Sun-induced fluorescence (SIF) retrieval

Sun-induced fluorescence (SIF) retrieval measures active photosynthesis by detecting chlorophyll emission at the O2-A (757-760 nm) and O2-B (687 nm) Fraunhofer absorption lines in solar backscatter spectra. SIF constitutes only 0.5-2% of top-of-canopy radiance, so the method requires spectral resolution of approximately 0.1-0.5 nm and signal-to-noise ratios above 400:1. Retrieval algorithms exploit the Fraunhofer line depth infilling principle, typically implemented via DOAS or PCA-based approaches. The measurement is distinct from reflectance-based vegetation indices: SIF tracks the instantaneous rate of photochemical activity rather than canopy greenness, giving it direct sensitivity to gross primary production and vegetation stress.

GOSAT (launched 2009) produced the first global satellite SIF time series using its TANSO-FTS spectrometer at 0.012 nm resolution.^[1] OCO-2 demonstrated SIF as a high-spatial-resolution by-product of its CO2 mission, retrieving fluorescence at 757 nm and 740 nm with L2 Lite products available at NASA GES DISC.^[2] OCO-3, deployed on the International Space Station, inherits the same spectrometer design and adds a snapshot area mapping mode.^[1] GOSAT-2 continues the GOSAT SIF record with the TANSO-FTS-2 instrument.^[1] Sentinel-5P TROPOMI delivers global daily SIF at 3.5 x 5.5 km via the TROPOSIF project, operating in the 743-758 nm window at 0.37 nm resolution since August 2019.^[3] MetOp-A, MetOp-B, and MetOp-C each carry the GOME-2 instrument; L2 daily SIF products at 40 x 80 km footprint are archived at ORNL DAAC and processed with the SIFTER algorithm, with SIFTER v3 improving cross-platform consistency.^[4][5] ESA's FLEX mission, carrying the FLORIS spectrometer at 0.1 nm sampling across 500-780 nm, is designed specifically for SIF at 300 m resolution as ESA Earth Explorer 8; the mission was pre-launch as of 2026.^[6]