# A Study of Landfast Ice with Sentinel-1 Repeat-Pass Interferometry over the Baltic Sea
## 1 Introduction
In addition, there are beacons that guide ships and debatching landfast ice can move or destroy them. Having knowledge about these displacements in landfast ice zone can improve our understanding and modeling of landfast ice mechanics, and this progress is useful for making hazard coastal maps as well as improving sea ice models in freezing basins [11; 12]. The landfast ice zone has been examined with different multi-parametric InSAR techniques in several prior studies, using X-, C- and L-bands [10; 11; 12]. European Remote Sensing satellite ERS-1 C-band SAR instrument was used in the northern Baltic Sea in 1998 to study the relation between InSAR coherence, SAR backscatter, phase changes and mechanical events [11]. L-band InSAR technique was employed to map landfast ice extent in the Alaskan coastal zone based on the Japanese Advanced Land Observing Satellite (ALOS) with a temporal baseline of 46 days. There, both interferometric phase pattern and coherence images were used to extract the landfast ice extent [12]. In a recent study in the Baltic Sea, X-band SAR of the Italian Cosmo-Sky Med constellation was used during the winter of 2012 [10], with a temporal baseline of one day. Shorter wavelengths (such as X-band) are in general more suitable for detection of small displacements of the fast ice. The InSAR coherence is a valuable remote sensing observable that can provide information about surface processes. There is a connection between coherence and backscattering images [10; 11; 13]. [PERSON] et al. [10] assessed the relationship between coherence and backscattering in more advanced shape than in the prior study in 1998 [10; 11]. They observed higher coherence along with high backscatter intensity at X-band [10], while an opposite trend was observed at C-band (studied regions were partly overlapping) [11]. Thus, the relationship between backscatter and coherence seemed to be case dependent with several possible explanations [10], although lack of field data complicated the interpretation. With the launch of ESA (European Space Agency) Sentinel-1 satellite in 2014, a new unprecedented capability for C-band InSAR has emerged, as Sentinel-1 data are provided at no cost for users [14]. The Sentinel-1 SAR sensor is operating at four imaging modes (Extra Wide Swath (EW), Interferometric Wide (IW), Stripmap (SM), and Wave (WV)) and with different resolution from 5 to 400 m. In this study, we use IW mode of the C-band SAR radar aboard Sentinel-1 with 12-day temporal baseline to map landfast ice displacement. The IW mode acquires data with a 250-km swath at 5 m \(\times\) 20 m spatial resolution, capturing three sub-swaths with Terrain Observation with Progressive Scans SAR (TOPSAR) technique [15]. This work is the first of its kind, in the examining of landfast ice properties and dynamics in the Baltic Sea with Sentinel-1. A crucial factor in the suitability of satellite sensor for sea ice monitoring is the temporal baseline. In two prior studies [10; 11], the temporal baseline was one and three days, respectively. Applying a long baseline leads to a strong temporal decorrelation which lowers the ability to measure displacements, but on the other hand, it is useful for separating the fast ice from the drift ice and detecting long-term trends in sea ice displacements. In this work, we examine the potential of Sentinel-1 data, even with a temporal baseline of 12 days, to detect, monitor and quantify the landfast ice displacement under certain conditions. Furthermore, in this paper, we examine and provide the interpretation of an InSAR image pair acquired by Sentinel-1 over the northeast Bay of Bothnia in February 2015, with a temporal baseline of 12 days. In Section 2, we provide basic information about the InSAR in the context of ice-measurements, and in Section 3, study site, reference data and InSAR methodological approach in the study are described. Results are provided and discussed in Section 4.