## 1 Introduction
A solution would be a constellation as available by COSMO-Skymed. Such data are, however, scarce compared to the acquisitions by C-band missions such as Sentinel-1, which only feature repeat intervals of 6 or 12 days. The latter is more common due to global acquisition strategies. A method to adjust for the early season data gap is required to allow the use of Sentinel-1 or similar data in the extensive region of shrub tundra and unsaturated soils. We hypothesize that knowledge gained from a multi-satellite constellation such as COSMO-Skymed, which provides dense time series, could lead to a suitable approach to account for the early thaw data gap in time series from Sentinel-1. This requires comparability of the displacement measurements from the two different wavelengths. So far only a few studies are available for comparisons between X and C-band over permafrost terrain, (e.g., [4; 19]). These efforts have been limited to qualitative assessment due to the differences in acquisition timing between the missions. Mostly spatial patterns are assessed visually. A comparison of L-band and X-band demonstrated the impact of vegetation on applicability of TerraSAR-X with comparably long revisit intervals [20]. We have selected a long-term monitoring site on Central Yamal, Russia, to test our hypothesis. This area features a high range of ALT (up to 170 cm and locally more), unsaturated moisture conditions over larger areas and shrubundra as dominating land cover. Tabular ground ice is known to extensively occur on central Yamal [21]. ALT is measured since 1993 over a CALM grid and several boreholes are in the surrounding area. ALT as well as Mean Annual Air Temperatures (MAAT) have been increasing continuously since the start of the records [21]. Anomalously high temperatures (and thaw indices) have been observed for 2012 as well as 2016 with unprecedented levels at Mare Sale at least since 1947 [22]. In situ measurement of subsidence started within the last decade. Landslides and resulting thermodenudation cirques are typical features on central Yamal [21] and are also partially represented on the CALM grid. They shape not only terrain but also the distribution of vegetation communities [23]. Landslides are usually initiated episodically in certain years. All known thermodenudation cirques are regularly surveyed and documented [24]. The latest year which triggered abundant thermodenudation was 2012 [21] as it was an exceptionally warm year. Several thermodenudation cirques have been reactivated in 2016 [22]. A further specific feature of central Yamal is ice-rich permafrost at the base of the active layer. It is in general expected that penetration of thaw into such layers is accompanied by loss of volume due to consolidation and the cause of surface subsidence [25]. A further feature on central Yamal are deep craters first suggested to be triggered by the extreme conditions in 2012 [26] and more in 2016. Satellite derived displacement measurements may provide advanced insight into the impact of temperature extremes in combination with in situ measurements. A range of SAR time series from different missions (including X-, C and L-band) are available for central Yamal, but no acquisitions exist for the year 2012 from for science commonly used missions such as TerraSAR-X, COSMO-Skymed, ALOS (failed in 2011), Sentinel-1 (not yet launched), ENVISAT ASAR (failed in April 2012), Radarsat (ScanSAR only). COSMO-Skymed, Sentinel-1 and ALOS-2 acquisitions are, however, available for 2016 and may allow for assessment of the warmer than average conditions on ground thaw on central Yamal. A constraint is the lack of spatial overlap of COSMO-Skymed and the CALM site what needs to be considered. Furthermore, a regularly year to year connection with C-band is not feasible over this region, as the abundant shrub cover is causing strong decorrelation over one year.