## 5 Discussion  
The two different track algorithms (OCOG and SMLE) were incorporated in parallel in the HY-2A/B ALT in order to prevent unlocked tracking above the no-oceanic surfaces. As detailed in Figures 5 and 6, the echoes from the seawater all had basic characteristics. For example, the spike-like waveforms' maximum from the sea ice of the OCOG was greater than that of the SMLE. Although there were differences in the waveform amplitudes between the two different packages, it was found that the shapes of the waves could be used to examine the separations between the sea ice and seawater in the polar regions. In previous studies, many researchers had used airborne altimeters to obtain their results [24]. They found that the echoes from old ice had smaller trailing edge slopes than the echoes from young ice. The results of this study were similar to the previous findings. However, not all of the echoes from the HY-2A/B for the old ice were observed to display the aforementioned phenomenon. As shown in Figure 12, spike-like echoes had existed in the regions of old ice, with the exception of the \"ocean-like\" echoes. The biggest differences between the radar altimeters used by [PERSON] and [PERSON] were that the footprints on the ground were much smaller than the ground footprint of the HY-2A/B ALT. In addition, the sources of the signals for the bigger footprint were more complicated than those for the smaller footprint. For example, the echoes were a mixture of signals from many different types of ice in the footprint. In the Antarctic region, \"ocean-like\" waveforms had existed in the centers of the large ice floes. Although the footprint of the C Band was larger than that of the Ku Band, the C Band had been correctly flagged on the same larger ice floes which had been identified as a diffuse surfaces in the Ku Band. The difference was the number of identifications. The attenuations of the echoes from the Ku Band were greater than those of the C Band above the ocean surface. The averages of the diffused echoes in the regions of the old ice were similar to that of the seawater. However, the trailing edges of the seawater from the C Band were found to be almost parallel to the axis, and the trailing edges of the diffused echoes in the regions of old ice were not. Also, there were many effects of the trailing edges observed, which had resulted from the direction angles, physical properties in the scattering cells, and the noise of the echoes. These effects could not be ignored by using a simple least square fitting method for the trailing edges. Meanwhile, the research results had provided a basis for the study of re-tracking algorithms for the altimeters of the HY-2A/B ALT. Based on the previous analysis of the echoes, three methods (threshold segmentation, KNN, and LIBSVM) were presented with four waveform parameters (AGC and PP values of the Ku and C Bands) in order to distinguish between sea ice and seawater in the polar regions in Section 4. The accuracy rate of the LIBSVM was found to be the lowest of the three methods in both the Arctic and Antarctic regions. The KNN Method was able to maintain a stable high accuracy rate with the different K values, and the single parameter threshold segmentation was determined to also have a good accuracy rate. For the purpose of discussing the distinguishing results of the three examined methods, the error points of results and OSI-SAF are plotted in Figures 18 and 19. In the figures, the previously mentioned ice chart was utilized to correct the results of the classifications. The AARI ice chart for 11-13 November of the Arctic region, and the ice chart for 15 November of the Antarctic region, were used in this study as references. The error points of the Arctic regions were mainly located in the old ice around Queen Elizabeth Island. The areas where the error occurs were consistent with the positions of the 'ocean-like' waveforms. The error distribution of LIBSVM was the most dispersed. The errors of the KNN had occurred near the old ice and sea ice boundaries.