# The DLR FireBIRD Small Satellite Mission: Evaluation of Infrared Data for Wildfire Assessment
## 1 Introduction  
Wildfires represent a predominant disturbance factor in a variety of biomes worldwide. They have played a crucial role in affecting the Earth's surface for millions of years [1]. [PERSON] and [PERSON] could show that forest loss has increased significantly in many parts of the world since 2000, and that these dynamics are strongly linked to fire [2]. Next to direct fire effects such as impact on human lives and property, wildfire events have been found to considerably contribute to global warming through GHG (greenhouse gas) emissions [3, 4]. Consecutively, the quantification of the amount of gaseous and particulate pollutants emitted through burning vegetation represents a major focus on global change studies [5]. Satellite remote sensing provides the means to monitor large geographical areas, with recurrence cycles of several times per day. The use of spaceborne, thermal sensors does not only allow the derivation of the spatial extent and temporal duration of ongoing fire events, but also enables the determination of the burn intensity. This measure is usually derived in the form of the Fire Radiative Power (FRP). It is directly related to theamount of ignited biomass at the moment of the satellite's overpass, which in turn can be used as a proxy for the amount of emitted pollutant [6]. FireBIRD is a satellite constellation consisting of two small satellites, which are specifically designed for the detection of high temperature anomalies. The mission is operated by the German Aerospace Center (DLR). Both satellites carry an infrared camera system receptive in the mid- and long wave infrared ranges (MWIR/LWIR), as well as in the visible and near-infrared (NIR) ranges [7]. The sensor system has been named HSRS (Hot Spot Recognition System). Opposed to the MWIR and LWIR instrument, the NIR sensor is not oriented in nadir direction, but faces a 6\({}^{\circ}\) backward orientation. It can therefore not be used in conjunction with the aforementioned bands. The sensor system's spatial resolution of 178 m is superior to mid-resolution systems currently in operation, such as the MODIS (Moderate-resolution Imaging Spectroradiometer) instrument onboard the National Aeronautics and Space Administration (NASA) Aqua/Terra satellites and the VIIRS (Visible Infrared Imaging Radiometer Suite) sensor onboard the Suomi-NPP (Suomi National Polar-orbiting Partnership) as well as NOAA-20 (National Oceanic and Atmospheric Administration) satellites. The same applies to the SLSTR (Sea and Land Surface Temperature Radiometer) instrument onboard the Sentinel-3 A/B satellites, which are operated by ESA (European Space Agency). Although the thermal anomalies products of MODIS and VIIRS have been available for years and represent the de-facto standard in satellite-based, global active fire monitoring, the SLSTR product has been published only recently. With this dataset, active fire detections are available for the mid-morning and evening low-Earth orbit timeslots, similar to MODIS Terra. The used algorithm has been found to detect fires of lower FRP compared to MODIS [8]. It is available through the ESA World Fire Atlas [9], which is still in a prototypical state at the time of writing. Comparisons between MODIS and VIIRS regarding FRP have been performed by [PERSON] et al. [10; 11], who found that especially fires of low intensity are regularly missed in the MODIS MOD14A1 product (see [12]). Moreover, in the wildfire domain, FireBIRD FRP data has been used in the field of volcano activity assessment (see [13; 14]). Satellite missions with higher spatial resolution, such as Landsat-TIRS (Thermal Infrared Sensor) or Sentinel-2 MSI (MultiSpectral Instrument), feature a prolonged recurrence cycle of several days before they can observe a region of interest a second time. Since wildfires often only last for some days, these systems are not suited for fire monitoring purposes. Unlike the mentioned missions, FireBIRD is not a continuously recording system.