In-flight calibration and validation for Level 1 Products of GEMS

The GEO-ring constellation of atmospheric composition missions was established to deliver continuous, diurnally resolved observations of trace gases and aerosols at regional scales. Since the launch of GK2B in 2020, the constellation has expanded to include TEMPO in 2023 and Sentinel-4 in 2025, with additional missions planned. This growing number of operating sensors emphasizes the need for cross-mission consistency and long-term radiometric stability across the GEO-ring framework.As the first GEO-ring sensor, the Geostationary Environment Monitoring Spectrometer (GEMS) onboard GK-2B has been operating for more than five years under harsh space environmental conditions. Over this period, the optical and electronic components of the sensor have experienced degradation of up to 30%, particularly at shorter wavelengths around 300 nm. Given that this sensor has undergone the longest period of degradation, its characterization and correction provide a valuable basis for other GEO-ring sensors with similar instrumental characteristics. In this regard, this study presents post-launch calibration and evaluation methodologies for GEMS with a focus on Level 1 products, including solar irradiance and Earth reflectance. Calibration updates address long-term degradation along with angular dependence and systematic biases of the onboard solar diffuser.For validation of the updates, GEMS is inter-calibrated with both geostationary and low Earth orbit sensors, including the Advanced Meteorological Imager (AMI) onboard the twin satellite GK2A, as well as the Tropospheric Monitoring Instrument (TROPOMI) and Ozone Mapping and Profiler Suite (OMPS). Each reference instrument provides unique strengths in spatial, spectral, and temporal coverage, enabling a comprehensive assessment of GEMS performance. The validation results indicate that the updated GEMS reflectance exhibits spectral biases within 5%, except at wavelengths below 320 nm, where straylight effects dominate.These results demonstrate that the applied calibration and inter-calibration strategies effectively improve the radiometric consistency of GEMS Level 1 products. Building on these approaches, this work highlights the importance of in-flight calibration of Level 1 products for accurate Level 2 retrievals and long-term GEO-ring consistency.