maps to the global Terrestrial Reference Frame and for SAR absolute determination. During a test phase, the orientation of the CR was changed in order to be aligned toward all possible orbits of Sentinel-1A/1B satellites. On the different SAR images, the CR exhibits a high backscattering signal, and provides a Signal-to-Clutter Ratio larger than 26 dB. Since December 2018, the CR is specifically oriented toward the relative orbit 88. It is clearly detected as a PS in our InSAR analyses and as expected, the standard deviation of displacement measured on the CR is lower than on surrounding PS. A first local survey was performed to locate precisely this CR with respect to the existing geodetic instruments and annual campaigns have been carried out since then to insure its stability over time.

Monitoring of large and inconsistent deformation in coal mining area using point-like target offset tracking

ABSTRACT Ground deformation in the coal mining area of Northwestern China is usually featured by large and inconsistent subsidence. Large subsidence brings great challenge to Differential Interferometric Synthetic Aperture Radar (DInSAR) and Permanent Scatterer Interferometry (PSI) technology due to the pixel-level offset in the range direction after co-registration, which could directly result in complete coherence loss even for the artificial Corner Reflectors (CRs). Offset tracking provides an alternative to capture the large deformation although the results could be biased due to the large deformation gradient and use of fixed correlation window size. In this paper, the point-like target (PT) offset tracking method is introduced to deformation monitoring in coal mining area with a levelly decreasing correlation window size. For each PT-centered SLC patch in the reference SAR image, the estimate from the larger correlation window will serve as a guide for the smaller one to locate the matching SLC patch in the slave SAR image, thus providing a guarantee that the residual offsets to be estimated remain to be a small fraction of the decreasing patch size. The offset estimates were evaluated by ground surveying results at 34 CRs obtained using single reference real-time kinematic (RTK) GPS technique, with a RMS error of 1/13-1/14 SLC pixel in both the range and azimuth directions.

Performance Analysis of Dihedral Corner Reflectors for Slope Movements: A Case Study From Aniangzhai Landslide in China

Using SAR and InSAR technology, artificial corner reflectors (CR) are popular coherent targets for monitoring ground instability with sub-centimeter accuracy in non-urban areas. In this letter, we investigate the performance of a newly designed small dihedral corner reflector for monitoring post-failure creep at the Aniangzhai landslide in Danba County, China. The new double geometry CRs consist of two sets of semi-circular metal plates, each 30-40 cm in radius and perpendicular to each other. Six such CRs are installed for Corner Reflector Interferometric SAR (CR-InSAR) analysis using both TerraSAR-X (TSX) High-resolution Spotlight (HS) data and medium-resolution Sentinel-1 (S1) SAR images. The CRs are first identified in SAR images using a probability model by taking into three factors. These are (1) inverse of amplitude dispersion, (2) intensity increment after the installation, (3) an upper empirical bound derived from the ensemble average of pixel intensities in post-deployment SAR images. Experimental results show that the CRs improve the background intensity in TSX images by around 30 dB, with signal-to-clutter ratio (SCR) exceeding 25 dB. Furthermore, the radar cross-section (RCS) of CRs in both TSX and S1 images remains relatively stable, ranging from 15 dB to 23 dB, making them suitable for CR-InSAR analysis using double-difference phase observations.

Calculating the Influence of Refractions and Reflections from Curvilinear Surfaces of Shells of Revolution on Acoustic Field

In the ray optics approximation, we have calculated the effect that changes in the directivity (defocusing) of an acoustic field due to refraction on a curvilinear entry surface and/or single or multiple reflections from curved inner surfaces of the shells of revolution have on the echo signal amplitude. The coincidence of the calculation results with the results of available studies is shown for special cases. A formula for calculating changes in the amplitude makes it possible to determine a defocusing-related change in the size of the echo pulse beam at the entry surface. The correctness of the calculation is confirmed experimentally, using specimens of pipes with embedded artificial corner reflectors.

Small Reflectors for Ground Motion Monitoring With InSAR

In recent years, synthetic aperture radar interferometry has become a recognized geodetic tool for observing ground motion. For monitoring areas with low density of coherent targets, artificial corner reflectors (CRs) are usually introduced. The required size of a reflector depends on radar wavelength and resolution and on the required deformation accuracy. CRs have been traditionally used to provide a high signal-to-clutter ratio (SCR). However, large dimensions can make the reflector bulky, difficult to install and maintain. Furthermore, if a large number of reflectors are needed for long infrastructure, such as vegetation-covered dikes, the total price of the reflectors can become unaffordable. On the other hand, small reflectors have the advantage of easy installation and low cost. In this paper, we design and study the use of small reflectors with low SCR for ground motion monitoring. In addition, we propose a new closed-form expression to estimate the interferometric phase precision of resolution cells containing a (strong or weak) point target and a clutter. Through experiments, we demonstrate that the small reflectors can also deliver displacement estimates with an accuracy of a few millimeters. To achieve this, we apply a filtering method for reducing clutter noise.

Multi-Temporal X-Band Radar Interferometry Using Corner Reflectors: Application and Validation at the Corvara Landslide (Dolomites, Italy)

From the wide range of methods available to landslide researchers and practitioners for monitoring ground displacements, remote sensing techniques have increased in popularity. Radar interferometry methods with their ability to record movements in the order of millimeters have been more frequently applied in recent years. Multi-temporal interferometry can assist in monitoring landslides on the regional and slope scale and thereby assist in assessing related hazards and risks. Our study focuses on the Corvara landslides in the Italian Alps, a complex earthflow with spatially varying displacement patterns. We used radar imagery provided by the COSMO-SkyMed constellation and carried out a validation of the derived time-series data with differential GPS data. Movement rates were assessed using the Permanent Scatterers based Multi-Temporal Interferometry applied to 16 artificial Corner Reflectors installed on the source, track and accumulation zones of the landslide. The overall movement trends were well covered by Permanent Scatterers based Multi-Temporal Interferometry, however, fast acceleration phases and movements along the satellite track could not be assessed with adequate accuracy due to intrinsic limitations of the technique. Overall, despite the intrinsic limitations, Multi-Temporal Interferometry proved to be a promising method to monitor landslides characterized by a linear and relatively slow movement rates.

Landslide deformation monitoring using point-like target offset tracking with multi-mode high-resolution TerraSAR-X data

Many landslides in the Three Georges area have shown active deformations associated with water level fluctuations since the full operation of the Three Georges Dam. Such active deformations of landslide bodies need to be closely monitored for disaster prevention and warning. One way to do this is to employ SAR pixel offset tracking, a powerful technique that can be used for measuring two-dimensional large displacements. As an improvement of the original pixel offset tracking technique, the point-like target (PT) offset tracking method focuses on stable point-like targets and thus can obtain more reliable measurements. In this paper, the PT offset tracking method is employed to investigate historical evolution of the Shuping landslide, using time series TerraSAR-X data acquired in both Stripmap (SM) and High-resolution Spotlight (HS) modes. Artificial corner reflectors (CR) installed at/near the landslide are identified as PTs and used to analyze the spatial–temporal pattern of landslide deformations. Results showed that the maximum accumulative deformation of the Shuping landslide reached more than 1.5m over a time span of two years. A correlation analysis between the deformation trends and the fluctuation of reservoir water level indicated that most deformations of the landslide happened during the water level declining period.

Some problems of landslide monitoring using satellite radar imagery with different wavelengths: Case study of two landslides in the region of Greater Sochi

The problems of processing and interpreting the data provided by radar satellite interferometry for the conditions of landslides covered by vegetation are analyzed in two case studies of landslides in the Northern Caucasus in the region of Kepsha and Mamaika villages in the vicinity of the railway tunnels. The estimates of the displacement fields are obtained by the method of persistent scatterers using the StaMPS program package. The five-year experience of landslide monitoring shows that in the unfavorable conditions of satellite radar interferometry, proper selection of the strategy of satellite image processing is vital. In the present paper, we discuss, in particular, the crop selection, the selection of the master image, reference area, and digital elevation model. For the landslide located in the sparsely populated region near Kepsha village, we used the data from the ascending and descending tracks of the long-wavelength ALOS and shorter-wavelength ENVISAT satellites. For the landslide in the region of Mamaika village with a large number of different buildings serving as good scatterers for radar signals, we used the images from the ENVISAT and from TerraSAR satellite, which transmits even shorter waves. The average line-of-sight (LOS) displacement velocities VLOS for the landslide near Kepsha village measure at most 10 mm per annum, which means that this landslide has remained stable at least since 2004. The landslide in Mamaika village is significantly more active. The average LOS displacement velocities in the active part of this landslide attain 60 mm per annum. The artificial corner reflector installed on the segment of the landslide where natural scatterers of radar signal are absent made it possible to estimate the LOS displacement velocity on this segment of the slope at 49 mm per annum.

Discontinuous GBSAR deformation monitoring

This paper is focused on deformation monitoring using the Ground-Based SAR (GBSAR) technique and a particular data acquisition configuration, which is called discontinuous GBSAR (D-GBSAR). In the most commonly used GBSAR configuration, the radar is left installed in situ, acquiring data periodically, e.g. every few minutes. Deformations are estimated by processing sets of GBSAR images acquired during several weeks or months, without moving the system. By contrast, in the D-GBSAR the radar is installed and dismounted at each measurement campaign, revisiting a given site periodically. This configuration is useful to monitor slow deformation phenomena. This paper outlines the D-GBSAR data analysis procedure implemented by the authors. This is followed by a discussion of some specific aspects of D-GBSAR monitoring. Two successful examples of D-GBSAR monitoring are discussed: one concerns an urban area, while the second one involves a rural area where the monitoring requires the use of artificial corner reflectors.

A Noninterferometric Procedure for Deformation Measurement Using GB-SAR Imagery

Deformation monitoring using ground-based synthetic aperture radar (GB-SAR) data usually exploits the interferometric phases. In this letter, a new noninterferometric procedure is proposed, which exploits the geometric content of GB-SAR amplitude imagery and estimates deformation through image matching. This letter describes, step by step, this procedure. In order to achieve acceptable deformation measurement performances, the technique needs special targets, which have to guarantee a good image matching quality. If the available natural targets are insufficient, artificial corner reflectors are required. The new approach overcomes some of the main limitations of GB-SAR interferometry; it yields aliasing-free deformation estimates, is insensitive to atmospheric effects, and provides 2-D displacement measurements while interferometry only has a mono-dimensional measurement capability. Several experiments focusing on the performances of the new procedure are described. The procedure is validated using different scenarios. On a real-size landslide scenario, a mean absolute error over 12 corner reflectors of 0.59 cm is achieved, i.e., 1/85th of the pixel size. These encouraging results can be of interest for several deformation monitoring applications.

RCS Simulation of Dihedral Corner Reflector Based FEKO

CR-InSAR is developing continuously in recent years. It is an important technology in the field of monitoring crustal micro-deformation. Radar cross section of corner reflector is an important work foundation because only radar cross section of artificial corner reflector achieves maximum, it is easier to recognize it from SAR image and increase monitoring accuracy. Dihedral corner reflector is the simplest type in proving ground of Northwest of Beijing; its RCS research is a beneficial attempt helping to build up reasonable corner reflector layout. In the paper, establishing geometric model and subdivision model of dihedral corner reflector are based on FEKO software, and choosing physical optics to simulate RCS. The simulation result is good consistency with theoretical max value. Through analyzing RCS curve, it can help to provide guidance on how to set up corner reflector in the proving ground.

Subsidence detection by TerraSAR-X interferometry on a network of natural persistent scatterers and artificial corner reflectors

The German satellite TerraSAR-X (TSX) is able to provide high-resolution synthetic aperture radar (SAR) images for mapping surface deformation by the persistent scatterer interferometry (PSI) technique. To extend the application of PSI in detecting subsidence in areas with frequent surface changes, this paper presents a method of TSX PSI on a network of natural persistent scatterers (NPSs) and artificial corner reflectors (CRs) deployed on site. We select a suburban area of southwest Tianjin (China) as the testing site where 16 CRs and 10 leveling points (LPs) are deployed, and utilize 13 TSX images collected over this area between 2009 and 2010 to extract subsidence by the method proposed. Two types of CRs are set around the fishponds and crop parcels. 6 CRs are the conventional ones, i.e., fixed CRs (FCRs), while 10 CRs are the newly-designed ones, i.e., so-called portable CRs (PCRs) with capability of repeatable installation. The numerical analysis shows that the PCRs have the higher temporal stability of radar backscattering than the FCRs, and both of them are better than the NPSs in performance of radar reflectivity. The comparison with the leveling data at the CRs and LPs indicates that the subsidence measurements derived by the TSX PSI method can reach up to a millimeter level accuracy. This demonstrates that the TSX PSI method based on a network of NPSs and CRs is useful for detecting land subsidence in cultivated lands.

The Design and Experiments on Corner Reflectors for Urban Ground Deformation Monitoring in Hong Kong

PSInSAR technology has been proved to be a powerful tool for monitoring urban ground displacement information to a millimetric accuracy. When it comes to the validation of PS-derived ground deformation, artificial corner reflector (CR) can be very useful due to its relative stability and high signal-to-noise ratio (SNR). In this paper, we will evaluate some general criteria for designing and setting up corner reflectors, including the shape, size, material, location, and others. An ideal prototype known as the rectangular trihedral with special designs is brought up in this paper, and validation experiments were conducted in Hong Kong to demonstrate the ability of the proposed prototype. The field data agreed with theoretical analysis, bringing up an economical and applicable approach for CR application in urban ground deformation monitoring.

TERRASAR-X INSAR PROCESSING IN NORTHERN BOHEMIAN COAL BASIN USING CORNER REFLECTORS (PRELIMINARY RESULTS)

Abstract. The area of Northern Bohemian coal basin is rich in brown coal. Part of it is undermined, but large areas were mined using open-pit mines. There are numerous reclaimed waste dumps here, with a horse racetrack, roads and in some cases also houses. However, on most of the waste dumps, there are forests, meadows and fields. Above the coal basin, there are the Ore mountains which are suspected to be sliding down to the open mines below them. We installed 11 corner reflectors in the area and monitor them using the TerraSAR-X satellite. One of the reflectors is situated in the area of radar layover, therefore it cannot be processed. We present preliminary results of monitoring the remaining corner reflectors, with the use of 7 TerraSAR-X scenes acquired between June and December 2011. We process whole scene crops, as well as the artificial reflector information alone. Our scene set contains interferometric pairs with perpendicular baselines reaching from 0 to 150 m. Such a configuration allows us to distinguish deformations from DEM errors, which are usual when the SRTM (Shuttle Radar Topography Mission) DEM (X-band) is used for Stripmap data. Unfortunately, most of the area of interest is decorrelated due to vegetation that covers both the Ore mountains and the reclaimed waste dumps. We had to enlarge the scene crop in order to be able to distinguish deformations from the atmospheric delay. We are still not certain about the stability of some regions. For the installed artificial reflectors, the expected deformations are in the order of mm/year. Generally, deformations in the area of interest may reach up to about 5 cm/year for the Ervěnice corridor (a road and railway built on a waste dump). When processing artificial corner reflector information alone, we check triangular sums and perform the processing for all possible point combinations – and that allows us to correct for some unwrapping errors. However, the problem is highly ambiguous.

Analysis of artificial corner reflector’s radar cross section: a physical optics perspective

Among many of the differential interferometric synthetic-aperture radar technologies, artificial corner reflectors (ACR) are widely used in monitoring earthquake deformation and urban subsidence due to their relative stability on synthetic-aperture radar (SAR) acquisition. Apparently, the detection and extraction of ACRs on remotely sensed images would be of utmost importance. Many different geometric types of ACRs are designed to achieve maximum detection. Among them, dihedral, rectangle trihedral, and pyramidal ACRs are the most commonly employed. The cost and difficulty of deploying ACRs in the field, however, render comparison among the three types rather impractical, if not impossible. The current study attempts to tackle the issue from a physical optics perspective. Adopting radar cross section (RCS) as the measure of ACRs’ detectability, we examined the relationships between the ACRs’ RCS under vertical polarity with various parameters including the radar incident angles, width and heights of the ACRs and the azimuthal angles. The analyses indicate that under vertical polarity, among the three types of ACRs, the rectangle trihedral ACR is the most tolerant to its deploying surroundings. To verify the physical optics analysis results, we collected ENVISAT data from a variety of deployed ACRs in the Yan-Huai Basin, China, and derived their reflectance characteristics. The field data agree with the theoretical analyses. From this practice, it seems that the physical optics method might prove to be a rather economical and effective approach to design and select appropriate ACRs in field deployment.

The selection of artificial corner reflectors based on RCS analysis

Artificial corner reflectors (ACRs) are widely applicable in monitoring terrain change via interferometric synthetic aperture radar (InSAR) remote sensing techniques. Many different types are available. The choice of the most appropriate ones has recently attracted scholarly attentions. Based on physical optics methods, via calculating the radar cross section (RCS) values (the higher the value, the better the detectability), the current study tested three ACRs, i.e., triangular pyramidal, rectangular pyramidal and square trihedral ACRs. Our calculation suggests that the square trihedral ACR produces the largest RCS but least tolerance towards incident radar ray’s deviation from optimal angle. The triangular pyramidal trihedral ACR is the most geometrically stable ACR, and has the highest tolerance towards incident radar ray’s deviation. Its RCS values, however, are the least of the three. Due to the high cost of deploying ACRs in the fields, the physical optics method seems to provide a viable way to choose appropriate ACRs.

Landslide Monitoring in the Three Gorges Area Using D-INSAR and Corner Reflectors

The differential INSAR technique has the potential for monitoring centimeter-scale ground motion in an accurate and costeffective manner. Probably the most important limiting factor in the application of INSAR is temporal change in the complex reflectivity of the ground surface during the period between radar acquisitions. This can be due to changes in such parameters as moisture content or vegetation. The stable artificial corner reflectors can be identified from long temporal series of interferometric SAR images even with large baselines, and therefore decrease the risk of image decorrelation. This paper will discuss the following questions: 1) how to obtain the true phase of a corner reflector in a SAR complex image; 2) how to co-register the corner reflector pixels when the coherence of its surrounding area is extremely low; 3) how to select the interpolation kernel to resample the SAR image; and, 4) how to compute the interferometric phase of two co-registered corner reflectors without flat earth term and corners’ height contribution. In order to demonstrate the results, a practical example of landslide monitoring in the Three Gorges area in China is presented.