In-flight calibration and performance monitoring of Geostationary Environment Monitoring Spectrometer (GEMS)

Geostationary Environmental Monitoring Spectrometer (GEMS), on-board Geostationary Korea Multi-Purpose Satellite-2B (GK-2B), is the first geostationary environmental instrument to be the Asian component of the global geostationary constellation for pollution monitoring together with the European Sentinel-4 and the North American Tropospheric Emissions: Monitoring of Pollution (TEMPO).  GEMS is a hyper grating spectrometer that hourly measure backscattered solar spectral radiance from the ultraviolet to visible (300 to 500 nm) with 0.6 nm spectral resolution and 3.5 (7 for trace gases) * 8 km2 spatial resolution over the Seoul in the daytime. These radiances are used to retrieve spatial and temporal distributions of aerosol and trace gases such as O3, NO2, SO2 and HCHO.  Since the performance of trace gas retrieval strongly depends on the quality of raw data, the in-flight characteristics have been analyzed and calibrated in detector, radiometric and spectral aspects. Furthermore long-term performance of GEMS have been monitored and analyzed for the dark current, electronic offsets, non-linearity, diffuser and the output of the internal light sources. The orbital performance of GEMS shows that the number of incurred dead and bad pixels due to cosmic-ray impacts is 0.05% increased over the three full year since the operations. The dark signal distributions of on-orbit dark images show 9.02% increased dark mean which indicates a gradually degraded detector CCD performance. The orbital electronic biases from averaging trailing pixels show quite stable status in orbit around 900-1050 counts depending on each quadrant. In-orbit PRNU (< 7%) and non-linearity(< 2%) of 5-95% CCD full well signals meet the system requirements and show stable status. However, there is a possibility that remaining 10% non-linearity effects on the shortwave signals under the 5% CCD full well in early morning or late afternoon. Trends over the three full year of nominal operations indicates stable status of GEMS with gradually degraded detector and diffuser performances. The detailed results and the orbital and long-term stability for internal light source (LED), dark currents and solar irradiance measurements are going to be presented.