# A Study of Landfast Ice with Sentinel-1 Repeat-Pass Interferometry over the Baltic Sea
## 3 Materials and Methods  
### Study Site, Reference Data, and InSAR Methodological Approach  
In this study, two Sentinel-1 IW Single Look Complex (SLC) products from 6 and 18 February 2015 over the Bay of Bothnia were used. Landfast ice extent did not change between the two acquisitions, and the ice chart on 7 February represents the landfast ice condition in the period of the study (Figure 2). The SAR data consist of IW images on 6 and 18 February 2015, at 04:47:04 local time (EET) (Figure 3). IW swath mode has three sub-swaths, named IW1, IW2, and IW3. Our study area with high coherence is a part of IW2 sub-swath. It is located between Oulu and Kemi on the Finnish coast of the Bay of Bothnia. The normal (perpendicular) baseline is 51.21 m, and its incidence angle \(\theta\) for IW2 is from 36.47\({}^{\circ}\) to 41.85\({}^{\circ}\)[21]. Sentinel-1 IW swath mode information is provided in Table 1.  
The winter of 2015 was mild in the Baltic Sea. The peak of ice cover was on 23 January when the ice covered an area of 51,000 km\({}^{2}\). At this stage, the whole Bay of Bothnia was covered by the ice. Ice formation in the innermost bays of the northern Bay of Bothnia began in the middle of November. At the beginning of December, there was 1-10 cm thick level ice in the inner archipelago. Then, a cold spell began, lasting until 23 January, resulting in the maximum ice extent during this ice winter. There was another cold period around 5 February, and then the extent of ice reached 50,000 km\({}^{2}\)  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Characteristic** & **Value** \\ \hline Swathwidth & 250 km \\ Incidence Angle Range & 29.1\({}^{\circ}\)–46.0\({}^{\circ}\) \\ Sub-Swaths & 3, IW1, IW2, IW3 \\ Azimuth Steering angle & \(\pm\)0.6\({}^{\circ}\) \\ Azimuth and Range looks & Single \\ Polarization Options & Dual HH + HV, VV + VH \\  & Single HH, VV \\ Maximum Noise Equivalent Sigma Zero (NESZ) & –22 dB \\ Radiometric Stability & 0.5 dB (3\(\sigma\)) \\ Radiometric Accuracy & 1 dB (3\(\sigma\)) \\ Phase Error & 5\({}^{\circ}\) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Characteristics of Sentinel-1 interferometric mode [15].  
After this, the weather became milder, and southerly winds pushed the ice fields together toward the northeast. The rest of February was unusually mild. At the beginning of March, the ice extent was only 20,000 km\({}^{2}\). The maximum thickness of the landfast ice was 55 cm in the Bay of Bothnia. The thickness of drift ice was 15-40 cm [22].  
The weather station at Kemi harbor, Ajos illustrates the conditions between acquisitions on 6 and 18 February 2015 over the Bay of Bothnia (Figure 4). The formation of permanent ice cover started on 24 December 2014, and the end of permanent ice cover was on 5 May 2015 [22].  
Figure 3: _Cont_.  
Figure 2: Ice chart of 7 February 2015 for the Bay of Bothnia [22]. The SAR images cover a 250 km swath at 5 m \(\times\) 20 m spatial resolution. IW swath is marked with a square. Landfast ice is shown by the grey area.  
Figure 4: Weather information recorded by the Kemi_Ajos weather station located at 65\({}^{\circ}\)40\({}^{\prime}\)23.48\({}^{\prime\prime}\) N and 24\({}^{\circ}\)30\({}^{\prime}\)54.72\({}^{\prime\prime}\) E during the experiment: (**a**) mean, minimum and maximum temperature information; and (**b**) wind direction, wind speed and cumulative precipitation information. The red squares in precipitation subfigure represent missing data.  
Figure 3: (**a**) An overview of the Baltic Sea with IW image in this study, showing the enlarged geometry of image IW (IW1, IW2, and IW3). (**b**)The IW2 swath from 6 February 2015, with backscatter values in decibels. (**c**) The IW2 swath 18 February 2015, with backscatter values in decibels.