# Independent System Calibration of Sentinel-1B
## 10 Radiometric Cross-Check between Sentinel-1A and Sentinel-1B  
Following the completion of all calibration tasks outlined in Figure 1, the final step was focused on the question of how precisely Sentinel-1B and Sentinel-1A are matched to each other. For this purpose, a cross-check of the radiometric accuracy achieved for both SAR system was performed, i.e., for S-1A and S-1B. This cross-check focused on the IW mode, which is the main SAR mode of operations and currently the best calibrated one for both systems.  
Concentrating on the absolute calibration factor of the IW mode, as shown in Figure 32a for all polarization channels (triangles and squares for co-, crosses for cross-polarization) and distinguishing between the transmit polarization (blue for H and the red for V on transmit), no significant dependencies for the derived Sentinel-1B calibration factor across the entire IW swath can be observed: neither a dependency from beam to beam, i.e., from sub-swath to sub-swath, nor over look angle across the sub-swath, i.e., within one beam. There is only a slight offset of about 0.4 dB for the VH channel related to the other polarization channels. However, correcting the SAR data with the mean value of all these measurements, that is the absolute calibration factor, then the remaining standard deviation of the measurements is a measure for the absolute radiometric accuracy. In this case, for Sentinel-1B, an absolute radiometric accuracy of 0.36 dB for the IW mode has been achieved, whereby additional error sources are taken into account (instrument stability: 0.05 dB [20], dynamic range error: 0.067 dB [20] and target accuracy: 0.2 dB [9]). This accuracy can be even further improved if the observed gain-offset of 0.4 dB for the VH channel is compensated for.  
Considering now the similar plot of the absolute calibration factor for Sentinel-1A shown in Figure 32b, which was derived likewise from measurements of DLR targets and performed during  
Figure 31: Measured azimuth mis-registration between master and slave images by using the enhanced spectral diversity (ESD) approach [22] for every overlap area of the data take.  
Figure 30: Sentinel-1A/B cross-interferogram acquired over east Europe for a long data take (range is vertical, azimuth is horizontal): (**top**) Reflectivity image; (**middle**) coherence and (**bottom**) DEM-flattened interferometric phase. The Sentinel-1A/B images were acquired on 13 June (S1A) and 19 June 2016 (S1B). The data take consists of eight slices with a total of 72 bursts and is about 1400 km long. The data take duration was 3 min and 20 s. The residual fringes observed in the interferogram are mainly related to atmospheric artifacts.  
the Sentinel-1B commissioning phase, a similar spread of the measurements can be observed yielding an absolute radiometric accuracy of 0.38 dB. That means, for both SAR systems, more or less exactly the same absolute radiometric accuracy has been achieved. Hence, this radiometric cross-check between both systems for the IW mode shows how accurately both systems can be adjusted and demonstrates the compatibility of both SAR systems providing an almost identical absolute radiometric accuracy and as such high image quality. In other words, the user will not be able to distinguish between Sentinel-1A and Sentinel-1B calibrated SAR images.