In-Orbit Calibration and Validation of HY-2B Altimeter Using an Improved Transponder

HY-2B (Haiyang) is the follow-on mission to HY-2A, the Chinese first sea satellite for oceanography, launched in October 2018. The main payloads onboard are similar to HY-2A. For the altimeter, an in-orbit calibration mode was designed to achieve higher calibration precision. An improved transponder was built and deployed for the calibration campaign. The purpose of this article is to describe the in-orbit calibration and validation of HY-2B altimeter based on an improved transponder. A signal-rebuilt transponder was developed and employed in HY-2A altimeter calibration. It played an important role in the quantitative analysis of HY-2A altimeter data products. For HY-2B altimeter, a newly designed calibration mode was added, and an improved transponder was built. It has some advantages over the previous one in the rebuilt signal forms and modulation ways. Two calibration campaigns were carried out, in April 2019 and October 2020 at different sites. In the two campaigns, the precision of the altimeter range calibration is obtained less than 1 cm. For further validation and assessment of the result, a comparison between HY-2B and Jason-3 was performed in terms of sea surface height on the tens of cross track points in the open ocean. The comparison result shows high degree of accuracy and stability of HY-2B altimeter instrument. In China's next five-year plan (2021–2025), many more transponders and related calibration facilities would be installed in the ocean calibration fields, serving a variety of different satellite altimetry missions.