## 4 Results  
### In Situ Observations in Comparison to C-Band Retrievals at the CALM Grid  
Satellite derived displacements in LOS are mostly lower than in situ measurements (Table 3). Sentinel-1 displacements show larger differences between the years. The average over the CALM grid is more than twice in magnitude in 2016 compared to the other years. The comparison with the individual measurements (Figure 4) gives insight into the heterogeneity over the CALM grid. The maximum in situ subsidence in 2016 is similar to 2017, the average is however 0.8 cm lower (larger subsidence) in 2016 (see Table 3). The variations across the CALM grid in 2016 are also visualized in Figure 5. The northwestern part which is located on the ridge shows lower subsidence than the lower part on the slope. The outlier as visible in Figure 4 can be in the lower southern part close to a depression, an area affected by landslides (number 5).  
Although that there are differences among the in situ measurements in 2018, Sentinel-1 displacements show little variations between the pixels overlapping with the locations of the in situ measurements. The underestimation in normal years (2017 and 2018) also remains after correction for viewing geometry based on a scale factor (Figure 4, Table 3). Satellite derived displacements are on average 1 cm (about 20%) lower over the CALM grid sites in these years. Differences within the season observed with C-band can be confirmed in case of 2018, when early season in situ measurements are available. Average subsidence was 2.9 cm in July and 4.1 cm at the end of August. This corresponds to a rate of 3.9 mm over 100 DDT. Sentinel-1 LOS displacement rate is similar but lower with approximately 3.6 mm (Figure 4) and higher with 4.3 mm after application of the scale factor.  
\begin{table}
\begin{tabular}{l c c c c c} \hline \hline
**Parameter** & **2013** & **2016** & **2017** & **2018** & **Average** \\ \hline In situ subsidence, average & nd & \(-5.5\) & \(-4.7\) & \(-4.1\) & \(-4.8\) \\ Sentinel-1 LOS average over CALM grid & nd & \(-5.4\) * & \(-2.6\) & \(-2.0\) * & \(-3.3\) \\ Sentinel-1 scaled LOS average over CALM grid & nd & \(-5.8\) * & \(-3.1\) & \(-2.5\) * & \(-3.8\) \\ \hline Sentinel-1 LOS average over overlap area & nd & \(-5.4\) * & \(-2.4\) & \(-2.5\) * & \(-3.4\) \\ COSMO Skymap LOS average over overlap area & \(-3.8\) & \(-5.8\) & nd & nd & na \\ \hline Active-layer depth in cm over CALM grid (minimum) & 103 & 119 & 108 & 95 & 103 (1999–2018) \\ Total DDT (thaw index) & 759 & 1443 & 827 & 720 & 780 (1999–2018) \\ Cumulative DDT with early season heave (LOS) & 95 & 183 & 0 & nd & na \\ Cumulative DDT when 100 cm in borehole are reached & 119 & 408 & 241 & 191 & 242 (2008–2018) \\ \hline Precipitation sum [mm] Autumn of preceding year & 28 & 33 & 19.3 & 19.3 & 26.4 (2012–2018) \\ Backscatter [dB] from Autumn of preceding year & \(-11.4\) & \(-11.8\) & \(-12.5\) & \(-12.5\) & \(-11.9\) (2012–2018) \\ \hline \hline \end{tabular}
\end{table}
Table 3: Characterization of seasonal maximum subsidence by in situ and satellite data analyses. * denotes offset corrected displacements based on 2017 early season rates. The averaging period differs by parameter. subsidence and line of sight displacement (LOS) values represent the full season and are given in cm. Overlap refers to the common area of Sentinel-1 and COSMO-Skymed acquisitions (Figure 1). Active-layer depth measurements on the CALM (Circumpolar Active-Layer Monitoring) grid exceeded in some cases/years the maximum measurable depth of 170 cm. Degree-days of thawing (DDT) and precipitation is based on measurements at Mare Sale. Autumn is defined as the last 20 days with unfrozen soil surface. nd—no data, na—not applicable.  
Figure 4: In situ (CALM grid) versus Sentinel-1 derived displacement in cm: **left—**line of sight (RMSE 2.42 excluding new sites), **right—**scaled with respect to viewing geometry (RMSE 2.35 excluding new sites). * offset corrected for thawing before start of records based on 2017 records.