Xiaomin Ye1,2*, Jianqiang Liu1,2, Mingsen Lin1,2 , Bin Zou1,2, Jing Ding1,2 , Qingjun Song1,2, Yue Teng1,2 1National Satellite Ocean Application Service, Ministry of Natural Resources, Beijing, China; 2Key Laboratory of Space Ocean Remote Sensing and Application, MNR, Beijing, China *E-mail: yexiaomin2011@foxmail.com；ORCID: 0000-0003-1044-0125 Introduction of HY-1C and HY-1D satellites The Chinese Haiyang-1C (HY-1C) and Haiyang-1D (HY-1D) satellites, the follow-on mission to the HY-1A and HY- 1B satellites, was launched in September 2018 and June 2020, respectively. This satellites are both equipped with the COCTS and a coastal zone imager (CZI), as well as an ultraviolet imager (UVI), a satellite calibration spectrometer (SCS) and a satellite-based automatic identification system (AIS) receiver. The orbit altitude and tilting angle are 782 km and 98.5 degrees, respectively. The overpass times are10:30 AM±30 min local time at the descending node for HY-1C and are 1:30 PM±30 min local time at the ascending node for HY-1D, respectively. The joint observations of HY-1C and HY-1D increase the observation time and improve the global coverage. The COCTS instruments on the two satellites can detect the global ocean and land four times every day and provide daily ocean color and daytime and nighttime SST data. The COCTS is a moderate-resolution imaging scanner with a nadir spatial resolution of 1.1 km and a viewing swath width of more than 2900 km. The satellite measures signals in 8 visible and near-infrared (VNIR) bands and 2 thermal infrared (TIR) bands (10.3~11.3μm，11.5~12.5μm). Evaluation of sea surface temperatures derived from the HY-1C and HY-1D satellites Global SST have been detecting by the COCTS instruments since September 2018. The BTs (Level-1 data) and SST products (Level-2 and Level-3 data) of SST can be distributed to public for free. Examples of data products are shown in Fig.1 to Fig.4. Level-3 SST data are the gridded products with a grid resolution of 4 km and 9 km(see Fig.4). Fig.1 Global quasi true-color image from COCTS on HY-1C (acquired on October 8, 2019) A nonlinear SST algorithm (NLSST) is applied to derive SSTs from the brightness temperatures (BTs) of the two COCTS thermal infrared bands at 10.8 and 12.0 μm. The SST retrieval results from the multichannel SST algorithm (MCSST) are used as the first guess for the input of the NLSST algorithm. The coefficients of both the MCSST and NLSST algorithms are regressed from the matchup datasets of the COCTS BT observations and the measurements from the in situ Quality Monitor system (iQuam). The algorithm coefficients of HY-1C and HY-1D can be found in Ye et al (2021) and Ye et al (2022). ___________________________________ Acknowledgements The authors would like to thank the NASA Goddard Space Flight Center, Ocean Ecology Laboratory, and Ocean Biology Processing Group (OBPG) for providing access to the MODIS and VIIRS SST data, as well as the Center for STAR and NOAA National Environmental Satellite, Data, and Information Service (NESDIS) for providing access to the in situ iQuam data for validation in our study. References [1] X. Ye,, J. Liu, M. Lin, J. Ding, B. Zou，Q. Song, and Y. Teng, "Evaluation of Sea Surface Temperatures Derived From the HY-1D Satellite," in IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, vol. 15, pp. 654-665, 2022, DOI: 10.1109/JSTARS.2021.3137230. [2] X. Ye, J. Liu, M. Lin, J. Ding, B. Zou and Q. Song, "Sea Surface Temperatures Derived From COCTS Onboard the HY-1C Satellite," in IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, vol. 14, pp. 1038-1047, 2021, DOI: 10.1109/JSTARS.2020.3033317. Fig.4 (a) Daily gridded nighttime SST of HY-1C satellite on October 8, 2019 and (b) Monthly gridded daytime SST of HY-1D satellite in June 2021. Fig5. Validation results of SST from COCTS against in situ measurements from iQuam during (a) daytime and (b) nighttime in the period from Ja