# Retrieving Freeze/Thaw Cycles Using Sentinel-1 Data in Eastern Nunavik (Quebec, Canada)
## 3 Results
### Accuracy and Analysis  
This section mainly focuses on the comparison of the results from four test rounds. These four rounds differ by the settings of preprocessing of the Sentinel-1 data, i.e., the original backscatter signals without any spatial filter, the backscatter signals with a simple 3 \(\times\) 3 window filter, the backscatter signals with a simple 5 \(\times\) 5 window filter, and the backscatter signals with a 3 \(\times\) 3 gamma filter. To reduce the repeatability, the comparison between the four rounds of tests focused only on the VV timeseries. The differences between the experimental results based on VV and VH data are presented under the setting with a gamma filter. As shown in Figure 6, overall, relatively good results were achieved. On most tested layers, accuracy higher than 70% was achieved; the accuracy of the best-performing sites was higher than 90%. All the layers from the test round with a 3 \(\times\) 3 window filter (green) had accuracies higher than 65%, except one with 64.7%, and the average accuracy reached 78.9%. The test round with a 5 \(\times\) 5 window filter (purple) showed similarly good results,  
Figure 5: Best-performing threshold at different test soil depths at each site (0 cm, green; 2 cm, blue; 5 cm, yellow; 10 cm, red). with the lowest accuracy of 65.2% and an even higher mean accuracy of 80.8%. The accuracy from the other two test rounds was overall lower, both with an average of approximately 71%, while there were very few layers with accuracies lower than 60%. The standard deviations of all four test rounds were not higher than 0.1; the lowest one was 0.072 from the test round with a 3 \(\times\) 3 window filter, and the highest one was nearly 0.085 from the test round with a 5 \(\times\) 5 window filter. The different layers of the same sites again showed high similarity, which means that their highest accuracy was reached not only by setting the same threshold but also was very close or even the same value. In contrast, between different sites, the accuracy results were quite different. The ups and downs between the stations were relatively consistent in the three rounds of testing; in particular, the curves of the test rounds with the gamma filter and without the filter almost overlapped in multiple segments. This shows that the differences brought by different spatial filtering were relatively systematic adjustments. On the whole, the two window filters were most effective for this area, while each of them had better- and worse-performing sites, and the improvement effect brought by the gamma filter was less than expected. Under the same filtering setting (gamma), there was also a difference in the sensibility between VV and VH polarization to be observed. Contrary to the observation in the previous section that the backscatter signals from VH polarization were generally weaker than those from VV, the accuracy achieved using VH data was almost always better than that achieved using VV data. As shown in Figure 7, there were a total of four layers where a higher accuracy was achieved in VV. There was no obvious community between these four layers, which may help to determine a common trigger for better performance of VV data. The accuracy difference of VV exceeding VH (green) was mainly within 10%, while the opposite difference (VH-VH, red) was approximately 10%, but no more than 20%. In general, VH data were not affected by a weaker signal and showed good usability in this approach. To further verify this statement, several test sites with remarkable but acceptable numbers of data points that were lower than \(-\)30 were chosen for testing. The test compared the accuracy of retaining and excluding data less than \(-\)30, and the results are summarized in Table 1. No significant differences were observed; in most layers, the result of not removing the too small value was even better than the result of removal. This shows that, in the range of the approach of this study, the lower VH value did not bring the expected adverse effects.