## 3 Results
### Characteristics of GNSS-R Features  
The distribution characteristics of six feature parameters (RESC, RESI, RESD, REWC, REWI and REWD) for sea ice and water are shown in Figure 6. The vertical height of the boxes represents the interquartile range of the samples, while the parallel line depicted in red inside the boxes is the median value of the samples for each feature. The green dotted line represents the threshold obtained by the method proposed in [28] for distinguishing sea ice from water. It is clear that sea ice shows a distinct difference between sea water for all the features considered. This is because the reflection over the sea ice surface is usually more coherent. The sea water surface is often rougher than that of sea ice and easily affected by ocean winds, which results in wider scattering. The median values of each parameter for sea ice and water are significantly different. However, the distribution of features of sea ice and water is more or less overlapped. As shown in Figure 6a, the RESC values of sea ice range from 0.15 to 1 and those of sea water range from 0.02 to 0.99; the threshold is 0.745. If RESC \(<\) 0.745, it is regarded as sea water; if RESC \(>\) 0.745, it is regarded as sea ice. However, some points of RESC below 0.745 appear in sea ice, and some points above 0.745 appear in sea water; these points are overlap. This indicates that simple thresholding of each feature may result in some false discrimination between sea ice and water.  
This study uses the combination of six features derived from the delay waveforms of different Doppler spread characteristics to describe the characteristics of reflecting surface. The six features of samples are composited into sequences, which are applied as input variables to train the sea ice monitoring model. The six features are combined into sequences in order. RESC, RESI, RESD, REWC, REWI and REWD values are presented from bottom to top in the y-axis. The feature sequences of samples in the Arctic and Antarctic regions are presented in Figure 7.  
Figure 5: TDS-1 data availability in (**a**) 2015, (**b**) 2016, (**c**) 2017 and (**d**) 2018. The rectangles filled in blue represent the available TDS-1 data, whereas the rectangles without filled color represent the unavailability of TDS-1 data.  
Figure 6: Box plots of six features (i.e., RESC, RESI, RESD, REWC, REWI and REWD) over sea ice and water using the data collected over the Arctic region from January 2015 to December 2018. The vertical height of the boxes indicates the interquartile range of the samples. While the parallel line (red) inside the boxes represents the median value of the samples for each parameter, the dotted line (green) represents the threshold obtained by the method proposed in [28] for distinguishing sea ice from water.  
Figure 7: Feature sequences composited with RESC, RESI, RESD, REWC, REWI and REWD in (**a**) Arctic (**b**) and Antarctic regions. The upper plot in each figure represents the feature sequences of sea ice and the lower plot represents those of sea water. The color scale represents the value of each feature parameter.  
As shown in Figure 7a,b, the feature sequences of 40,000 samples for sea ice (upper plot) and water (lower plot) show distinct differences, which provide the opportunities of monitoring sea ice. Moreover, the feature sequences can describe the characteristics more accurately than individual features.