# Independent System Calibration of Sentinel-1B
## 7 Radiometric Calibration
### Absolute Radiometric Calibration  
According to the overall Sentinel-1 calibration strategy [4; 5], the objective of the absolute radiometric calibration is to derive one calibration factor that is applicable to all SAR modes and sub-swaths. This strategy is only feasible if all relative radiometric corrections, i.e., the internal calibration and the antenna pattern correction (shape and beam-to-beam gain offset) are properly applied during SAR data processing. Then, no relevant variation of the derived absolute calibration factor is expected for data from different beams, modes and polarization channels. To verify whether these conditions are fulfilled with the required radiometric accuracy, a suitable set of acquisitions covering different modes, beams and polarization channels was chosen for the only three-month commissioning phase of Sentinel-1B. Following this approach, the absolute calibration factor was derived from measurements in all SAR modes acquired over the point targets. The results for the co-polarized channels (HH and VV) are depicted as a function of look angle in Figure 24; the SAR mode is indicated by color, the used target type by symbol (corner reflector: triangle, transponder: square). In Figure 24, the absolute calibration factor varies up to 2.3 dB. In the following, the mode and polarization dependencies are investigated in order to identify systematic trends. Figure 23: Timeline of the standard deviation of the absolute calibration factor for each overpass for all acquired products (blue: SM, red: IW, green: EW) for co-polarization channels. The number of covered targets within the scene is denoted in the circle center. The mean value and standard deviation of the absolute calibration factor for each SAR mode is listed in Table 6; beside the results for all polarization channels (right), the co- and the cross-polarization results are extracted separately (left and center columns). Referring to Figure 24 and Table 6, a different behavior for the StripMap modes compared to TOPS (IW, EW) can be seen. The absolute calibration factors derived for both TOPS modes (IW and EW) are well balanced with mean values of 1.31 dB and 1.45 dB, respectively. In particular, no relevant beam-to-beam offsets are visible for IW, the most operationally used SAR imaging mode. For the StripMap mode, the calibration factor shows high beam-to-beam offsets in the order of 1.5 dB which are visible in Figure 24; the mean values for the co-polarized channels vary between 0.07 dB (S5) and 1.13 dB (S2) (see Table 6). These systematic biases were based on a normalization issue that occurred during SAR data processing. This issue was identified and resolved during the CP and is no longer visible in the data acquired during Sentinel-1 routine operations. By taking into account all polarization channels (right column in Table 6), almost all values are below 3/10 of a dB (1 dB 3\(\sigma\)). Higher standard deviations are indicated for S1 and S5. To investigate this behavior, the polarization and elevation dependency is further analyzed within the following section. However, based on this standard deviation, the absolute radiometric accuracy of Sentinel-1B can be determined for each mode. In order to derive this overall absolute radiometric accuracy for Sentinel-1B products over mission time, additional error contributions have to be taken into account:  
* a reference target accuracy of 0.2 dB [9],
* a dynamic range error of 0.067 dB [20], and
* a long-term stability of the instrument of 0.05 dB [20]. Considering all these contributions for the IW mode, an absolute radiometric accuracy of 0.36 dB (1\(\sigma\)) for Sentinel-1B has been achieved.