# Performance Evaluation of China's First Ocean Dynamic Environment Satellite Constellation
## 4 Discussion  
In past studies, only one or both of the altimetry parameters were usually evaluated. However, in this study, the SLA, SWH, and SSWS of the HY-2B/C/D satellite were systematically evaluated, and the evaluation of all elements was realized, demonstrating the accuracy consistency between the three satellites. The standard deviation of the SLA between HY-2B and Jason-3 satellites at the dual-crossovers is less than 4 cm with good results, which shows that the data quality of HY-2B is more stable with high accuracy, and it is similar to the international equivalent satellite radar altimeter of Jason-3 satellite. Therefore, HY-2B altimetry data can be used to verify the SWH and SSWS data of HY-2C/D quality. The validation results of the self-crossing SLA data demonstrate that the accuracy of the HY-2D satellite's height measurement data is somewhat different from that of HY-2B/C satellites. The validation time interval is located in the early launch stage of the HY-2D satellite, and the observation data is unstable and missing. The standard deviation of multiple cycles of the dual-crossing SLA data is 4.25 cm, which is significantly different \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**SSWS** & **N** & **RMSE (m/s)** & **STD (m/s)** & **R** & **Mean SWH Div (m/s)** \\ \hline HY-2B/NDBC & 842 & 1.48 & 1.48 & 0.92 & \(-\)0.05 \\ HY-2C/NDBC & 1053 & 1.85 & 1.59 & 0.89 & 0.95 \\ HY-2D/NDBC & 494 & 1.45 & 1.35 & 0.95 & \(-\)0.52 \\ HY-2B/Jason-3 & 693 & 1.07 & 0.85 & 0.97 & \(-\)0.66 \\ HY-2C/HY-2B & 948 & 1.27 & 0.95 & 0.97 & 0.85 \\ HY-2D/HY-2B & 722 & 1.16 & 1.13 & 0.96 & \(-\)0.25 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: HY-2B/C/D satellites’ SSWS data verification. from the standard deviations of 3.40 cm and 3.48 cm of the other two satellites' SLA data. It is hypothesized that the large bias in the HY-2D radar altimeter data is not significantly related to the inversion method, but to the instrument itself. Therefore, in the next version of this product, this study suggests addressing the relevant corrections. The accuracy levels of the SWH data products of the HY-2B/C/D satellite radar altimeters were observed to be 0.23 m, 0.25 m, and 0.26 m, respectively. The accuracy levels of the SSWS data products of the HY-2B/C/D satellite radar altimeters were observed to be 1.48 m/s, 1.59 m/s, and 1.35 m/s, respectively. Future research could further explore the differences and underlying reasons between the HY-2D and HY-2B satellite data and conduct more in-depth analyses to improve the quality and accuracy of the data. Furthermore, the validation of HY-2C/D satellites' altimeter data could be based on the comparison with altimeter data from other satellites, such as Sentinel-3(A, B). In previous studies, satellite data quality assessment has been limited to the evaluation of altimetry parameters, while the assessment of satellite altimetry data coverage has been neglected. Altimetry parameters evaluation typically focuses on the accuracy and precision of the SSH and SLA information provided by satellite radar altimeters to ensure their reliability. However, this assessment methodology does not fully take into account the spatial coverage of satellite data. Coverage assessment of satellite altimetry data is critical for many applications. For example, in the prediction and evaluation of ocean circulation and sea surface wind fields, if satellite altimetry data only covers a limited area or a specific geographical area, it cannot provide comprehensive information, thus limiting the accuracy and reliability of relevant research. Therefore, in order to assess more fully the quality of satellite radar altimeter data, the coverage of satellite altimetry data must be taken into account. The results of this study also prove the scientific design of the orbit of China's first dynamic environment satellite constellation. The minimum radius of HY-2B/C/D three satellites to find the observation point in the global ocean surface shrank from 471.13 km to 90.33 km in the 14-day cycle.