## 2 Operational Retrievals of ET from EO
### Operational Capability of EO Technology in ET Estimation  
#### 2.2.1 Spinning Enhanced Visible and Infrared Imager (SEVIRI)  
Land Surface Analysis Satellite Applications Facility (Landsaf) distribute an operational product of the latent heat (LE) flux, based on observations from the MSG-2 Spinning Enhanced Visible and Infrared Imager (SEVIRI) radiometer, a co-funded space mission between the European Space Agency (ESA) and EUMETSAT Space Agencies. This product has been developed in the framework of the EUMETSAT \"Satellite Application Facility\" (SAF) on Land Surface Analysis (LSA). The spin-stabilised Meteosat Second Generation (MSG) is a geostationary satellite with a temporal coverage of 15 min. SEVIRI is a multispectral radiometer with a spatial resolution of 3 km at nadir (1 km for the high-resolution visible channel) and 12 spectral channels from the visible to TIR. An example of the ET product is shown in Figure 1. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Pixel Scale** & **Spatial Resolution** & **Temporal Resolution** & **Past Sources** & **Current Sources** & **Future Sources** \\ \hline Coarse & 5–20 km & 15 min & MSG & GOES & GOES \\  & & & MSG & MSG \\  & & & AIRS & CrdS \\ Moderate & 1 km & Daily & AVHRR & MODIS & SENTINEL-3 \\ Fine & 60–120 m & Once every 5–16 days & LANDSAT & LANDSAT & LDCM \\ \hline \end{tabular} Note: GOES (Geostationary Operational Environmental Satellite), MSG (Meteosat Second Generation), AIRS (Atmospheric Infrared Sounder), MODIS (Moderate Resolution Imaging Spectroradiometer), AVHRR (Advanced Very High Resolution Radiometer), ATSR (Along Track Scanning Radiometer), ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer), CrIS (Cross-track Infrared Sounder), VIIRS (Visible/Infrared Imager Radiometer Suite), LDCM (Landsat Data Continuity Mission), HyspIRI (Hyperspectral-Infrared Imager).  
\end{table} 
Table 1: Examples of current and future satellite-based TIR imaging systems, along with characteristics spatial and temporal resolutions (after [25]). Briefly, the method developed by LSA-SAF allows estimation of both the instantaneous and daily total ET by the MSG SEVIRI radiometer. It follows a physical approach and can be described as a simplified Soil Vegetation Atmosphere Transfer (SVAT) model modified to accept EO data combined with data from other sources as forcing. The SVAT model employed is essentially a simplified version of the SVAT model TESSEL [27], which computes land surface processes taking both EO and atmospheric parameters as inputs [28]. The algorithm is then adapted to accept real-time data from meteorological satellites as forcing. The main forcing to the model comes from the remote sensing inputs including the daily albedo [29] and half-hourly short-wave [30] and long-wave fluxes [31]. In order to provide ET with a limited amount of missing values, a gap filling procedure is also adopted in the operational algorithm. The daily ET operational product is derived by temporal integration of instantaneous ET operational product values. The integration limits correspond to the first (theoretically at 00:30 UTC) and last (theoretically at 24:00 UTC) existing slots for a given day and the integration step is 30 min. These two products are provided for the full disk divided in four sub regions (Europe, North Africa, South Africa and South America) and are distributed by the Satellite Application Facility (SAF) on Land Surface Analysis (LSA) [32], from where a series of other operational products are also distributed. A detailed description of the operational algorithm for ET estimation from SEVIRI is available in [33; 34]. The MET product contains instantaneous values of ET (in mm h\({}^{-1}\)) plus an associated quality flag.