On-Orbit Radiometric Calibration and Performance of GF-5A WTI

The Wide-swath Thermal Infrared Remote Sensor (WTI) is the primary payload on Chinese Gaofen-5A(GF-5A) satellite. WTI is a cross-track infrared scanner developed by the Beijing Institute of Space Mechanics & Electricity(BISME). It uses the measurements of high temperature blackbody(HTBB) and low temperature blackbody (LTBB) as the on-orbit radiometric calibration reference sources allowing to update the calibration coefficients on a scan-by-scan basis. There is scan-angle effect, different calibration coefficients for different scan angles, for WTI. The method to get the angle-dependent calibration coefficients based on HTBB and LTBB calibration data is presented in this paper. The scan angle correction coefficients (SACs) are applied to account for the scan-angle effect. The SACs are calculated based on cross-calibration which utilize the Advanced Geostationary Radiation Imager(AGRI) aboard on FY-4B as reference. The calibration site data with the infrared ground temperature automatic measurement equipment and the atmospheric data from ERA5 are utilized to evaluate the absolute calibration accuracy. The root mean square error(RMSE) of WTI are as follows: 0.82K for 8.2μm, 0.74K for 8.6μm, 0.75K for 10.8μm, 0.85K for 12.0μm. To assess on-orbit radiometric performances of WTI, the key parameters including temperature stability, Noise Equivalent Temperature Difference (NETD) and System Radiation Stability(SRS) are calculated. Since the launch of GF-5A in December 2022, the radiometric performances have been monitored based on routine measurements of the onboard calibration data. There are only 0.4mK, 7mK and 8mK temperature fluctuation respectively for IRFPA , LTBB and HTBB for a single imaging time about 1200 sconds. The fluctuations of NETD and SRS are less than 1% and 0.002% respectively. The results indicate that there are no significant attenuation in WTI on-orbit radiometric performance during the first two years following its launch.