On-Orbit Radiometric Calibration and Stability Assessment of the Visible to Near-Infrared Bands of Geostationary Ocean Color Imager-II (GOCI-II)

Abstract

While most previous ocean color missions relied on polar-orbiting satellites, geostationary platforms are gaining attention for their capacity to continuously monitor dynamic coastal ocean. In Northeast Asia, Geostationary Ocean Color Imager-II (GOCI-II)—the successor to the first geostationary ocean color satellite, GOCI—was launched in February 2020. Here, we present an improved on-orbit radiometric calibration (RC) framework of GOCI-II to ensure radiometric stability and consistency for multiyear ocean color observations. Following an in-orbit test period, gain values <inline-formula><tex-math notation="LaTeX">$( {{{G}_{\mathrm{IOT}}}} )$</tex-math></inline-formula> were incorporated into the radiometric correction model. Using solar diffuser measurements obtained from June 2020 to June 2023, we develop a new model to add angle correction <inline-formula><tex-math notation="LaTeX">$( {{{C}_\theta }} )$</tex-math></inline-formula> and temporal trend correction factors <inline-formula><tex-math notation="LaTeX">$( {{{C}_t}} )$</tex-math></inline-formula> to model seasonal variation and long-term changes in detector response. Unlike conventional methods, residual calibration errors are minimized through the iterative application of <inline-formula><tex-math notation="LaTeX">${{C}_\theta }$</tex-math></inline-formula> and <inline-formula><tex-math notation="LaTeX">${{C}_t}$</tex-math></inline-formula>. Results reveal that sensor responses differ across wavelengths, with the green band (555 nm) showing the highest gain loss up to about ∼6%, while the blue and near-infrared (NIR) bands are less affected. Reprocessed top-of-atmosphere radiances increase across all spectral bands. However, sea-surface remote sensing reflectance <inline-formula><tex-math notation="LaTeX">$( {{{R}_{\mathrm{rs}}}} )$</tex-math></inline-formula> trends exhibit a wavelength-dependent response, with <inline-formula><tex-math notation="LaTeX">${{R}_{\mathrm{rs}}}$</tex-math></inline-formula> increases at longer wavelengths and decreases at shorter ones, driven by the NIR band ratio in atmospheric correction. As part of the 2024 update, a new RC model was implemented for operational use, supporting the stable, long-term GOCI-II ocean color products for environmental applications.