## 1 Introduction
### Main Features and Parameters of Operational Polar Orbiting IR Sensors  
This chapter shows selected application examples where data from BIRD, TET-1 and BIROS are used in comparison with IR sensor data registered nearly simultaneously by operational satellite systems. Table 1 shows features and parameters of three operational IR sensors: Moderate Resolution Imaging Spectroradiometer (MODIS), Visible Infrared Imaging Suite (VIIRS) and Sea Land Surface Temperature Radiometer (SLSTR), in comparison with the Hot Spot Recognition System (HSRS), flown on BIRD, TET-1 and BIROS. The BIRD demonstrator mission as well as the FireBIRD mission with the two satellites TET-1 and BIROS was defined, developed and operated as a science mission. BIRD and FireBIRD were operated on user demand and did not generate data with a duty cycle of 100%, i.e., in \"24/7\" operation, as it is the case for the operational IR sensors MODIS, VIIRS and SLSTR. An important parameter of sun-synchronous orbiting satellites is the local time of their upward or downward equator crossing, the so-called _Local Time of Ascending Node_ (LTAN) or _Local Time of Descending Node_ (LTDN), respectively. The LTAN and/or LTDN can be used to find out the observation time difference of various satellites. Table 2 shows the LTAN and LTDN of the operational satellites carrying the IR sensors MODIS, VIIRS and SLSTR, as well as the LTAN and LTDN of BIRD, TET-1 and BIROS. This overview permits a \"grouping\" N\({}^{\circ}\), considering possible years of common utilization and low observation time differences. The selection of the LTAN and LTDN of the operational satellites carrying MODIS, VIIRS and SLSTR was driven by the requirements of a great number of applications. Their orbits are not optimized for fire detection. In particular, the NOAA-20 (VIIRS) system with its LTAN at 13:30 h must fulfill the requirements of a great number of important and higher-ranking utilizations than fire applications, such as ocean color analysis [7]. The LTAN and LTDN of the operational satellites, mentioned in Table 2, are kept stable by orbit maintaining using propulsion systems. Due to the better availability of affordable and well-timed piggyback launch opportunities (worldwide) into LEO for the dedicated fire detection satellites BIRD, TET-1 and BIROS, morning orbits resulted for all satellites. The morning orbits, however, allow the detection of so-called incipient fires early in the day, i.e., the anti-meridiem detection of small HTEs, representing a further \"unique selling point\" of BIRD and FireBRID. However, in this sub-chapter and in Table 2, we do not aim at \"orbit optimization for fire detection\", but \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Feature/Parameter** & **MODIS** & **VIIRS** & **SLSTR** & **HSRS** \\ \hline \multirow{3}{*}{Launched in the year with _Satellite_} & 1999 _”Terra\"_, & 2011 _“Suomi_ & 2015 & \multirow{3}{*}{2001 _“BIRD”_} \\  & 2002 _“Aqua\"_, i.e., & \(\text{NPP}^{*}\), & \(\text{NPP}^{*}\) & & 2018 _“TET-1”_ \\  & two nearly identical & _”NOAA-20\"_ & _“Sentinel-3B\"_ & 2016 _“BIROS”_ \\ \hline Orbit altitude & 705 km & 829 km & 815 km & 500–560 km \\ \hline Swath width & 2330 km & 3060 km & 1407 km & 162–178 km \\ \hline Ground sampling distance (at the nadir) & 1000 m & 375 m & 1000 m & 170–185 m \\ \hline MIR and TIR pixel saturation temperature [1] & MIR; 450 K, TIR: 367 K, TIR: 400 K & MIR: 500 K, TIR: 400 K & MIR: 630 K \\ \hline Minimum detectable area of a 1000 K THE [2] & \(\sim\)150 m\({}^{2}\) & \(\sim\)20–30 m\({}^{2}\) & \(\sim\)150 m\({}^{2}\) & 15–20 m\({}^{2}\) \\  & 1000 K & 12 h (achieved with two instruments) & 12 h & 24 h & 12 h–3 d  \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Geometric features and further sensor parameters of the operational IR sensors MODIS, VIIRS and SLSTR in comparison with HSRS, flown on BIRD, TET-1 and BIROS. we look for orbit communalities only with respect to possible years of common utilization and low observation time differences.