I. Introduction
Photosynthesis is possibly the most important biochemical process on Earth. It provides the energy that powers ecosystem processes, thus it is critically important for the understanding of ecosystem function [1]. Photosynthesis is one of the key processes removing CO2, an important greenhouse gas, from the atmosphere, providing an important feedback to the climate system [2]–[5]. While it is well known that the global carbon cycle is changing [6]– [8] with terrestrial ecosystems contributing higher carbon uptake, identifying the mechanisms and locations responsible for high carbon uptake remains an important challenge for predicting future carbon–climate interactions [6], [9] . Satellite observations have the potential to provide timely data for regions where it is difficult or impossible to collect in-situ measurements, however the existing satellite-based observing systems cannot provide the long term, calibrated, and spectrally sensitive observations required to quantify global carbon-budget changes [10].