## 2 Data  
This assessment was specifically designed for AVHRR. The AVHRR sensors (AVHRR/1, 2, and 3) are mounted on the NOAA-series of satellites, since 2006 AVHRR/3 is also onboard of the MetOp series. AVHRRs carries one band in the red (band 1), one in the near infrared (band 2), and one in the shortwave infrared (band 3) domain. Since AVHRR/3, band 3 can also contain acquired radiation in the mid infrared wavelength. Main bands for LST estimation are however the bands in the thermal infrared. AVHRR/2 and 3 contain two channels in the thermal infrared, while AVHRR/1 features only one band in this domain (Table 1). Although the different AVHRR sensors measure in the same bands, their spectral responses are not identical, as each sensor is a unique instrument, which is subject to some form of degradation. The degradation of a sensor and therewith possible overall changes in the spectral responses are accounted for by calibration. However, no information about possible changes in the spectral form is available after lunch [27]. Figure 1 shows the spectral response curves of band 4 and band 5 of the different sensors. The response curves may differ substantially not only between the different models, but also within one model (especially AVHRR/3). For the method development of this assessment, it was necessary to compile a dataset of atmospheric profiles. For this, a selection of a training database of global profiles (called SeeBor Version 5.0) was used [28]. This database consists of 15.704 global profiles of temperature, moisture and ozone at 101 pressure levels--all for clear sky conditions. The profiles were all quality checked and a surface temperature was assigned to each profile based on a physically based scheme. The profiles of the database are compiled from five different existing databases (NOAA-88, ECMWF 60L training set, TIGR-3, ozone sondes from 8 NOAA Climate Monitoring and Diagnostics Laboratory sites, and radiosondes from 2004 in the Sahara Desert) [28]. From this database a selection was made for this assessment, which ensured having profiles in all ranges of surface temperature and columnar water vapour (see Table 2), as well as from as many land cover types (referring to the International Geosphere-Biosphere Programme (IGBP)) as possible. Due to the uneven distribution of land cover classes in the profile database, the number of profiles for each land cover class differs. A total of 2662 profiles were selected, featuring the following characteristics: 811 profiles are taken from TIGR-3, 317 profiles from Radiosondes, 479 profiles from Ozonesondes, and 1055 profiles are taken from ECMWF. The profiles were taken in a range of different land cover classes as defined in Table 3. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Channel**} & **AVHRR/1** & **AVHRR/2** & **AVHRR/3** \\  & **(NOAA-6,8,10) (\(\upmu\)m)** & **(NOAA-7,9,11,2,14) (\(\upmu\)m)** & **(NOAA-15,16,17,18,19 MetOp A and B) (\(\upmu\)m)** \\ \hline
1 & 0.58–0.68 & 0.58–0.68 & 0.58–0.68 \\
2 & 0.725–1.1 & 0.725–1.1 & 0.725–1.0 \\
3A & - & - & 1.58–1.64 \\
3B & 3.550–3.93 & 3.550–3.93 & 3.55–3.93 \\
4 & 10.5–11.5 & 10.3–11.3 & 10.3–11.3 \\
5 & 10.5–11.5 & 11.5–12.5 & 11.5–12.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Spectral bands of the AVHRR sensors. Figure 1: Spectral response curves of band 4 and 5 of the different AVHRR/1, 2, and 3 sensors onboard the NOAA and the MetOp satellite series. AVHRR/1 curves are given in red tones, AVHRR/2 curves in green tones, and AVHRR/3 curves in blue tones and black. Further, a land use classification [29] was used to retrieve emissivity values for the application of the algorithm on real AVHRR data. The emissivity estimation was done using the Vegetation Cover Method (VCM) from [30]. For the comparison with MODIS data, the MOD11A1 V6 Land Surface Temperature and Emissivity product (MOD11A1) was used. The data was retrieved from the USGS HTTP-Server [26]. The MODIS data, as well as the AVHRR data, were projected to a common 1 km lat/lon grid.