Triangular Trihedral Corner Reflectors Research Articles

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.

Novel Corner-Reflector Array Application in Essential Infrastructure Monitoring

High precision monitoring of infrastructure using artificial reflectors is possible with freely available Sentinel-1 data, but large reflectors are needed. We find that a triangular trihedral corner reflector should typically have at least 1 m inner leg length. As such large reflectors are often not feasible for use in urban areas for essential infrastructure monitoring, we designed a multiple corner-reflector array to replace a single corner reflector with an inner leg length of 1 m. In this case, we use four reflectors where each of them is a truncated triangular trihedral with an inner leg length of 0.33 m. We measured InSAR amplitude, phase and coherence of this reflector array with various configurations of alignments of the array. We find that as long as great care is taken in the relative positioning of the four corner reflectors, so that they constructively interfere, each horizontal or vertical configuration provides the expected amplitude, coherence and phase stability. Applications of multiple small corner reflectors in urban areas range from essential infrastructure monitoring (e.g bridges, overpasses, tunnel constructions), through assessment of structural health of buildings, to monitoring highway and railway embankments. We show that the multiple corner array works when placed in a single InSAR resolution cell, but depending on the application, the number and projection of corner reflectors can be varied, as long as sufficient signal-to-clutter ratio is achieved in the area of interest.

On the Efficacy of Compact Radar Transponders for InSAR Geodesy: Results of Multiyear Field Tests

Compact and low-cost radar transponders are an attractive alternative to corner reflectors (CR) for SAR interferometric (InSAR) deformation monitoring, datum connection, and geodetic data integration. Recently, such transponders have become commercially available for C-band sensors, which poses relevant questions on their characteristics in terms of radiometric, geometric, and phase stability. Especially for extended time series and for high-precision geodetic applications, the impact of secular or seasonal effects, such as variations in temperature and humidity, has yet to be proven. Here we address these challenges using a multitude of short baseline experiments with four transponders and six corner reflectors deployed at test sites in the Netherlands and Slovakia. Combined together, we analyzed 980 transponder measurements in Sentinel-1 time series to a maximum extent of 21 months. We find an average Radar Cross Section (RCS) of over 42 dBm2 within a range of up to 15 degrees of elevation misalignment, which is comparable to a triangular trihedral corner reflector with a leg length of 2.0 m. Its RCS shows temporal variations of 0.3--0.7~dBm2 (standard deviation) which is partially correlated with surface temperature changes. The precision of the InSAR phase double-differences over short baselines between a transponder and a stable reference corner reflectors is found to be 0.5-1.2 mm (one sigma). We observe a correlation with surface temperature, leading to seasonal variations of up to +/-3 mm, which should be modeled and corrected for in high precision InSAR applications. For precise SAR positioning, we observe antenna-specific constant internal electronic delays of 1.2-2.1 m in slant-range, i.e., within the range resolution of the Sentinel-1 Interferometric Wide Swath (IW) product, with a temporal variability of less than 20~cm. Comparing similar transponders from the same series, we observe distinct differences in performance. Our main conclusion is that these characteristics are favorable for a wide range of geodetic applications. For particular demanding applications, individual calibration of single devices is strongly recommended.

ABSOLUTE LOCALIZATION OF CORNER REFLECTORS WITH TERRASAR-X SPOTLIGHT IMAGES

Abstract. Synthetic aperture radar, capable of imaging the Earth surface from space in nearly all-weather conditions and high spatial resolution, has shown its outstanding capability for a variety of ground mapping applications. With well-controlled orbits of the new generation SAR satellites, high accuracy absolute localization with multiple SAR images has been demonstrated and become one of the hot spots with increasing attention. In this paper, high-resolution Spotlight-mode TerraSAR-X images acquired from the single orbit track were applied to 3D absolute positioning of three triangular trihedral corner reflectors. In order to overcome the limitation imposed by the acquisitions with very short baselines, a height constraint was introduced and the sub-meter accuracy was derived after carefully compensating for the known error sources, such as atmospheric delays and solid earth tide shifts.

On the Design of Radar Corner Reflectors for Deformation Monitoring in Multi-Frequency InSAR

Trihedral corner reflectors are being increasingly used as point targets in deformation monitoring studies using interferometric synthetic aperture radar (InSAR) techniques. The frequency and size dependence of the corner reflector Radar Cross Section (RCS) means that no single design can perform equally in all the possible imaging modes and radar frequencies available on the currently orbiting Synthetic Aperture Radar (SAR) satellites. Therefore, either a corner reflector design tailored to a specific data type or a compromise design for multiple data types is required. In this paper, I outline the practical and theoretical considerations that need to be made when designing appropriate radar targets, with a focus on supporting multi-frequency SAR data. These considerations are tested by performing field experiments on targets of different size using SAR images from TerraSAR-X, COSMO-SkyMed and RADARSAT-2. Phase noise behaviour in SAR images can be estimated by measuring the Signal-to-Clutter ratio (SCR) in individual SAR images. The measured SCR of a point target is dependent on its RCS performance and the influence of clutter near to the deployed target. The SCR is used as a metric to estimate the expected InSAR displacement error incurred by the design of each target and to validate these observations against theoretical expectations. I find that triangular trihedral corner reflectors as small as 1 m in dimension can achieve a displacement error magnitude of a tenth of a millimetre or less in medium-resolution X-band data. Much larger corner reflectors (2.5 m or greater) are required to achieve the same displacement error magnitude in medium-resolution C-band data. Compromise designs should aim to satisfy the requirements of the lowest SAR frequency to be used, providing that these targets will not saturate the sensor of the highest frequency to be used. Finally, accurate boresight alignment of the corner reflector can be critical to the overall target performance. Alignment accuracies better than 4° in azimuth and elevation will incur a minimal impact on the displacement error in X and C-band data.

Interference Analysis for Synthetic Aperture Radar Calibration Sites with Triangular Trihedral Corner Reflectors

The typical method for performing an absolute radiometric calibration of a Synthetic Aperture Radar (SAR) System is to analyze its response, without interference, to a target with a known Radar Cross Section (RCS). To minimize interference, an error-free calibration site for a Corner Reflector (CR) is required on a wide and flat plain or on an area without disturbance sources (such as ground objects). However, in reality, due to expense and lack of availability for long periods, it is difficult to identify such a site. An alternative solution is the use of a Triangular Trihedral Corner Reflector (TTCR) site, with a surrounding protection wall consisting of berms and a hollow. It is possible in this scenario, to create the minimum criteria for an effectively error-free site involving a conventional object-tip reflection applied to all beams. Sidelobe interference by the berm is considered to be the major disturbance factor. Total interference, including an object-tip reflection and a sidelobe interference, is analyzed experimentally with SAR images. The results provide a new guideline for the minimum criteria of TTCR site design that require, at least, the removal of all ground objects within the fifth sidelobe.

대기 굴절률을 이용한 원형레일 기반 지상 SAR 자료의 대기보정

In this paper, an atmospheric effect of repetitive measurements of X-band (9.65 GHz) arc-rail type GB-SAR (ArcSAR) system was quantitatively analyzed. Four artificial triangular trihedral corner reflectors as stationary targets for getting stable back scattered signal during 43 hours continually. The results of the analysis showed that the phase of those stationary targets had changed maximum of 5 radian (12.4 mm) and total RMS error had was 1.62 radian (4 mm) during 65 repeated measuring time. The refractive index of air which was calculated using the temperature, humidity and pressure of atmosphere showed very close relationship with the phase difference. We could check the atmospheric correction was fulfilled by the correction of an atmospheric effect using refractive index during the selected 16 hours period showed that RMS error was dropped from 1.74 radian (4.3 mm) to 0.10 radian (0.24 mm).

GB-SAR 간섭기법으로 측정된 X-밴드 대기 굴절률의 상대습도, 기온 및 기압에 따른 변화

이 논문은 X-밴드 Ground-Based Synthetic Aperture Radar (GB-SAR) 시스템을 이용하여 5개의 삼각삼면반사체(triangular trihedral corner reflector)의 시간에 따른 위상변화를 분석하였다. 각각의 반사체는 시스템으로부터의 거리를 다르게 설정하여 고정시켰고 모든 편파에 대한 영상을 20 분 간격으로 123 회, 총 40 시간 동안 연속 측정하였다. SAR 간섭기법(interferometry)을 이용한 분석 결과 반사체는 고정된 상태임에도 불구하고 시간에 따라 최대 2 radian의 위상차가 나타났으며 기상요소와 유사한 추세를 보였다. 대기 중 굴절률을 고려한 GB-SAR 위상관계식을 통하여 대기 중 습도, 기온 및 기압의 변화에 따른 마이크로파 굴절률의 변화 양상을 회귀분석을 통하여 살펴보았다. 그 결과 X-밴드의 굴절률은 습도 및 기온과 비교적 높은 선형적 관계를 보였으나(평균 결정계수 각각 0.72, 0.76) 기압과는 그렇지 않았다(결정계수 0.34). 실험 기간 동안 습도는 약 50 %에서 90 %까지, 기온은 <TEX>$-1^{\circ}C$</TEX>에서 <TEX>$9^{\circ}C$</TEX>까지 변하였으며 X-밴드 굴절률의 변화는 약 <TEX>$3.14{\times}10^{-5}$</TEX>였다. 향후 보다 다양한 기상조건에서 실험이 이루어진다면 습도, 기온 및 기압 조건에 따른 굴절률의 포괄적 관계식을 유추할 수 있을 것으로 기대된다. In this paper, we analyzed the phase change of 5-triangular trihedral comer reflectors by using X-band Ground-Based Synthetic Aperture Radar (GB-SAR) system. Each reflector was set as a stationary target at a different distance from the system. We obtained total 123 full-polarization images during 40 hours continuously at 20 minute interval. Results of SAR interferometric analysis showed phase changes of maximum 2 radians and followed similar pattern with atmospheric data. Through a GB-SAR phase formula that includes refractive index in the air, we performed regression analysis for refractive index as a function of atmospheric humidity, temperature and pressure. As a result, refractive index of air in X-band showed relatively high coefficient of determination with humidity and temperature (0.72 and 0.76 on average, respectively) but not so with pressure (0.34). The refractive index of air in X -band changed by <TEX>3.14\;{\times}\;10^{-5}$</TEX> during the measuring time with a humidity range of 50% ~ 90% and a temperature range of <TEX>$-1^{\circ}C$</TEX> ~ <TEX>$9^{\circ}C$</TEX>. We expect that a total expression of refractive index of air including humidity, temperature and pressure can be calculated when more extensive data would be collected at various atmospheric conditions.

Computer Analysis of RCS Characteristic of Rectangle Trihedral Corner Reflector

This Method of physical optics is used to simulate radar cross section characteristic of rectangle triangular trihedral corner reflectors. The reliability of RCS characteristic analysis for rectangle triangular trihedral corner is verified. The experiment and analysis shows that: the rectangle triangular trihedral corner reflector has higher peak values, the further it deviate from the 90°, the peak value dropped, the more obvious; the bottom surface area of rectangle trihedral corner reflector is 1m × 1m, side surface area is 1m × 1.5m, elevation angle are 45° and 90°, the RCS value has reached maximum and the RCS curve present symmetrical distribution, and it is one of the most recognizable objects in SAR images. Index Terms—Rectangle trihedral corner reflector, Radar Cross Section (RCS), physical optics, computer simulation

Direction differentiating cluster reflector for marine/air navigation

A cluster reflector formed by a perfect electric conductor coated trihedral corner reflector, and soft and hard surface coated triangular trihedral corner reflectors, can be used in marine/air navigation. The cluster reflector scatters back circularly polarised waves of one or the other handedness depending on which reflector is illuminated.

Radar Cross Section of Trihedral Corner Reflectors: Theory and Experiment

ABSTRACT Two high-frequency methods are used to compute the scattered fields from both square and triangular trihedral corner reflectors. Scattering from a trihedral corner reflector is dominated by single, double, and triple reflections. Both methods use Physical Optics (PO) to calculate the single reflected fields. For the evaluation of the double and triple reflected fields, the first method uses Geometrical Optics (GO) for the initial reflections and PO only for the last reflection. On the other hand, the second method uses PO for the last two consecutive reflections. In addition to the reflected fields, the two methods use the Method of Equivalent Currents (MEC) to calculate the single diffracted fields from all the exterior edges. Higher-order terms, such as multiple diffractions, reflection-diffractions and diffraction-reflections, are not included in the analysis. The backscattered fields are computed for incident angles along both elevation and conical paths. The predicted results are compared wi...