## 2 Data and Methods
### Ground-Based LST Measurements  
The KIT operates four permanent validation stations for satellite-based LST retrieval. The stations, being part of LSA-SAF's validation effort and supported by EUMETSAT, were specifically chosen and designed to validate LST derived from MSG/SEVIRI. They are located in large homogenous areas within the field of view of the METEOSAT satellites and lie in different climate zones, which provides a broad range of atmospheric conditions for product validation [34]. The locations of the four validation stations on the SEVIRI Earth disk are indicated in Figure 1. An overview of the KITs validation sites is provided in Table 1.  
In principle, LST datasets can readily be validated with ground-truth radiometric measurements. However, this so-called 'temperature-based validation' is largely complicated by the spatial scale mismatch between satellite- and ground-based sensors: areas observed by ground radiometers usually cover about 10 m\({}^{2}\), whereas satellite measurements in the thermal infrared typically cover between 1 km\({}^{2}\) and 100 km\({}^{2}\)[34]. Furthermore, natural land covers and the corresponding land surface temperatures are spatially quite heterogeneous: therefore, for validation measurements to be representative for satellite-derived LST, they have to be performed in areas that are homogenous at the satellite pixel scale. The size of the area that needs to be viewed by the validation instrument at the ground depends on the within-pixel variability of the surface and on how well measurements of several \"end members\" can be  
Figure 1: Locations of the Karlsruhe Institute of Technology’s (KIT) validation stations on the Meteosat Second Generation (MSG)/Spinning Enhanced Visible and Infrared Imager (SEVIRI) disk. mixed in order to obtain a representative value for the satellite pixel. This so-called end-member-cover method is based on a linear spectral mixing approach and assumes that the total IR radiance emitted by the land surface within a satellite pixel can be reasonably well approximated by a linear mixture of the IR radiance emitted by the relevant surface cover types within that area [35]. The mixing of measurements obtained for different end-members requires information on their respective fractions within the sensor's field of view and also on scene emissivity [26, 36, 37]. At KIT's validation sites, the relevant spectral end-members (e.g., trees, grassland and background soil) were determined from an independent component analysis of high-resolution satellite data (visible and near-infrared). The fractional coverages of the determined end-members were then obtained by land cover classification [35]. The main instrument for the _in situ_ determination of LST at KIT's validation stations is the precision radiometer \"KT15.85 IIP\" produced by Heitronics GmbH, Wiesbaden, Germany. KT15.85 IIP radiometers measure thermal infra-red radiance between 9.6 \(\upmu\)m and 11.5 \(\upmu\)m, have a temperature resolution of 0.03 K and an accuracy of \(\pm\)0.3 K over the relevant temperature range [38]. The KT15.85 IIP has a drift of less than 0.01% per month: The high stability is achieved by linking the radiance measurements via beam-chopping (a differential method) to internal reference temperature measurements and was confirmed by a long-term parallel run with the self-calibrating radiometer \"RotRad\" from Commonwealth Scientific and Industrial Research Organisation (CSIRO), which is continuously stabilized with 2 blackbodies [37]. The parallel run at the Evora site started in April 2005; a year later, the agreement between the instruments was still excellent (correlation 0.99). Due to the KT-15.85 IIP's narrow spectral response function and the small distance between the radiometers and the surface atmospheric attenuation of the surface-leaving, thermal infrared radiation is negligible.