## 2 Background and International Context
### ESA/Copernicus Current and Future Land-Focused Optical Sensors
Future expansion of this portfolio includes the S2 harmonised and fused products in combination with Landsat-8, which are currently in the prototype phase [12]. Supplementary land products derived from thermal observations of the S3 SLSTR instrument, include, notably, the Land Surface Temperature (LST) and Fire Radiative Power (FRP), which are key variables to study evapotranspiration processes and vegetation fire disturbances. Future land-focused missions are currently in preparation; they will be launched in the coming years to complement and expand ESA/Copernicus observation capabilities. We recall here, in particular, the following missions:  
* The Fluorescence Explorer (FLEX) mission [13; 14], which aims at providing global maps of vegetation fluorescence for studying photosynthetic activity and plant health and stress. The FLEX satellite will orbit in tandem with one of the Sentinel-3 satellites, taking advantage of its optical and thermal sensors to provide complementary measurements, in particular for atmospheric correction and cloud screening. The FLEX satellite will carry on board the FLuORescence Imaging Spectrometer (FLORIS), a high-resolution spectrometer acquiring data in the 500-780 nm spectral range, with a sampling of 0.1 nm in the oxygen bands (759-769 nm and 686-697 nm) and 0.5-2.0 nm in the red edge, chlorophyll absorption and PRI (Photochemical Reflectance Index) bands. * The Copernicus Hyperspectral Imaging Mission (CHIME) [15] will carry a unique visible to shortwave infrared spectrometer to provide routine hyperspectral observations to support new and enhanced services for sustainable agricultural and biodiversity management, as well as soil property characterisation. The mission will complement Sentinel-2 for applications such as land cover mapping. * The Copernicus Land Surface Temperature Monitoring (LSTM) [16] mission will carry a high-spatiotemporal-resolution thermal infrared sensor to provide observations of the land surface temperature. Land surface temperature measurements and the derived evapotranspiration are key variables to understand and respond to climate variability, manage water resources for agricultural production, predict droughs and to address land degradation, natural hazards, coastal and inland water management as well as urban heat island issues. \begin{table}
\begin{tabular}{c c c c c} \hline
**Mission** & **Sensor** & **Level** & **Description** & **Reference** \\ \hline \multirow{4}{*}{Sentinel-2} & \multirow{4}{*}{MSI} & L1-C & Top-Of-Atmosphere (TOA) reflectances radiometric and geometrically corrected & [17] \\ \cline{3-5}  & & L2-A & Bottom-Of-Atmosphere (BOA) reflectances1 corrected for atmospheric effects & [17; 18] \\ \hline \multirow{4}{*}{Sentinel-3} & OLCI & L1-B & TOA radiances in the instrument grid radiometrically corrected & [19] \\ \cline{2-5}  & SLSTR & L1-B & TOA radiances (VIS, NIR, SWIR channels) and TOA brightness temperatures (thermal infrared channels) & [20] \\ \cline{1-1} \cline{2-5}  & \multirow{2}{*}{OLCI} & \multirow{2}{*}{L2 Land} & OLCI Global Vegetation Index (OGVI renamed in the future as GI-FAPAR, Green Instantaneous-fAPAR) providing the Fraction of Absorbed Photosynthetically Active Radiation (fAPAR) & [21; 22] \\ \cline{1-1} \cline{2-5}  & OLCI & L2 Land & OLCI Terrestrial Chlorophyll Index (OTCI) providing indication of the content of Chlorophyll in the canopy & [23; 24] \\ \hline \end{tabular}
\end{table} 
Table 1: List of current ESA Optical Land imaging satellites with their operational land products.