## 1 Introduction  
Increasing ground temperatures are expected to have serious consequences for ecosystems, hydrological systems, and infrastructure integrity in the Arctic [1]. A network of in situ monitoring sites exists but many regions are underrepresented [2]. Satellite measurements can only indirectly support permafrost monitoring [3]. Apart from ground temperature, the variation of the seasonal thaw depth on top of permafrost has been addressed with various remote sensing methods including land cover as proxy and displacement monitoring. The latter relies on Synthetic Aperture Radar interferometry (InSAR). Displacement in line of sight can capture the subtle changes (millimeter to centimeter) of ground subsidence and heave, (e.g., [4; 5; 6]). These seasonal variations in ground displacements are expected to reflect changes in thaw depth and active-layer thickness (ALT) respectively in permafrost areas [5]. The surface changes result from melt of pore ice or ice lenses. Drainage may also lead to secular subsidence. The water/ice content of a soil therefore determines the amount of displacement. If the soil profile is uniform and is fully saturated, ALT can be directly inferred [7]. Models which reflect the varying soil layer properties and variations in wetness (precipitation) need to be used in other cases, (e.g., [8]). An example for an account of different soil layers is described in [PERSON] et al. [8] and [PERSON] et al. [9] (also includes the impact of precipitation). This does, however, require detailed knowledge of the soil stratigraphy, which is rarely available across the Arctic. A further shortcoming is the lack of in situ measurements of active-layer thickness. Such measurements are often sporadic when related to expeditions and therefore do not allow detailed interpretation of findings from InSAR analyses, (e.g., [10]). Measurement grids of long-term monitoring sites which are part of the Circumpolar Active-Layer Monitoring (CALM) network provide very valuable data in this context. In situ measurements of subsidence itself are also challenging. No in situ monitoring network for ground displacements in permafrost areas exists to date. Satellite data of all common frequencies (X-, C-, and L-band) have been shown to be applicable to detect line of sight (LOS) displacements related to thaw subsidence. Constraints caused by the wavelength exist for the comparably short X-band (\(\sim\)3.1 cm) due to its sensitivity to changes on the ground between acquisitions [11] (e.g., for high latitude tundra). This is especially the case if acquisitions come from a single satellite constellation only [12], what results in case of TerraSAR-X in 11 day repeat intervals. An alternative provides for example COSMO-Skymed (COnstellation of small Satellites for Mediterranean basin Observation), which currently consists of four satellites and allows for repeat measurements of one day. Such data have been so far not investigated for permafrost related applications. Longer wavelengths (specifically L-band, wavelength 23 cm) are of advantage for connecting the seasonal time series across the years [4]. All so far available long-term application studies make therefore use of L-band data, specifically the Japanese ALOS (Advanced Land Observing Satellite)-missions, (e.g., [5; 13]). It has been recently demonstrated that also C-band (wavelength 5.6 cm) can be used to establish long-term records in areas with limited vegetation growth [14; 15]. Large parts of Arctic permafrost areas are, however, characterized with tundra shrubs. Relating observations between seasons allows also to account for data gaps at the beginning of the soil thaw season. Data pairs usually start several days to weeks later due to the disturbance of snowmelt for repeat InSAR retrieval [16] and mission specific repeat intervals. This limits the monitoring based on separate seasons, (e.g., [17]). The majority of ground thaw in areas with unsaturated soils does typically occur right after snowmelt, (e.g., [18]).