# Independent System Calibration of Sentinel-1B
## 11 Conclusions  
An independent SAR system calibration of Sentinel-1B was successfully performed by DLR in summer 2016. In addition to the Sentinel-1B commissioning phase implemented by ESA, several calibration activities were executed by DLR: starting from planning and coordinating of calibration campaigns, the deployment, initialization and alignment of reference targets up to the analysis and evaluation of a multitude of data takes and measurements. The Sentinel-1B instrument is very stable, similar to that of Sentinel-1A [3]. Irrespective of the imaging mode, transmit and receive polarizations, it can be observed that the instrument behaves very stably even for long data acquisition periods of about 25 min. This stability of the instrument provides the basis on which to achieve an accurate calibration of the whole SAR system. Moreover, in-orbit RFC measurements show that the TRMs are working well, verifying the health and proper performance of the antenna after launch. Furthermore, the pixel localization accuracy is up to 1 m in azimuth and better than 0.5 m in range, i.e., within the sub-pixel domain for all Sentinel-1B products. The observed pointing offsets should be compensated for in order to improve the antenna pattern compensation during SAR data processing and consequently to improve the relative radiometric accuracy. Furthermore, a cross-talk of less than \(-35\) dB for all modes confirms the very good isolation of both polarization receiving channels. The analysis of the antenna patterns and the absolute calibration factor shows gain slopes of up to 1 dB and beam-to-beam gain offsets of up to several dBs, especially for steep look angles. The reason for the offsets is a gain normalization issue during SAR data processing, which could be solved and will be updated within the next processor version. The gain slope and especially the so-called smiling effect observed for the channel imbalance arises from the antenna reference patterns derived by the antenna model and their application during SAR data processing. Figure 32: Absolute calibration factor derived from point targets for the IW mode (triangle: corner reflectors, square: co- transponders, crosses: cross- transponder) and distinguished between transmit polarization (blue: H on transmit, red: V on transmit). (**a**) for Sentinel-1B of the IW mode; (**b**) for Sentinel-1A of the IW mode. However, for the Interferometric Widespread mode, the main mode of SAR operations, Sentinel-1B already shows an absolute radiometric accuracy of 0.36 dB (1\(\sigma\)). The cross-check with Sentinel-1A reveals an almost identical accuracy of 0.38 dB (1\(\sigma\)) and confirms the high compatibility of both SAR systems. In other words, for the IW mode, the user will not be able to distinguish between Sentinel-1A and Sentinel-1B calibrated SAR images. The good performance of the system also applies to interferometry which could be performed without major efforts with Sentinel-1B SLC data as well as cross-platform interferometry (Sentinel-1A-Sentinel-1B). No InSAR phase artifacts were detected and the burst synchronization is better than 5 ms for both the Sentinel-1B-Sentinel-1B interferometry and in the case of the cross-platform interferometry. This confirms the very good interferometric performance of the Sentinel-1B unit, as well as the excellent cross-InSAR compatibility with the Sentinel-1A SAR instrument. Further efforts should be concentrated on improving the accuracy of the antenna reference patterns and their application for shape compensation during SAR data processing. In addition, the radiometric performance across the swath could be further improved by correcting a potential mispointing of the elevation antenna pattern. Concentrating on these topics, the stringent requirement of 0.33 dB (1\(\sigma\)) for the radiometric performance should also be achievable for the other SAR imaging modes. The absolute radiometric accuracy should be re-assessed with the final version of the SAR processor once the mispointing corrections have been applied.