## 1 Introduction
No civil space-borne IR sensor fulfilled this requirement before the year 2000 because the Moderate Resolution Imaging Spectroradiometer (MODIS) on the \"Terra\" satellite, the first system which fulfilled this requirement, was launched at the end of 1999. The small satellite Bi-spectral Infrared Detection (BIRD) was developed and built by the German Aerospace Center (Deutsches Zentrum fur Luft unf Raumfahrt (DLR)) and successfully operated from October 2001 until February 2004 [2; 3]. The small German satellites Technologie-Erprobungs-Trager N\({}^{\circ}\) 1 (TET-1) and Bi-spectral InfraRed Optical System (BIROS) were piggyback launched in July 2012 and June 2016, respectively. Both satellites are part of the space segment of the FireBIRD mission, led by the DLR [4; 5]. The IR sensors of BIRD, TET-1 and BIROS provided image data in the mid-IR (MIR, at 3.8 \(\upmu\)m) and thermal IR (TIR, at 9.2 \(\upmu\)m) bands, with ground sampling distances (GSD) of \(\sim\)180 m in both spectral bands, and noise equivalent difference temperatures (NEDTs) of 0.4 K (at 300 K) in the MIR band and 0.2 K (at 300 K) in the TIR band. Both bands are also composed of a very high radiometric dynamic, necessary to avoid signal saturation from strong high-temperature events (HTEs), such as big wildfires or major volcanic eruptions. The combination of these features allows for the estimation of fire attributes such as the fire temperature, the fire area or the FRP of small and huge vegetation fires, as explained in detail in Chapters 4 and 5 of [3]. These unique features of the push broom-type MIR/TIR sensor of BIRD, TET-1 and BIROS, called the \"Hot Spot Recognition System (HSRS)\", are achieved by the real-time adjustment of their integration time: if, after sampling with the regular exposure time, the real-time sensor onboard data handling indicates that the signals of some detector elements are saturated (or close to saturation), a second exposure is performed within the same sampling interval, but with an exposure time to be reduced by an order of magnitude, i.e., by a factor of \(\sim\)12. The data of both exposures are digitized and stored by the onboard data handling system. After data transmission to the ground station, the data samples with these double exposures are merged during the on-ground processing. This smart \"Hot Area (HA) reconstruction\" procedure preserves the 0.4 K and 0.2 K radiometric resolution in the MIR and TIR channels, respectively, and eliminates detector saturation for huge wildfires and strong volcanic lava eruptions [5; 6]. These capabilities of BIRD and FireBIRD are a unique feature compared with other existing IR sensors, i.e., imaging infrared sensors with at least one spectral band in the mid-IR band (MIR at 3.8 \(\upmu\)m) and one spectral band in the thermal IR band (TIR, between 8 and 12 \(\upmu\)m), currently used for HTE observations. However, imaging IR sensors with one or more spectral bands in TIR, but without a band in MIR, are not capable of providing data for the estimation of the above-mentioned fire attributes. The MIR band is used as the \"leading channel\" for the detection of active fires and their quantitative analysis. This is mainly due to the fact that the signal contrast of fire-occupied pixels to \"cold\" background pixels is much higher in the MIR band than in the TIR band, which is illustrated in Figure 4 of [3]. Furthermore, an MIR band NEDT of \(<\)0.5 K (at a 300 K blackbody temperature) is a very important prerequisite for its \"leading channel\" function. One objective of this paper was to demonstrate the capability of BIRD- and FireBIRD-type IR sensors with higher spatial resolutions (\(<\)300 m) and radiometrically accurate MIR and TIR bands (with NEDTs \(<\) 0.4 K) that do not suffer from saturation effects caused, for example, by large fires.