## 2 Materials and Methods
### Data and Field Work  
#### 2.2.1 Satellite Data and Preprocessing  
To achieve a high temporal resolution, Sentinel-1A and SPOT-5 Take-5 images were requested from ESA (European Space Agency) and from CNES (_Centre National d'Etudes Spatiales_), respectively, within the scope of the ESA Alcantara initiative project (Ref: 14-P13) and the SPOT-5 Take-5 project (ID: 29142) for the time period from March to September 2015. A total of 37 satellite images were made available for this study: 28 SPOT-5 Take-5 images from 10 April to 12 September and 9 Sentinel-1A images from 26 March to 4 October (Table 1 and Figure 2). \begin{table}
\begin{tabular}{c c c} \hline
**Satellite** & **Acquisition Date** & **DOY 2015** \\ \hline Sentinel-1A & 26 March & 85 \\ SPOT-5 & 10 April 1 & 100 \\ SPOT-5 & 15 April 1 & 105 \\ Sentinel-1A & 19 April & 109 \\ SPOT-5 & 30 April & 120 \\ SPOT-5 & 5 May & 125 \\ SPOT-5 & 10 May & 130 \\ Sentinel-1A & 13 May & 133 \\ SPOT-5 & 15 May & 135 \\ SPOT-5 & 20 May & 140 \\ SPOT-5 & 25 May & 145 \\ SPOT-5 & 4 June & 155 \\ Sentinel-1A & 6 June 2 & 157 \\ SPOT-5 & 9 June & 160 \\ SPOT-5 & 14 June & 165 \\ SPOT-5 & 19 June & 170 \\ SPOT-5 & 24 June & 175 \\ SPOT-5 & 29 June & 180 \\ Sentinel-1A & 30 June & 181 \\ SPOT-5 & 4 July & 185 \\ SPOT-5 & 9 July & 190 \\ SPOT-5 & 14 July & 195 \\ SPOT-5 & 19 July & 200 \\ Sentinel-1A/SPOT-5 & 24 July 2 & 205 \\ SPOT-5 & 29 July 1 & 210 \\ SPOT-5 & 8 August & 220 \\ SPOT-5 & 13 August & 225 \\ Sentinel-1A & 17 August 2 & 229 \\ SPOT-5 & 18 August & 230 \\ SPOT-5 & 23 August 1 & 235 \\ SPOT-5 & 28 August & 240 \\ Sentinel-1A & 29 August & 241 \\ SPOT-5 & 2 September 1 & 245 \\ SPOT-5 & 7 September & 250 \\ SPOT-5 & 12 September & 255 \\ Sentinel-1A & 4 October & 277 \\ \hline \end{tabular}
\end{table} 
Table 1: Sentinel-1A and SPOT-5 Take-5 acquisition dates (day of the year (DOY) 2015). The Sentinel-1 mission comprises a constellation of two polar-orbiting satellites, operating day and night and performing C-band synthetic aperture radar (SAR) imaging, enabling image acquisition regardless of the weather. Sentinel-1A was launched on 3 April 2014, while Sentinel-1B was launched on 25 April 2016. The Sentinel-1A mission includes SAR imaging in four exclusive imaging modes with different resolutions (down to 5 m) and coverages (up to 400 km). This mission provides dual polarization capability, very short revisit times and rapid product delivery. A single Sentinel-1 satellite can potentially map the global landmasses in interferometric wide (IW) swath mode once every 12 days in a single pass (ascending or descending). The two-satellite constellation offers a 6-day exact repeat cycle at the Equator. Because orbit track spacing varies with latitude, the revisit rate is significantly greater at higher latitudes than at the Equator. All of the Sentinel-1A C-band SAR images were made available in IW mode with a dual polarization scheme (VV + VH). These images were distributed as Level-1 products, as single look complex (SLC), except for the first two acquisition dates, and as ground range detected (GRD) for all dates. SLC image ground resolution is 5 m \(\times\) 20 m, while that for GRD images is 10 m. All of the images were acquired in ascending mode with incidence angles ranging from 38.87\({}^{\circ}\) to 39.26\({}^{\circ}\). Sentinel-1A Level-1 GRD products were used in this study due to their improved quality. However, because typical SAR data processing, which produces Level-1 images, does not include radiometric corrections and because significant radiometric bias remains, it is necessary to radiometrically correct the SAR images. Moreover, radiometric correction is also required when comparing SAR images acquired from the same sensor, but at different times, as in this study. Radiometric calibration was applied using the following equation (Equation (1)) [26]:  
\[\gamma_{i}=\frac{(DN_{i}^{2}+b)}{A_{i}} \tag{1}\]  
where \(A_{i}\) is the gamma calibration vector _(i)_, \(b\) is a constant offset and \(DN_{i}^{2}\) is the intensity.