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. Solar-induced fluorescence (SIF) data from satellites are increasinglyused as a proxy for photosynthetic activity by vegetation and as aconstraint on gross primary production. Here we report on improvementsin the algorithm to retrieve mid-morning (09:30 LT) SIF estimates onthe global scale from the GOME-2 sensor on the MetOp-A satellite (GOME-2A)for the period 2007–2019. Our new SIFTER (Sun-Induced Fluorescence ofTerrestrial Ecosystems Retrieval) v2 algorithm improves over aprevious version by using a narrower spectral window that avoidsstrong oxygen absorption and being less sensitive to water vapourabsorption, by constructing stable reference spectra from a 6-yearperiod (2007–2012) of atmospheric spectra over the Sahara and byapplying a latitude-dependent zero-level adjustment that accounts forbiases in the data product. We generated stable, good-quality SIFretrievals between January 2007 and June 2013, when GOME-2Adegradation in the near infrared was still limited. After thenarrowing of the GOME-2A swath in July 2013, we characterised thethroughput degradation of the level-1 data in order to derivereflectance corrections and apply these for the SIF retrievals betweenJuly 2013 and December 2018. SIFTER v2 data compare well with theindependent NASA v2.8 data product. Especially in the evergreentropics, SIFTER v2 no longer shows the underestimates against othersatellite products that were seen in SIFTER v1. The new data productincludes uncertainty estimates for individual observations and isbest used for mostly clear-sky scenes and when spectral residualsremain below a certain spectral autocorrelation threshold. Our resultssupport the use of SIFTER v2 data being used as an independentconstraint on photosynthetic activity on regional to global scales.

Improved SIFTER v2 algorithm for long-term GOME-2A satellite retrievals of fluorescence with a correction for instrument degradation