Characterization of GEMS level 1B based on inter-comparison using the visible channel of AMI

             The Geostationary Korean Multi-Purpose Satellite (GK-2) program consisting of GK-2A and GK-2B provides consistent monitoring information in the Asia Pacific region, including the Korean peninsula. The Geostationary Environment Monitoring Spectrometer (GEMS) onboard GK-2B in particular provides information on the atmospheric composition and aerosol properties, retrieved from the calibrated radiance (Level 1B) with high spectral resolution in 300-500 nm. GEMS started its extended validation measurement after the in-orbit test (IOT) in October following the launch of the satellite in February 2020. One of issues found during the IOT is that GEMS shows a spatial dependence in the measured solar irradiance along the north-south direction, albeit the solar irradiance does not have such a dependency. Thus, such a dependence should be from the optical system or the solar diffuser which is placed in front of the scan mirror. To clarify the root cause of the dependence, we utilize inter-comparison of the Earth measurement between GEMS and the Advanced Meteorological Imager (AMI), a multi-channel imager onboard GK-2A for meteorological monitoring. As the spectral range of GEMS fully covers the spectral response function (SRF) of the AMI visible channel having a central wavelength of 470 nm, spectral matching is properly done by convolving the SRF with the hyperspectral data of GEMS. By taking advantage of the fact that the position of GK-2A and GK-2B is maintained within a 0.5 degree square box centered at 128.2°E, match-up data set for the inter-comparison is prepared by temporal and spatial collocation. To reduce spatio-temporal mis-match and increase the signal to noise, zonal mean is applied to the collocated data. Results show that the north-south dependence occurs in the comparison of reflectance, the ratio between the earth radiance and solar irradiance, while not in the comparison of radiance. This indicates the dependence occurs due to the characteristics of the solar diffuser, not because of optical system. It is further deduced that dependence of diffuser transmittance on the solar azimuth angle is the main cause of the north-south dependency which was not characterized during the pre-flight ground test.