# Radiometric Comparison within the Sentinel-1 SAR Constellation over a Wide Backscatter Range
## 3 Results
### Transition to Lower Backscatterer (Greenland Region)  
In order to further investigate the radiometric intercomparability of both instruments for lower backscatterer, distributed targets with stable SNRs are sought which can be used for long-term evaluations covering a sufficient backscattering range. For this purpose ice areas within the Greenland region are chosen which cover a wide range of related radar brightness. These values are strongly dependent on the incidence angle and reach values between \(-12\) dB and \(-5\) dB for the copolarized channels and between \(-22\) dB and \(-15\) dB for the cross-polarized channels. Five different distributed targets are defined by selecting observation areas of 1000 by 700 pixels (range, azimuth) covering all three IW subswathes with vignettes of about 23 km\({}^{2}\) in slantrange geometry (Figure 5). The radar brightness \(\beta^{0}\) is calculated for each target from related SAR images using co- and cross-polarized channels. The HV polarization results for both S-1A and S-1B are depicted as a function of observation time within Figure 6; the mean values and standard deviations as well as the differences between S-1A and S-1B are summarized in Table 3.  
As documented in Figure 6 and Table 3, the selected ice areas show a very low backscatter variation for the HV polarization channel over time; the highest standard deviation found is 0.13 dB. These values are in particular lower compared to the rainforest or Lake Constance observation area (shown in the next subsection). The differences between S-1A and S-1B measurements are between 0.03 dB and 0.61 dB. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Target Center [lat,lon] @ Subswath** & \begin{tabular}{c} **pol** \\ **(\(\mu\pm\sigma\))** \\ \end{tabular} & \begin{tabular}{c} **S-1A \(\beta^{0}\) [dB]** \\ **(\(\mu\pm\sigma\))** \\ \end{tabular} & \
\begin{tabular}{c} **S-1B \(\beta^{0}\)[dB]** \\ **(\(\mu\pm\sigma\))** \\ \end{tabular} &
\begin{tabular}{c} **S-1A/S-1B** \\ **Difference [dB]** \\ \end{tabular} \\ \hline \(7.18^{\circ}\) S, \(66.61^{\circ}\)W & HH & \(-4.75\pm 0.18\) & \(-4.92\pm 0.20\) & 0.17 \\
**IW1** & HV & \(-10.81\pm 0.15\) & \(-10.86\pm 0.21\) & 0.05 \\
7.08\({}^{\circ}\) S, \(65.76^{\circ}\)W & HH & \(-5.54\pm 0.21\) & \(-5.87\pm 0.19\) & 0.33 \\
**IW2** & HV & \(-11.57\pm 0.21\) & \(-11.75\pm 0.17\) & 0.18 \\
6.85\({}^{\circ}\) S, \(65.02^{\circ}\)W & HH & \(-6.29\pm 0.19\) & \(-6.73\pm 0.15\) & 0.44 \\
**IW3** & HV & \(-12.25\pm 0.18\) & \(-12.43\pm 0.32\) & 0.18 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Mean radar brightness and standard deviations (\(\mu\pm\sigma\)) from three different observation areas within the Amazon rainforest. Figure 4: Observed radar brightness \(\beta^{0}\) as a function of acquisition time for three distributed target areas within the Amazon rainforest for S-1A (blue) and S-1B (red) for the HV channel. Figure 5: Five distributed targets (red rectangles) selected within IW-mode acquisitions over Greenland. The background elevation map was derived from TanDEM-X 90 m DEM using GMT. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Observation** & **Sub** & **S-1A \(\beta^{0}\) [dB]** & **S-1B \(\beta^{0}\) [dB]** & **S-1A/S-1B** \\
**Area Center** & **Swath** & \((\mu\pm\sigma)\) & \((\mu\pm\sigma)\) & **Difference [dB]** \\ \hline A: 72.62\({}^{\circ}\)N, 33.70\({}^{\circ}\)W & IW1 & \(-\)15.09 \(\pm\) 0.05 & \(-\)15.12 \(\pm\) 0.11 & 0.03 \\ B: 72.68\({}^{\circ}\)N, 34.40\({}^{\circ}\)W & IW1 & \(-\)16.12 \(\pm\) 0.06 & \(-\)16.32 \(\pm\) 0.09 & 0.20 \\ C: 72.83\({}^{\circ}\)N, 34.92\({}^{\circ}\)W & IW2 & \(-\)18.29 \(\pm\) 0.13 & \(-\)18.90 \(\pm\) 0.15 & 0.61 \\ D: 72.88\({}^{\circ}\)N, 35.68\({}^{\circ}\)W & IW2 & \(-\)20.77 \(\pm\) 0.11 & \(-\)21.12 \(\pm\) 0.10 & 0.35 \\ E: 73.01\({}^{\circ}\)N, 38.32\({}^{\circ}\)W & IW3 & \(-\)21.32 \(\pm\) 0.05 & \(-\)21.72 \(\pm\) 0.10 & 0.40 \\ \hline \hline \end{tabular}
\end{table}