TanSat-2: a new satellite for mapping solar-induced chlorophyll fluorescence at both red and far-red bands with high spatiotemporal resolution

Abstract

Global observations of solar-induced chlorophyll fluorescence (SIF) serve as a robust proxy for monitoring vegetation photosynthetic activity and elucidating the terrestrial carbon cycle. To date, several atmospheric remote sensing satellites have been deployed to generate global SIF products. However, accurate mapping of dual-band (red and far-red) SIF with daily temporal resolution and kilometer-level spatial resolution remains a critical gap, despite its significance for various applications. The Chinese next-generation greenhouse gas monitoring satellite, TanSat-2, is set to succeed the original TanSat satellite, aiming to record the fraction of greenhouse gases, pollutants, and SIF measurements from space. According to current schedules, TanSat-2 is slated for launch in 2026. This satellite will feature a wide swath of 2900 km, a high spatial resolution of 2 km at an orbit altitude of 7000 km, and near-daily global coverage. TanSat-2 is equipped with two spectral channels, 747-777 and 672-702 nm, operating at a spectral resolution of 0.12 nm, thereby offering significant potential for mapping SIF in both the red and the far-red bands. In this study, we explore the prospective capabilities of TanSat-2 for SIF retrieval through simulation experiments. First, we simulated the satellite's radiative transfer processes using the Moderate Resolution Atmospheric Transmission 5 (MODTRAN 5) and Soil Canopy Observation of Photosynthesis and Energy (SCOPE) models. An end-to-end orbit simulation dataset for TanSat-2 was generated by aggregating global bottom-of-atmosphere (BOA) reflectance, meteorological datasets, and the global Orbiting Carbon Observatory-2 (OCO-2) SIF dataset (GOSIF). We then assessed the theoretical accuracy of TanSat-2 based on this spectral simulation dataset, yielding root mean square error (RMSE) values of 0.24 and 0.19 mW m-2 sr-1 nm-1 for SIF retrievals at 740 and 685 nm, respectively. Finally, we examined the global prospects of TanSat-2 SIF retrievals using the end-to-end orbit simulations. Comparisons between the anticipated TanSat-2 SIF retrievals and GOSIF inputs revealed excellent correlation at both bands, with R2 values of 0.88 and 0.61 and RMSE values of 0.082 and 0.061 mW m-2 sr-1 nm-1, respectively. Thus, TanSat-2 is poised to provide a valuable data resource for reliable SIF retrievals in the red and far-red bands, characterized by high spatiotemporal resolution on a global scale.