Enhancing the reliability of GCOM-C/SGLI data for red tide detection in the upper Gulf of Thailand

With the upcoming completion of the Moderate Resolution Imaging Spectroradiometer mission, this study applied 250-m-spatial-resolution ocean colour products from the Second-Generation Global Imager (SGLI) on the Global Change Observation Mission-Climate (GCOM-C) platform to continue monitoring red tides in the upper Gulf of Thailand. However, inaccuracies of GCOM-C/SGLI data on red tide detection were observed. To enhance its reliability, we first improved the accuracy of GCOM-C/SGLI remote sensing reflectance (<i>R</i>_rs) data by adjusting it to <i>in situ</i> data using wavelength-specific regression functions. This method significantly reduced the mean absolute percentage difference between <i>in situ</i> and GCOM-C/SGLI data by 25%–87% for single SGLI wavelengths and by 46%–83% for <i>R</i>_rs(λ)/<i>R</i>_rs(565) of the original data. Subsequently, we employed the corrected data of SGLI <i>R</i>_rs(λ) at 565 nm and the ratios of <i>R</i>_rs(490)/<i>R</i>_rs(565), <i>R</i>_rs(530)/<i>R</i>_rs(565), and <i>R</i>_rs(673)/<i>R</i>_rs(565) in local empirical algorithms to estimate chlorophyll-a(chl-<i>a</i>) concentration and identify red tide waters. SGLI chl-<i>a</i> improved substantially in areas affected by red tides, particularly as chl-<i>a</i> concentrations increased. Additionally, the corrected GCOM-C/SGLI accurately identified red tides caused by green <i>Noctiluca</i>, diatoms, and dinoflagellate blooms consistent with local reports, confirming the GCOM-C/SGLI reliability enhancement. This method may serve as an alternative strategy for monitoring the red tides.