# Independent System Calibration of Sentinel-1B
## 2 In-Orbit Calibration Overview  
As for Sentinel-1A [3], DLR's Microwave and Radar Institute performed an independent system calibration for Sentinel-1B. Based on an efficient calibration strategy, see [4] and [5], measurement campaigns were executed for 67 overpasses during the 3-month Sentinel-1B commissioning phase according to the calibration procedures depicted in Figure 1. To derive the different calibration parameters, at least four reference targets per overpass were initialized and aligned (in total, 274 reference targets), and more than 1200 data takes and measurements were analyzed and evaluated. The procedures depicted in Figure 1 were mainly conducted both for relating the SAR images to the geographic location on the Earth's surface and for transforming the image level into Radar Cross Section (RCS) or backscattering coefficients \(\sigma^{0}\). In addition to these procedures, the polarimetric characteristics of both receiving channels were analyzed in-flight. By means of internal calibration and of absolute radiometric calibration, i.e., by external measurements against suitable reference targets, this polarimetric characterization was focused on the determination of the channel imbalance  
Figure 1: SAR system calibration procedures required for absolute radiometric calibration of SAR data products. in amplitude and in phase between both receiving channels (H-pol and V-pol). In addition, the cross-talk between both receiving channels was also verified by means of external measurements using high precision corner reflectors. The successful completion of these calibration procedures requires a well equipped calibration facility [6]. For this purpose, different analyses and evaluation tools were developed considering the specifics of the Sentinel-1 SAR system. Furthermore, DLR developed and manufactured their novel generation of remote controlled reference targets:  
* DLR's novel C-band transponder, called \"Kalibri\", was designed for the Sentinel-1 mission. Despite the compact design, as shown in Figure 2a, \"Kalibri\" provides a RCS of 60 dBm\({}^{2}\)[7] at a center frequency of 5.405 GHz with a bandwidth of 100 MHz. As \"Kalibri\" serves as a radiometric reference, significant effort was spent on the radiometric calibration of these transponders. Different methods were analytically compared [8] and conducted in order to increase calibration confidence through cross-comparisons. Thus, a standard uncertainty of 0.2 dB has been achieved [9; 10; 11]. * DLR's remote controlled trihedral corner reflector is shown in Figure 2b. This corner reflector is turned up-side down, allowing the opening facing downwards to be rotated when the corner reflector is not operated. This parking position reduces the accumulation of dirt over time and protects the opening from different weather conditions when the corner reflector is not being operated. With an inner leg length of 2.8 m, the corner reflector provides a peak RCS of 49.2 dBm\({}^{2}\) at the Sentinel-1 center frequency according to the physical optics approximation [12]. A similar standard uncertainty of 0.2 dB has been achieved by a low shape tolerance. Precise laser-tracker measurements have confirmed mechanical deviations (e.g., plate deformations) from an ideal corner shape, which result in RCS changes that are negligible at below \(\pm 1\) mm. At the beginning of 2014, three transponders and three corner reflectors of these precision multi-mission SAR calibration point targets were installed within the DLR calibration site, located in South Germany to the west of Munich. Since then, the DLR calibration site has been maintained and successfully operated for Sentinel-1A and Sentinel-1B. Although the locations of these permanently installed targets are optimized for the Sentinel-1 coverage as shown in Figure 3, remote-controlled alignment for all targets allows a flexible assignment for different missions and tasks. Figure 2: Permanently installed, remote-controllable geometric and radiometric SAR calibration measurement standards.