# The DLR FireBIRD Small Satellite Mission: Evaluation of Infrared Data for Wildfire Assessment
## 2 Materials and Methods  
This study analyzes and evaluates thermal anomaly data acquired by the two experimental small satellites of the DLR FireBIRD mission, namely the Technology Experiment Carrier-1 (TET-1) and the Berlin InfraRed Optical System (BIROS). To assess the spatial and radiometric accuracy of the data, two widely used datasets are used as references. These are, first, the NASA MOD14A1 thermal anomaly product [12], derived from the MODIS sensor onboard the Aqua and Terra satellites. Secondly, the NASA VNP14A1 product [15], based on data from the VIIRS sensor onboard Suomi-NPP. These reference datasets can be acquired through the NASA Fire Information for Resource Management System (FIRMS [18]). The first satellite, TET-1, has been launched in July 2012, with BIROS following in June 2016. They are configured to fly sun-synchronously in a low-Earth orbit at approximately 560 km altitude. Although BIROS is still active, TET-1 has reached his end of life and has thus been decommissioned in 2020. With both satellites in orbit, the potential repetition rate was less than three days with \(\pm 30\lx@math@degree\) across track acquisitions. Table 1 shows an overview of the FireBIRD mission and the specifications for each HSRS sensor. The FireBIRD HSRS is a push broom sensor system with a ground sampling distance of 178 m for the infrared channels. Using double sampling, each pixel is recorded with two different integration times of 4000 \(\upmu\)s and 500 \(\upmu\)s, respectively. The first value is suited for normal temperature observations. However, acquisitions using this setting suffer from saturation of the infrared bands when high temperature events above 900 K are recorded [20]. This effect does not occur when the shorter integration time is used. To obtain high quality results, each recording is done using both integration times simultaneously. The acquisition lines of both integration times are then combined in a post processing step, yielding an increased radiometric dynamic range without band saturation. This process is called band staggering. It allows a spatial sampling step size of only half the effective pixel size [13]. Due to this high resolution combined with high radiometric sensitivity, the FireBIRD sensors allow a more detailed analysis of active fires compared to MODIS and VIIRS [21]. All processing steps, which were performed in this study in order to allow a comparison between the different systems, are depicted in Figure 1.  
Both the MWIR and the LWIR spectral range can be used to discriminate high temperature areas on the Earth's surface. However, according to the Wien's displacement law, the MWIR band is better suited for the detection than LWIR, since higher temperature events have a higher radiant power at shorter wavelengths. The method developed by [PERSON] [5] et al. exclusively uses MWIR band information to derive the FRP. This method is known to be very robust, and has thus been selected as the method of choice for the MOD14A1 and VNP14A1 products. However, this method is only satisfactorily applicable for high temperature events, which range between 600 K and 1500 K. For fire events of less intensity, the method is known to underestimate the actual intensity [6]. To circumvent this shortcoming, the bi-spectral method can be used, which uses both the MWIR as well as to LWIR band [17]. This method allows for a more accurate determination of FRP for lower intensity fire events. However, the requirement of an exact co-registration of both bands makes this method vulnerable to disturbances. Both methods, the bi-spectral one and the one by [PERSON] et al., are used and compared in this research. For convenience, the method by [PERSON] is referred to as _MWIR_, while the bi-spectral method is referred to as _MWIR/LWIR_ in consecutive sections.