## 3 Results
### Backscatter Characteristics
Figure 5: Boxplots (minimum, lower quartile (25%), median (50%), upper quartile (75%), maximum) of (**a–i**) the polarimetric channels (HH/HV/VV) of TSX, R-2, ALOS and ALOS-2; (**i–o**) Yamaguchi Decomposition features (DBL3/VOL3/ODD3) of R-2 and ALOS-2; (**p–x**) Kennaugh Matrix elements (K0/K3/K4) of TSX, R-2 and ALOS-2 for the land cover classes OL (Permanent Water Bodies), BS (Sand), BM (Mixed Sediment), BW (Driftwood Accumulation), HT (Herb Dominated Tundra), ST (Shrub Dominated Tundra), WI (Inundated Low Lying Tundra) and WT (Wetland) (see Table 2). The L-Band HV channel and the volume scattering intensity of the Yamaguchi decomposition showed the same median values and comparable data ranges (lower to upper quantile) for these two land cover classes. At L-Band, the difference between HT and ST was better expressed via the VV channel, and the surface scattering intensity of the Yamaguchi decomposition via lower intensities of the ST compared to HT. This indicates an absence of volume scattering processes and a full penetration of the vegetation by the L-Band microwaves. Assuming that this observation is not caused by temporal variations present in the ALOS-2 data--which were acquired in 2016, while C-, X-Band and the land cover reference data were acquired in 2010/2011, the signal can be interpreted to represent backscattering mostly from the ground, as it is assumed that major changes in land cover type present have not occurred in this time. The differences between the land cover classes WI and WT were characterized by increased DBL3, K0, HH and VV scattering in X- and C-Band and by K3 in X-, C- and L-Band. The statistics of WT showed a clear separation from the other land cover classes in the DBL3 and K3 of C-Band and the HH and K3 of X-Band. The difference between the WT's median value and the median value of any other land cover class exceeded +5 dB in the X-Band HH and C-Band DBL3. The differences between wetland (WT) and inundated low-lying tundra (WI) was observed as higher HH and VV intensities--and the Kennaugh Matrix element K4 accordingly--in X- and C-Band and the double bounce intensity of the Yamaguchi decomposition. Further, both classes were characterized by comparably low values of the Kennaugh Matrix element K3, which points to distinct double bounce scattering, since \(\Re\big{(}S_{HH}S_{VV}^{*}\big{)}\) is a known discriminator for this type of scattering (compare [27; 31]). In contrast, both classes showed low intensities in the HH, VV and double bounce of the Yamaguchi decomposition at L-Band. Accordingly, K3 and K4 were less distinct and no double bounce scattering was present, when using the L-Band, which again is most likely due to the relatively short statured vegetation, the high penetration depth, and the absence of interactions between incident microwaves and the water surface and vegetation canopy. In addition to the boxplots, Figure 6 shows scatterplots of the Kennaugh Matrix elements K0, K3 and K4 of X-, C- and L-Band data in order to investigate the scattering characteristics of the land cover classes in a multivariate feature space. Figure 6a-i shows the position of the land cover reference in the K0/K3 (left column), K0/K4 (center column) and K3/K4 (right column) feature spaces of X-Band (a-c), C-Band (d-f) and L-Band (g-i). These results show that the feature space K0/K3 of X-and C-Band facilitates the differentiation of the classes: OL, BS and WT; however, the position of values for BM, HT, ST and WI were indiscriminant from others. The feature space K0/K4 (Figure 6b,e,h) shows increased distance between samples of HT/ST and WI--especially at X-Band; however, a substantial degree of overlap between the samples of BM and BW, and the samples of HT, ST and WI was present. K0 provided the best separation between land cover classes at X-Band, C-Band, and L-Band. This is unsurprising since K0 of X- and C-Band showed a high positive linear correlation (\(R^{2}\) of 0.8) (Figure 6j).