## Data and Methods  
### Hy-2a/2b  
The electromagnetic waves passing through the ionosphere interact with mobile free electrons which changes the speed of the waves. Therefore, there will be some errors brought into oceanic measurement results when using the data of radar altimeter wave echoes [20]. In order to remove the impacts of ionospheric delays, the HY-2A/B ALT were designed with double-frequency (Ku 13.58 GHz and C 5.25 GHz bands) altimeters. The main paramenters of the HY-2A/B radar altimeters have been listed in Table 1.  
HY-2A/B ALT provides three levels of product. These include the Level 1A/B products (L1A/B) which calculates the tracker range and corrects the instrument errors; Level 2 products (L2) or Interim Geophysical Data Records (IGDR), Sensor Geophysical Data Records (SGDR), and Geophysical Data Records (GDR); and Level 3 products (L3) which calculates the monthly average and annual average meshing products.  
In this study, the L1B level product was used, which was provided by the NSOAS. Every single L1B folder contained two basic files: Primary data (.bin) and the instruction files (.meta.xml). In the.bin files, all of the 20 Hz packages along the satellite orbit could be accepted with some other auxiliary data (for example, track algorithm flags and calibration data). The 20 Hz records were the waveforms of the 50 ms averages. Approximately 20 of waveforms within each second were further averaged in order to obtain the 1 Hz records for this study's practical applications. Then, in those waveforms, quality control processes were carried out. First, remove the data which just included the leading edge or trailing edge. Second, remove the data whose the location of the max power were after bin 108 or before bin 20; then, mask the data using the land flags.  
Figure 2 provides an example of the spatial coverage of the HY-2A/B in November 2018 in the Arctic and Antarctic regions. The month of November is considered to be the freeze-up period of the Arctic and the melt-out period of the Antarctic.  
In this study, the data from 1 st to 30 th November 2018 were examined. The HY-2A/B ALT data had utilized approximately 470,000 surface points throughout all of November, respectively.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Items** & **Values** & \\ \hline Emission signal center frequencies & 13.58 GHz & 5.25 GHz \\ Bandwidth & & 102.4 us \\ AGC dynamic range & & 60 dB \\ Fast Fourier Transform bin & & 128 \\ number & & \\ Transmitter peak power & \(\geq\)10 w & \(\geq\)20 w \\ Chirp signal band width & Ku: 320/80/20 MHz (three bandwidths for adaptive adjustments) \\ C: 160/40/10 MHz (three bandwidths for adaptive adjustments) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Altimeter parameters (Source: [[http://www.nsoas.org.cn/](http://www.nsoas.org.cn/)]([http://www.nsoas.org.cn/](http://www.nsoas.org.cn/))).  
The ground footprint diameter of the Ku band of the HY-2A/B over the flat surfaces was 1.9 km, and the C band was approximately 10 km. Figure 3 showed the schematic diagram of the Ku band footprint of the HY-2A/B radar altimeters. The altimeter had continuously transmitted pulses to the ground. During the process, Millisecond-level pulse echo averaging was performed on the satellites. Then, the HY-2A/2B continuous measurements were separated by approximately 7 km along a track at 1 s intervals.