# Near Real-Time Freeze Detection over Agricultural Plots Using Sentinel-1 Data
## 2 Study Sites and Datasets
### Datasets  
#### 2.2.1 SAR Sentinel-1 Time Series  
Sentinel-1 is a constellation of two polar-orbiting SAR satellites (S1A and S1B) operating at C-band (\(f\) = 5.406 GHz), which were launched in April 2014 (S1A) and April 2016 (S1B). Sentinel-1 products are available free of charge, and can be downloaded one day after their acquisition from the Copernicus website ([[https://sciub.copernicus.eu/dhus](https://sciub.copernicus.eu/dhus)]([https://sciub.copernicus.eu/dhus](https://sciub.copernicus.eu/dhus))). Over our two study sites, two Sentinel-1 time series data were used. The first S1 time series, from 1 September 2017 to 31 May 2018, was used for threshold calculation. This S1 time series contains 288 S1 images acquired from the Sentinel-1A (S1A) and Sentinel-1B (S1B) satellite constellation in both ascending (afternoon \(\sim\)18h00 UTC) and descending modes (morning \(\sim\)06h00 UTC). Of these 288 images, 133 images were acquired over study site A (88 images in ascending mode, and 45 descending mode), and 154 images acquired over study site B (87 images in ascending mode, and 67 in descending mode). The second S1 time series, from 1 September 2018 until 31 May 2019, was used for the mapping of freezing events. The second S1 time series (1 September 2018 and 31 May 2019) contains 263 S1 images acquired from S1A and S1B in both ascending (\(\sim\)18h00 UTC) and descending modes (\(\sim\)06h00 UTC). In the second time series,  
Figure 1: Location of the two study sites situated in France. LC1 represents cereal grains, LC2 meadows, and LC3 orchards and vineyards. Figure 2: Temperature profiles over our study sites. (**a**) Study site A and (**b**) study site B. 130 images were acquired over study site A, of which 43 images acquired in descending mode and 87 images in ascending mode. For study site B, 133 images were acquired, of which 66 images and 67 images were acquired in descending and ascending modes, respectively. Figure 3 shows the repeat cycle of the S1 SAR in ascending \"A\" and descending \"D\" acquisition modes over both study sites for the month of October 2018. For study site A, two images were acquired in the ascending mode at incidence angles of 41.9\({}^{\circ}\) and 32.8\({}^{\circ}\), respectively, with a time difference of 2 days, followed after 12 h by an image in the descending mode at an incidence angle of 37.8\({}^{\circ}\) (Figure 3a). The pattern was repeated every 6 days for a total of 16 images. For study site B, two images in the ascending mode were acquired in the span of 24 h at incidence angles of 42.1\({}^{\circ}\) and 32.8\({}^{\circ}\), respectively, followed by one image in the ascending mode after 6 days at an incidence angle of 32.8\({}^{\circ}\). This pattern was repeated every 12 days (Figure 3b). The acquisition pattern for the images in the descending mode was two images (24 h difference) at incidence angles of 36.6\({}^{\circ}\) and 45.2\({}^{\circ}\), respectively, followed after 5 days by an image with an incidence angle of 36.6\({}^{\circ}\) (Figure 3b). This pattern was also repeated every 12 days for a total of 16 images in both acquisition modes. The time difference between the acquisition of images in ascending and descending modes varied between two and three days. The entirety of the S1 SAR images database was acquired in the Interferometric Wide swath mode (IW) in both VV and VH polarizations. These images were generated from the high-resolution Level-1 ground range detected (GRD) product, which were acquired at a spatial resolution of 20 m \(\times\) 22 m (range \(\times\) azimuth), and then resampled to produce the final product at 10 m \(\times\) 10 m pixel spacing. The images were calibrated using Sentinel SNAP toolbox, developed by the European Space Agency (ESA). The calibration aimed to convert the image pixel values into backscattering coefficients (\(\sigma^{\circ}\)) in linear units, while the geometric correction was used to ortho-rectify the SAR images using the 30 m Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM).