## 3 Quality Assessment
### Resolution Capability
The specific results are listed in Table 4. Generally, the recent HY-2 missions (HY-2B, HY-2C and HY-2D) have smaller resolution capabilities than HY-2A, which indicates stronger detective abilities of the geoid signals. HY-2D had the smallest average value of 18.98 km, while slightly higher values of 19.98 km and 19.35 km were derived from HY-2B and HY-2C. HY-2A data were from a geodetic mission with longer, seasonal time intervals. Time variations, as well as data quality, might have caused the largest average resolution capability of around 23.21 km. Figure 5: Variation trend of geoid heights along sample data sequences for HY-2A, HY-2B, HY-2C and HY-2D: Region 1, North Pacific; Region 2, South Pacific; Region 3, Mid-Pacific; Region 4, Indian Ocean; Region 5, North Atlantic; Region 6, South Atlantic. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Experimental Region** & **Satellite** & **Resolution** \\ \hline \hline  & HY-2A & 27.27 km \\  & HY-2B & 24.51 km \\
1 & HY-2C & 23.92 km \\  & HY-2D & 25.12 km \\ \hline  & HY-2A & 23.36 km \\
2 & HY-2B & 25.28 km \\  & HY-2C & 17.74 km \\  & HY-2D & 19.95 km \\ \hline  & HY-2A & 21.03 km \\
3 & HY-2B & 16.44 km \\  & HY-2C & 20.09 km \\  & HY-2D & 16.71 km \\ \hline  & HY-2A & 20.51 km \\
4 & HY-2B & 18.58 km \\  & HY-2C & 14.99 km \\  & HY-2D & 15.53 km \\ \hline  & HY-2A & 19.71 km \\
5 & HY-2B & 17.76 km \\  & HY-2C & 17.33 km \\  & HY-2D & 16.18 km \\ \hline  & HY-2A & 27.40 km \\
6 & HY-2B & 20.01 km \\  & HY-2C & 22.05 km \\  & HY-2D & 20.40 km \\ \hline \end{tabular}
\end{table} 
Table 4: Summary of resolution capabilities for HY-2 data sequences: Region 1, North Pacific; Region 2, South Pacific; Region 3, Mid-Pacific; Region 4, Indian Ocean; Region 5, North Atlantic; Region 6, South Atlantic. Figure 6: Coherence map and deduced resolution capabilities of repeat track data sequences for HY-2A, HY-2B, HY-2C and HY-2D over study regions: Region 1, North Pacific; Region 2, South Pacific; Region 3, Mid-Pacific; Region 4, Indian Ocean; Region 5, North Atlantic; Region 6, South Atlantic. Stacking is a common processing method for repeated periodic orbital altimetry data, which can effectively suppress the time-varying noise and further improve observational accuracy. At present, HY-2A carried out GM with 169-day repeat tracks for more than five years. HY-2B, HY-2C and HY-2D are running ERM and accumulated cycles of data with 14-day, 10-day and 10-day intervals. All these HY-2 missions have repetitive data sequences that can be stacked. We acquired more than 50 cycles of HY-2B ERM data that were split into two stacking data groups with relatively longer periods. The following experiments were designed to prove enhanced data quality after stacking in the six research regions. This procedure adopted weighted averaging for all repetitive passes. Resolution analysis was also carried out to assess the stacking results of 5, 10, 15 and 20 cycles. As shown in Figure 7, the resolution capabilities were remarkably enhanced along with the increased number of stacking cycles. Relevance and coherence were all improved at both long and short wavelengths. Table 5 indicates that the effect of every cycle improvement remained stable within 10 cycles; then, a slightly slower rate of improvement occurred, involving more and more cycles, indicating that stacking within 10 cycles was more efficient. In addition, HY-2A provided seven complete cycles of GM observations, which inspired us to incorporate stacking GM data in future studies. Figure 7: Coherence map and deduced resolution capabilities of HY-2B repeat track data sequences after stacking 5, 10, 15, 20 cycles’ data: Region 1, North Pacific; Region 2, South Pacific; Region 3, Mid-Pacific; Region 4, Indian Ocean; Region 5, North Atlantic; Region 6, South Atlantic.