Radiometric Assessment for FY-3D HIRAS Provisional Sensor Data Records

The High-Spectral Infrared Atmospheric Sounder (HIRAS) is a space-borne Fourier transform spectrometer (FTS) onboard the Polar-orbiting FengYun 3D (FY-3D) satellite, launched into orbit on 13 November 2017. HIRAS provides measurements of Earth view interferograms in three infrared spectral bands at 29 cross-track positions, each with a 2 × 2 array of field of views (FOVs). The HIRAS ground processing software transforms the measured interferograms into calibrated and navigated spectra in the form of Sensor Data Records (SDRs) that cover spectral bands from 650 to 1140 cm^-1 (Longwave Band, LW), 1210 to 1750 cm^-1 (Midwave Band, MW), and 2155 to 2550 cm^-1 (Shortwave Band, SW) with spectral resolutions of 0.625 cm^-1, 1.25cm^-1, and 2.5 cm^-1, respectively. During the time of the intensive calibration and validation (ICV) period from 1 March to 31 July of 2018, the HIRAS performance, including noise, spectral frequency accuracy and radiometric uncertainty were characterized under the framework of Global Space-based Inter-Calibration System (GSICS) and the requirements of the Numerical Weather Prediction (NWP) systems.

The Earth view spectra are radiometrically calibrated with the ICT and cold space measurements and including the corrections of the detector nonlinearity and scene selection mirror polarization effects. During the ground TVAC testing, significant nonlinearity was detected for the LW and MW bands and therefore a nonlinearity correction model was established. The nonlinearity correction model takes the second term into account. The nonlinearity for SW was negligible. During the ICV, the nonlinearity correction parameters were adjusted by analyzing the on-orbit measurements of the ICT, whose temperature varied slowly from its high stage to low stage over several orbits. The polarization effects significant for the MW band were detected by analyzing the scanning-mode cold space measurements as well as dependence of Earth view spectra bias on scan angle and scene temperature, and the correction coefficients were derived by fitting the cold space data to the correction model. Validations of the radiometric calibration are performed by using HIRAS and CrIS/IASI SNO observations as well as the observations collocated with RTM simulations. The results show that the radiometric uncertainties are better than 0.5K and 0.7K for LW and MW bands, respectively, and 0.5K for the SW band in the CO absorption and window regions. Also, the consistence between FOVs was estimated to be within 0.1K for most spectral domain.