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Passive calibration target: Trihedral corner reflector with an inner leg length of 1.5 m.  

Passive calibration target: Trihedral corner reflector with an inner leg length of 1.5 m.  

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Conference Paper
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The Microwaves and Radar Institute regularly performs calibration campaigns for spaceborne SAR systems, among which have been X-SAR, SRTM, and ASAR. In 2007, the German SAR satellite TerraSAR-X will be launched. Before it is ready for scientific and commercial use, the instrument has to be calibrated to ensure highly accurate data products. An abso...

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Citations

... Long-term monitoring results over the entire mission duration show that, all technical challenges that occurred over time, could be resolved without having a major influence on the SAR performance and consequently on the mission [12]. In addition, the instruments in terms of transmit and receive modules (TRMs) as well as the evaluation of accurate reference targets [13] to derive e.g. the absolute calibration factor, have not shown any considerable degradations over time. By estimating the on-board resources, it furthermore becomes apparent that the remaining consumables allow a further operation of both satellites for several years. ...
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The TanDEM-X mission, formed by the TanDEM-X satellite (TDX) in cooperation with its almost identical twin TerraSAR-X (TSX), has mainly been designed to acquire bistatic Synthetic Aperture Radar (SAR) images of the Earth. Initiated in 2010, the primary mission objectives were to generate a global Digital Elevation Model (DEM), to perform scientific measurements and to explore novel SAR techniques. Up to the present day all these objectives have been completed with outstanding results. In addition, the satellites maintained a stable image quality from the start throughout their entire lifetime. The ground segment, which was tailored to the capabilities of the space segment, as well as an excellent design and manufacturing of the satellites were the key elements to achieve such a performance. In addition, the elaborate calibration concept and a careful treatment of the on-board resources enabled full operability up to the present day and allowed additional mission objectives such as the generation of a Change-DEM layer as an update to the global DEM. The stable bus and SAR payload performance as revealed by long-term system monitoring provide the basis for further extension of the TanDEM-X mission for several more years. In the following paper these long-term system monitoring results are presented to provide an overview of the evolution of the radar system. The results and experience gained with more than a decade of TanDEM-X mission operation are a great value for both the scientific and commercial community and serve as a forerunner for the development of future SAR missions.
... There are several well-established techniques to determine the antenna beam pointing: homogeneous distributed targets like the Amazon rainforest are used to determine the antenna pointing in elevation [9]. Measurements performed with ground receivers [10,11] and the evaluation of the Doppler centroid are often applied to retrieve the antenna pointing in azimuth. This paper focuses on the determination of the antenna pointing in elevation, at any region, i.e., also regions without homogeneous distributed targets. ...
... Remote Sens. 2019,11, 320 ...
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... The processor generates well-defined level 1b radar images with each pixel referring to the time of closest approach (zero-Doppler time) and the corresponding two-way round trip time [38,39]. In order to accurately relate this 2-D timing information to a dedicated location within the image, one may create a bright point response by placing a trihedral CR, see Figure 5. From the SAR sensor perspective, the CR needs to be large enough to offer a strong signal return in the radar image, and it must fulfill tight limits in terms of plate orthogonality, planar surface geometries, and mechanical stability [40]. If such a reflector is permanently installed and the reference coordinates are known in the International Terrestrial Reference Frame (ITRF, for the latest release 2014 see [22]), then the SAR measurements may be verified on a pass by pass basis by analyzing the radar timings. ...
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... This LTSM has been ensuring a consistent product quality for more than ten years. The success of this performance is based on the innovative design and precise manufacturing of the satellite systems [8,9] on the one hand and on the other on innovative methods, accurate reference targets and the strategy for calibrating and monitoring TSX and TDX over lifetime [5,[10][11][12][13][14][15]. The radiometric performance over lifetime of both systems is analyzed in three steps: ...
... A typical measured uncorrected profile together with the antenna reference pattern is depicted on the top panel of Figure 5 for a single ScanSAR acquisition. Note that absolute radiometric calibration was performed by measuring both systems against accurate reference targets [5][6][7]11]. In the next step, the minimum and the maximum value as well as the standard deviation are derived from the difference between the reference and the measured antenna patterns, as shown on the bottom panel of Figure 5. Based on these statistic values derived from each set of ScanSAR beams acquired over the Amazon rainforest, the antenna patterns and consequently the front-end panels, especially the antenna wave guides have been monitored since launch of the satellites. ...
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... The factors that can act to reduce the RCS of a CR at boresight compared to the theoretical value are misalignment of the reflector, inter-plate orthogonality, plate curvature, size deviation, and surface irregularities. The reduction in RCS due to various factors is described in Zink and Kietzmann (1995), Ulander et al. (1991), and Döring, Schwerdt, and Bauer (2007). For uncertainty analysis in this study, the manufacturing tolerance specifications of the CRs (Table 10) were used. ...
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... This means that a trihedral design is much more forgiving of field alignment errors when compared to other reflector designs. Consequently, trihedral corner reflectors have been used for many years as targets suitable for calibration of SAR images (e.g., [19,[27][28][29]) and they are also the most common target type being deployed for use in PSInSAR analysis of ground deformation. ...
... Due care and consideration should be taken to ensure that the target design will not saturate the signal, particularly at higher frequencies. As an indication of 'safe' target sizes, the German Aerospace Center (DLR) report usage of 3.0 m TCR for calibrating the TerraSAR-X sensor without saturation [28]. Furthermore, DLR have designed a C-band transponder with an RCS of 60.8 dBm 2 for calibration of Sentinel-1 and tested using RADARSAT-2 [33]. ...
... There are several factors that can introduce a loss of RCS for a TCR compared to the theoretical values given by Equation (6), including inter-plate orthogonality, plate curvature and surface irregularities. To achieve an RCS accuracy of better than 1 dB with respect to theoretical values, DLR specify the following tolerances on their TCR manufacture process: Inter-plate orthogonality ≤ 0.2 • ; Plate curvature ≤ 0.75 mm; Surface irregularities ≤0.5 mm [28]. These tolerances apply to X-band, with less stringent tolerances applying to lower frequencies. ...
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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.
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... The verification of the antenna pattern in azimuth direction was performed for transmit pattern only using the DLR ground calibration equipment in form of ground receivers [7] as shown in Figure ?? These receivers record the amplitude of the pulses transmitted by the SAR antenna. ...
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... Especially the requirements in terms of accuracy and sta- bility represent a step forward with respect to transponders which were operated during the TerraSAR-X commission- ing phase [3]. ...
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... In addition to elevation, also the antenna patterns in the flight direction have to be verified. The verification of these azimuth antenna patterns was performed for a transmit pattern using the DLR ground receivers [10] as shown in Fig. 4. In contrast to the elevation pattern, where only the main lobe is measured, also several sidelobes can be determined in the azimuth. ...
... Here, in Fig. 17, a cyclic variation can be seen. This variation was also recognized in L-band and C-band images over rainforest, e.g., for RADARSAT-1 [10], where it evolves sinusoidal. The variation is caused by seasonal effects of the vegetation in the Amazon rainforest. ...
Article
The high flexibility and tight accuracy requirements of modern spaceborne synthetic aperture radar (SAR) systems require innovative technologies to calibrate and process SAR images. To perform accurate pattern correction during SAR processing, an antenna model can be used to derive the multitude of different antenna beams generated by active antenna steering. The application of such an antenna model could be successfully demonstrated for the TerraSAR-X mission, launched in 2007. The methodology and the results of the in-orbit verification with an achieved accuracy of better than +/-0.2 dB are reviewed in this paper in detail, showing its outstanding accuracy. Additionally, the results of the antenna pattern long-term monitoring are described, pointing out the high stability of the system.