Characterization of the in-flight spectral response function of Geostationary Environment Monitoring Spectrometer (GEMS) retrieved using observed solar irradiance

The successful launch of Geostationary Environment Monitoring Spectrometer (GEMS) onboard the Geostationary Korea Multipurpose Satellite 2B (GK-2B) opens up a new possibility to provide daily air quality information for trace gases and aerosols over East Asia with high spatiotemporal resolution. As a part of major efforts to calibrate and validate the performance of the GEMS, accurate characterization of the spectral response functions (SRFs) is critical. The characteristics of preflight SRFs examined in terms of shape, width, skewness, and kurtosis vary smoothly along both the spectral and spatial direction thanks to highly symmetrical optic system of GEMS. While the preflight SRFs are determined with high accuracy, there is possibility of changes of in-flight SRFs during the harsh launch processes and/or operations over the mission lifetime. Thus, it is important to verify the in-flight SRFs after launch and to continue monitoring of their variability over time to assure the reliable trace gases retrievals. Here, we retrieve the in-flight SRFs for all spectral and spatial domain of the GEMS using spectral fitting of observed daily solar measurement and high-resolution solar reference spectrum. A variety of analytic model functions including hybrid form of Gaussian and flat-topped function, asymmetric super Gaussian, Voigt function are tested to determine the best representative function for GEMS SRF. The SRFs retrieved from early solar irradiances measured during the in-orbit tests agree well with the preflight SRFs indicating that no significant change occurred during the launch process. Continuous monitoring of the in-flight SRF is planned, using daily solar irradiances to investigate the temporal variation along with spectral and spatial directions. The detailed results of the in-flight SRF retrieval are to be presented.