# Near Real-Time Freeze Detection over Agricultural Plots Using Sentinel-1 Data
## 4 Results  
### Calculation of Thresholds for Frozen Detection  
As described in Section 3.3, the thresholds over \(\Delta\)\(\alpha^{0}\) for the detection of frozen surfaces were obtained separately for each land cover class (three LCs) and each S1 polarizations (VH and VV). Figures 5 and 6 show the distributions of \(\Delta\)\(\alpha^{0}\) that allowed the calculation of the thresholds after fitting normal functions. The distributions of \(\Delta\)\(\alpha^{0}\) were plotted for each LC using Sentinel-1 SAR images at two temperature ranges: \(-3\)\({}^{\circ}\)C \(\leq\) t \(\leq\) 0 \({}^{\circ}\)C and t \(\leq\)\(-3\)\({}^{\circ}\)C. Then, from each fitted normal function, the mean was calculated (Table 1). The results show that the threshold values vary greatly depending on the land cover class and the polarization used. Table 1 shows that for LC1, which corresponds to cereals, severe freezing occurs when the calculated \(\Delta\)\(\alpha^{0}\) in VH polarization is greater than 5.3 dB, 3.5 dB for meadows (LC2), and 2.9 dB for orchards and vineyards (LC3). In addition, for VH, mild-to-moderate freezing occurs for \(\Delta\)\(\alpha^{0}\) between 3.5 and 5.3 dB for LC1, 2.8 and 3.5 for LC2, and between 2.1 and 2.9 for LC3. Finally, surface soils are considered as unfrozen if \(\Delta\)\(\alpha^{0}\) in VH polarization is lower than 3.5, 2.8 and 2.1 dB for LC1, LC2, and LC3, respectively. The thresholds in VV polarization, as shown in Table 1, are significantly lower in comparison to VH across the three land cover classes. For example, the threshold at which severe freezing occurs in VV is lower by 1.3 dB for both LC1 (4.0 vs 5.3 dB for VH), and LC2 (2.2 vs 3.5 dB for VH) in comparison to VH, and by 0.5 dB for LC3 (2.4 vs 2.9 dB for VH). The thresholds in VV polarization are also lower than those obtained for VH for the detection of mild-to-moderate freeze. For mild-to-moderate freeze, VV is lower than VH by roughly 1 dB for LC1 (2.5 vs 3.5 dB for VH) and LC2 (1.7 vs 2.8 dB for VH), and by 0.5 dB for LC3 (1.6 vs 2.1 dB for VH). These results are in accordance with the results obtained in the study of [PERSON] et al. (2018), and confirm the higher potential of VH in comparison to VV for the detection of freezing episodes at the plot scale. It is worth mentioning that choosing lower threshold values, as is the case with VV polarization, would lead to a greater number of false detections of freezing episodes over each plot. However, after applying the temperature filter, some of these false detections could be removed.  
Figure 5: Distributions of \(\Delta\)\(\alpha^{0}\) in VH polarization for the three main land cover classes: (**a**, **d**) cereals LC1, (**b**, **e**) meadows LC2, and (**c**, **f**) orchards and vineyards LC3. (**a**, **b**, and **c**) \(\Delta\)\(\alpha^{0}\) over LC1, LC2, and LC3, respectively, for temperatures \(-3\)\({}^{\circ}\)C \(\leq\) t \(\leq\) 0 \({}^{\circ}\)C, (**d**, **e**, and **f**) \(\Delta\)\(\alpha^{0}\) over LC1, LC2, and LC3, respectively, for t \(\leq\)\(-3\)\({}^{\circ}\)C. The Y-axis represents the frequency of different \(\Delta\)\(\alpha^{0}\) ranges over the entire plots. The red curve represents the fitted normal functions, while the red vertical line represents the mean.