# Radiometric Comparison within the Sentinel-1 SAR Constellation over a Wide Backscatter Range
## 5 Conclusions  
The radiometric performance of S-1A and S-1B was evaluated for different target types covering a wide backscatter range from point targets with a high SNR to distributed targets like rainforest, ice area, and calm waters with medium or low SNR values. Using the known RCS of the DLR point targets (corner reflectors and transponders), an absolute radiometric accuracy of 0.346 dB (1\(\sigma\)) for S-1A and of 0.302 dB (1\(\sigma\)) for S-1B was derived for the IW mode covering a two-year observation period. The long-term radiometric stability of both instruments was verified to be in the order of 0.1 dB by focusing on the backscatter variations of targets observed with a given acquisition geometry, i.e., with a (nearly) constant SAR antenna elevation angle. Besides point targets, ice areas were also found to be stable targets over time. The backscatter intercomparability was analyzed by a direct comparison between S-1A and S-1B for the same targets observed by the same acquisition geometry. For point targets only small differences remain between S-1A and S-1B of \(-\)0.08 dB and 0.21 dB for the VV and VH polarization channel, respectively. Similar values were found for distributed targets with a medium SNR above 10 dB (rainforest and ice areas).  
Furthermore, low backscatter targets with a SNR below 10 dB were analyzed by focusing on cross-polarized acquisitions of ice areas and calm waters. For these targets higher backscatter  
Figure 13: Transmit pattern gain of S-1A and S-1B measured by transponder recordings for different geometric configurations related to a certain look angle for S-1A (blue) and S-1B (red) acquisitions. The detected power matches well the shape of the theoretical SAR transmit elevation pattern (lines) derived from the Sentinel-1 antenna model (AM). The transmit power from S-1B is about 0.7 dB higher compared to S-1A.  
differences between S-1A and S-1B were found; e.g., up to 0.6 dB for the ice area and 1.1 dB for Lake Constance. The recommended noise subtraction step reduces these differences; nevertheless, remaining discrepancies are observed after noise subtraction in the order of 0.6 dB. The reason is that the accuracy of these noise subtracted backscatter signals is strongly dependent on the quality of the estimated noise levels. It was shown that in case of low SNR (e.g., 0.8 dB) a backscatter difference of 0.6 dB may arise from an inaccuracy of the estimated noise level in the order of only 0.1 dB.  
However, it was shown that for low SNR targets, S-1B shows systematically lower radar brightness compared to S-1A - in the extreme case, the observed NEBZ acquired for cross-polarized channels over calm waters differs by about 1.4 dB. From this fact it was concluded that S-1B is more sensitive compared to S-1A. From evaluations of transponder recordings, a higher transmit pattern gain of about 0.7 dB was observed for S-1B compared to S-1A. Taking furthermore the reciprocity of the passive radiation antenna elements into account, the receive pattern gain of S-1B must also be higher compared to S-1A by about 0.7 dB, yielding the highest observed backscatter difference of 1.4 dB over calm waters.  
Conceptualization and analysis, K.S.; supervision, M.S., writing-original draft preparation, K.S.; writing-review and editing, M.S, N.M., J.R. All authors have read and agreed to the published version of the manuscript.  
The work in this paper was partially funded by the ESA Contract no. 4000119795/17/I-BG.  
The authors acknowledge the DLR calibration team for their support related to the point target operation and maintenance (transponders and corner reflectors), namely [PERSON], [PERSON], [PERSON], and [PERSON]. The authors thank [PERSON] and [PERSON] for the review of earlier versions.  
The authors declare no conflict of interest.