This project was carried out at Valencia University in the Laboratory of Earth Observation (LEO), headed by Prof J. Moreno. This group is also the driver behind the Fluorescence Explorer (FLEX) mission, a European Space Agency (ESA)'s Earth Explore candidate with the task to measure vegetation chlorophyll fluorescence from Space and the exploitation of this signal to better understand the carbon cycle. Meanwhile ESA's Earth Science Advisory Committee (SAC) recommended the investigation of the FLEX concept as an in-orbit demonstrator carrying a fluorescence imaging spectrometer (FLORIS), to be flown as a tandem mission with Sentinel-3 (S-3). This tandem concept consists of a high-resolution spectrometer measuring in the O2-A and O2-B absorption features complemented by S-3 instruments.