# Radiometric Comparison within the Sentinel-1 SAR Constellation over a Wide Backscatter Range
## 4 Discussion
### Noise Data Analysis from Calm Ocean Acquisitions  
As stated in the previous section, noise estimation has a high impact on the radiometric accuracy in particular for backscatter with low SNR. In order to analyze this impact, the noise level of S-1A and S-1B is directly compared. Therefore, radar acquisitions are used covering a wide area of very calm waters. These areas can be found in particular within the ocean near islands. For the current study, an observation area located in the Pacific ocean south of the Hawaiian islands is chosen where dual-polarized radar data are available for S-1A and S-1B. In particular for the cross-polarization channel it is expected that most parts of the transmitted energy are reflected by the smooth surface away from the incidence direction so that the instrument receives almost no backscatter energy from the surface and the SAR receiving channel contains mainly the instrument noise.  
The radar brightness of the calm ocean region detected by the cross-polar channel is depicted in Figure 12 for two single IW mode acquisitions (S-1A and S-1B) with a six days time delay between S-1A and S-1B. In addition to the measured backscatter, the annotated noise levels are also plotted. In an ideal case, the annotated noise profiles should match the measured radar brightness as the received power in the cross-polar channel is expected to be instrument noise only. Note that instead of a constant noise level, the shape of the inverted two-way elevation antenna pattern is reflected within the figure as expected because these patterns have to be subtracted during SAR data processing  
Figure 11: Radar backscatter differences between S-1A and S-1B as a function of SNR for distributed targets (rainforest: green, ice area: blue, lake: dark blue) derived from copolarized (squares) and cross-polarized channels (diamonds). The point target results for copolarized (plus) and cross-polarized (cross) channels are depicted on the right as a comparison – the corresponding SNR is higher compared to the distributed targets and not related to the x-axis. For low SNR the noise subtracted backscatter results (open symbols) differ significantly from backscatter results without noise subtraction (full symbols).  
accordingly. Such observations were already evaluated in order to derive the noise equivalent sigma zero (NESZ) of the Sentinel-1 SAR instrument [40].  
The results within Figure 12 show that the annotated noise from S-1A nearly predicts basically the measured radar brightness \(\beta^{0}\). Only for some regions (e.g., between 29\({}^{\circ}\) and 30\({}^{\circ}\) look angle within IW1) the measured \(\beta^{0}\) is even lower than the predicted noise, i.e., the noise level may be slightly overestimated for these sections. For S-1B the measured backscatter is always higher compared to the annotated noise over the entire look angle range. This indicates that the noise level is not reached for S-1B within the selected calm ocean regions.  
Furthermore, Figure 12 shows that the lowest detectable radar brightness (which is equivalent to the NEBZ) differs significantly between both SAR instruments by about 1.4 dB on average. S-1B can detect a lower backscatter level than S-1A or in other words, S-1B is more sensitive than S-1A. In addition to a lower noise level, the higher sensitivity may also arise from a higher two-way antenna gain due to a higher transmit power or a higher pattern gain for the transmit or the receive path or both. From the calm ocean measurements alone it is not decidable if the different sensitivity levels are due to different system noise, transmit power levels, or pattern gains of both SAR instruments.