# Radiometric Comparison within the Sentinel-1 SAR Constellation over a Wide Backscatter Range
## 3 Results
### Point Target Evaluations  
The differences between the observed RCS derived from SAR images and the theoretical expected RCS of the DLR reference targets are depicted in Figure 1 for S-1A (top) and S-1B (bottom) as a function of time. The trihedral corner reflectors produce impulse responses only for copolarized products. Thus results from the VV-polarization channel (red) containing both transponder and corner reflector RCS appear more often compared to VH-polarization channel (blue) which represents transponder results only. The RCS differences spread around 0 dB and indicate that both SAR instruments are well balanced in terms of radiometric calibration in general. However, both subfigures show small drifts over time (up- and down) in the order of 0.3 dB over several months. This behavior may arise from season related environment effects, e.g., due to more rainy days in summer compared to winter or remaining SAR instrument drifts. Figure 1: Deviation between radar cross section (RCS) derived from synthetic aperture radar (SAR) images and the nominal RCS of DLR point targets for each acquisition within a two-year observation period. SAR products are acquired by S-1A (top) and S-1B (bottom) both using interferometric wide-swath mode (IW) mode with VV-polarization (red) and VH polarization (blue). Note that for the trihedral corner reflectors (CR) only RCS from copolarized products can be derived while for transponders (TR) both polarization channels are available. The overall mean values and standard deviations for both SAR instruments and polarization channels related to the observation period are summarized in Table 1. These values document a slightly higher radiometric variation seen for the VH- compared to the VV-polarization channels and also a slightly higher variation of S-1A compared to S-1B. However, the mean values verify the radiometric calibration and indicate the intercomparability of both SAR instruments. In particular, for the VV-polarization channel the overall mean RCS deviation is below 0.2 dB for both SAR instruments with a mean difference below 0.1 dB between S-1A and S-1B for this polarization channel. The highest RCS bias is seen for VH-polarization channel of S-1B with -0.24 dB which leads to a mean difference between S-1A and S-1B of 0.21 dB for this polarization channel. The absolute radiometric accuracy can be derived from these point target evaluations using the standard deviations from Table 1 and considering additional error contributions, i.e., the long-term stability of the instrument of 0.05 dB (1\(\sigma\)), and the reference target accuracy of 0.20 dB (1\(\sigma\)). For the IW mode and the two-year observation period, the absolute radiometric accuracy is obtained to be 0.346 dB (1\(\sigma\)) and 0.302 dB (1\(\sigma\)) for S-1A and S-1B, respectively. Note that the requirement in terms of absolute radiometric accuracy is 1 dB (3\(\sigma\)) for Sentinel-1; propagation effects are neglected. The next step focuses on the acquisition geometry by grouping the derived RCS differences as a function of look angle. Therefore, the mean RCS deviation for each reference target is plotted for the given look angle in Figure 2 for S-1A (left) and S-1B (right). The results cover all three subswathes (IW1 - IW3) over a wide look angle range of about 12\({}^{\circ}\) (from 27\({}^{\circ}\) to 39\({}^{\circ}\)) which corresponds to an antenna elevation angle range from about -3\({}^{\circ}\) to +9\({}^{\circ}\) for this observation area. Each target is depicted by a specific symbol in Figure 2 and appears at different look angles related to three different satellite tracks used. As before, VV-polarization results (red symbols) are available for both target types (corner reflectors and transponders), while the RCS deviation for VH-polarization channels (blue) can be derived for transponders only.