# A Study of Landfast Ice with Sentinel-1 Repeat-Pass Interferometry over the Baltic Sea
## 1 Introduction  
Landfast sea ice is a type of sea ice that is formed and fastened to the coastline, islands, grounded ridges or any fixed objects in waters. Further offshore, the drift ice moving by winds and sea currents is found and forms an interactive outer boundary for the landfast ice. In the Baltic Sea, this boundary can reach the isobaths of 5-15 m depending on how cold the winter is [1]. The landfast ice zone grows laterally during the winter, and its decay begins from the shoreline in the spring. Most of the winter, the landfast ice forms a stable ice cover. Annual ice season lasts 5-7 months in the Baltic Sea, from November to May. The landfast ice and its mechanics are of major concern in the archipelago areas, especially in the Bay of Bothnia, the northernmost basin. In extreme forcing conditions, the landfast ice may break and move horizontally, therefore scouring of the sea bottom, and loading of the coastline and man-made structures take place [1]. These events put coastal structures, beacons and on-ice traffic in danger, and compromise safety for working on the ice. After the start of the melting season in the spring, ice floes and ridges may drift easily, causing increased land-ice interaction and coastal erosion. However, the springice has lower strength than the colder ice in the winter. Analytic tools [2; 3] provide a first-order approach to examine the evolution of the landfast ice zone. These tools consist of simple models for ice formation, thickness, and breakage. With increasing thickness, the strength of the ice layer increases, and landfast ice boundary extends further out. The whole process is forced by air temperature, wind, and bathymetry. Physically based numerical models of the evolution of landfast ice zone do not yet exist. While the mechanism of landfast ice breakage is generally understood, its details are not fully known. In addition, the lateral growth of landfast ice is more complex than a moving single boundary, and no suitable analytical or numerical models are available for describing the deterioration of landfast ice in the spring [1]. The lack of physically based models is partly attributed to historical reasons, as the landfast ice, unlike the drift ice, is less of an issue for winter navigation. To better understand, predict and monitor the landfast ice zone, more well-documented cases and analyses of the observations are needed. One widely used tool for mapping ice conditions is spaceborne radar. Radar methods have advantages in remote sensing of the sea ice due to relatively strong backscatter contrast between different ice types [4]. Therefore, spaceborne imaging radar is widely applied in ice condition monitoring [5]. SAR has been widely examined for the ice/open water classification [6; 7]. Imaging radar, unlike optical instruments, has all-weather and day-and-night observation capability. In addition, imaging radars can provide better detail and spatial coverage compared to non-imaging radars [8]. Synthetic Aperture Radar (SAR) is a kind of imaging radar that employs advanced processing technology for achieving detailed images of underlying terrain and mapping its features. Furthermore, advanced SAR techniques such as the InSAR can be used for precise measurements of surface topography and displacements. Measurements of travel path variations as a function of the satellite position and time of acquisition allow generation of Digital Elevation Models (DEMs), and measurement of centimeter-scale surface displacements of the terrain [9] by differentiating the phase of two or more SAR images [10]. InSAR coherence and phase provide information of stability and changes in the underlying terrain with high accuracy. As landfast ice is nearly stationary, it may experience small displacements in order to cm to meters [11]. These displacements can make cracks and fractures, which are dangerous for local people who use the ice cover for traveling, hunting and fishing, as well as for shipping communication.