# A Study of Landfast Ice with Sentinel-1 Repeat-Pass Interferometry over the Baltic Sea
## 4 Results and Discussion
### Comparison with Previous Works  
To our knowledge, the InSAR techniques have been used at least twice before over the Baltic Sea to investigate the landfast ice mechanisms [10; 11], but with very different set-up. In our study, the temporal baseline was the 12 days while, in two previous studies, the temporal baselines were much shorter, three days [11] and one day [10]. Two reasons for losing coherence in our study were increasing temperature and precipitation which were not seen in the previous studies. Air temperature reached over 0 \({}^{\circ}\)C several times during this study period while it was between \(-2.7\)\({}^{\circ}\)C to \(-5\)\({}^{\circ}\)C [11] and mainly between \(-16\)\({}^{\circ}\)C and \(-1\)\({}^{\circ}\)C in [10], although we can find one case in [10], where temperature increased from 0 \({}^{\circ}\)C to 5\({}^{\circ}\)C and was the reason for losing much coherence. Dry snow cover retained coherence over the landfast ice in [11] while in our study snow cover may have experienced melting because of high temperature causing a loss of coherence.  
[PERSON] et al. [11] examined three InSAR pairs. The first interferogram was formed in the winter of 1992 by the image pair 24 and 27 March, the second one by the image pair 27 and 30 March, and the third by 24 and 30 March. The largest displacement was seen over an ice patch that was cut by two icebreaker tracks; 94 cm compression towards north was detected. The ice patch was compressed mostly by theoutheasterly wind and boundary forces of the drift ice floe south of the landfast ice edge. There was a small contribution also from the sea surface tilt. The icebreaker cuts were obvious from the change direction and form the fringes. In our case, we only had some small jumps in the fringe pattern which showed shearing and cracking. However, these cracks were not as strong as icebreaker fractures and did not change the fringe pattern.  
[PERSON] et al. [10] interpreted two interferograms from two image pairs, A (15 and 16 March 2012) and B (31 March and 1 April 2012). They evaluated the ice rheology image pair A and had a closer look at the compression in the center of the interferogram. There, fringes had high density. An ice patch was seen at the edge of the fast ice. A part of the drift ice moved towards the fast ice about 100 m in the one-day period between the overflights and then it was connected to the fast ice. By this connection, a pressure force was applied on the fast ice from the drift ice, although some forces may be applied by winds. The relative compression movement in LOS direction was three fringes over a distance of 1800 \(\pm\) 25 m. Based on ice thickness and weather information, these fringes and compression could not produce only with wind and water currents. In addition, sea level tilt was small, approximately 5 mm, over the whole scene corresponded to a change in radar LOS of almost 4 mm, and thus compression should have been another source that was due to the force exerted by the impinging drift ice. In our case, with a particular 12-day period, there could be several kinds of forcing background for the fringes. Sea level tilt, currents, and thermal expansion played a small role here, but a likely realization was forcing by drift ices toward the landfast ice boundary by pushing the southwest winds. [PERSON] et al. [11] showed that the fast ice compression could be described by viscous rheology used in sea ice modeling. This kind of rheology may also be the case in our study.