# Performance Evaluation of China's First Ocean Dynamic Environment Satellite Constellation
## 1 Introduction  
In 2021, China built the first ocean dynamic environment satellite constellation, which included the HY-2B, HY-2C, and HY-2D ocean dynamic environment satellites. The HY-2B satellite is a sun-synchronous orbit satellite equipped with a radar altimeter, a radiometer, a correction radiometer, a data collection system, and an automatic ship identification system. HY-2B has a 14-day repeat cycle. The HY-2C and HY-2D satellites are non-sun-synchronous orbit satellites with an inclination of 66\({}^{\circ}\). They are equipped with all payloads same as HY-2B except the radiometer and have repeat cycles of 10 days. The satellite radar altimeter is the main payload of China's ocean dynamic environment satellite constellation. Table 1 details the main information of China's first ocean dynamic environment satellite constellation ([[https://osdds.nsoas.org.cn](https://osdds.nsoas.org.cn)]([https://osdds.nsoas.org.cn](https://osdds.nsoas.org.cn)), accessed on 23 September 2023). The calibration and validation of the altimeter data were critical steps in the altimetry mission. Accurate and objective calibration and validation can provide reliable information and assurance to the users of the data products. [PERSON] et al. [1] evaluated the quality of the sea surface height (SSH), significant wave height (SWH), and SSWS data products using cross-calibration of HY-2B radar altimeter data with the Jason-3 data. It was concluded that the HY-2B altimeter data products had good stability and high accuracy. In addition, the quality of the HY-2B SWH products is evaluated by comparing with the SWH obtained from NDBC buoys and Jason-2/Jason-3 satellite altimeters. Their root-mean-squared errorwas 27 cm, 26 cm, and 23 cm, respectively [2]. Therefore, the comparison results confirmed that the SWH data of the HY-2B are of higher quality. To assess the data quality and the performance of the altimeter systems, the data from the Jason-1 [3; 4; 5], Jason-2, Envisat [6], and SARAL/Altika satellites [7; 8] were calibrated and validated using a cross-calibration approach. [PERSON] et al. [9] used the Geophysical Data Records (GDR) of HY-2B and Jason-3 satellites observed during the same period as a reference, and quality analysis was conducted on the SSWS data in HY-2C satellite Sensor Geophysical Data Records (SGDR). The average difference in SLA at the dual-crossover point of HY-2C satellite SGDR data and HY-2B satellite and Jason 3 satellite GDR data is \(-\)0.47 cm and \(-\)0.3 cm, respectively, with standard deviations of 5.32 cm and 5.32 cm. These data indicate that the height measurement accuracy of HY-2C satellite is consistent with that of HY-2B satellite and Jason 3 satellite. [PERSON] et al. [10] evaluated the data quality of the SSWS product and carried out by the HY-2D satellite Ku-band scatterometer. SSWS observed by the scatterometer (HSCAT-D) were validated by comparing with wind data from the U.S. National Data Buoy Center (NDBC) buoys and European Centre for Medium-Range Weather Forecasts (ECMWF) model. The statistical results show that the HSCAT-D winds have a good agreement with the buoys' wind measurements, and in comparison with buoy winds, the wind speed standard deviation (STD) and root-mean-squared errors (RMSE) of direction were 0.78 m/s and 14.10\({}^{\circ}\), respectively. In previous studies, only the accuracy of mono-mission satellite radar altimeter data was evaluated, and the accuracy of the satellite constellation was not comprehensively and systematically evaluated. The full elements (SWH, SLA, and SSWS) of the HY-2C/D satellites radar altimeter GDR data have not been verified. The difference between this article and previous articles is that the validation parameters are different, and the validation data category and the satellite payload are different. In this study, the quality levels of the HY-2B/C/D satellites' along-track SLA, SWH, and SSWS data were validated as a whole in order to perform a comprehensive and systematic evaluation.