# Small Satellite Tools for High-Resolution Infrared Fire Monitoring
## 3 The FireBIRD Main Payload  
The design concept of the FireBIRD main payload had to fulfill the FireBIRD mission requirements and it refers to the proven BIRD sensor system, which consists of [4]:  
* a two-channel infrared Hot Spot Recognition Sensor system (HSRS); and
* a Wide-Angle Optoelectronic Stereo Scanner adapted to the BIRD mission (WAOSS-B). The HSRS is a push-broom scanner with spectral bands in the mid-infrared (MIR) and thermal infrared (TIR) spectral ranges. The detectors of the bi-spectral sensor are two Cadmium Mercury Telluride (CdHgTe) photodiode line arrays--with an identical layout in the MIR and the TIR--comprising 2 \(\times\) 512 elements each in a staggered structure [5]. The WAOSS was initially developed for the Mars-96 mission and its Flight Spare Model was modified to be used as BIRD's visible-near IR (VIS-NIR) push-broom camera. Table 1 shows the main characteristics of the BIRD sensor system. The HSRS sensor head components of both of its spectral channels are based on identical technologies to provide good pixel co-alignment. Both spectral channels have the same optical layout but with different wavelength-adapted lens coatings. Radiometric investigations of thermal anomalies require (a) a large dynamic range to not be saturated by high-temperature events (HTE) occupying a large part of a pixel and (b) a high signal-to-noise ratio to be able to observe small thermal anomalies and detect small sub-pixel HTE. A unique feature of the BIRD MIR and TIR sensor channels is that the respective integration times can be adjusted to the maximum radiance temperature of the scene. This capability, controlled in real time, is related to one of the most important sensor features, namely maintaining an adequate signal-to-noise ratio over ambient pixels while providing unsaturated data over pixels exposed to even the most intense fire events. If on-board processing of the HSRS data indicates that detector elements are saturated during the first exposure (or are close to saturation such that they may be outside the range for which the channel calibration is most accurate), then a second exposure is performed with a reduced integration time [6; 7]. The FireBIRD main payloads, i.e., the multi-spectral camera systems of TET-1 and BIROS, are in their bi-spectral IR sensors nearly identical with BIRD's Hot Spot Recognition System, but they comprise a new VIS-NIR CCD push-broom camera whose entrance lens has a focal length of 90.6 mm, which is quite different to the 21.65 mm focal length WAOSS used in the BIRD mission (see Table 1). This allows for a ground sampling distance (GSD) of up to 40 m in the visible and near-IR bands, giving us the opportunity to support the sub-pixel technique of the bi-spectral method [8]. The bi-spectral cameras of the FireBIRD satellites TET-1 and BIROS were developed and built at the DLR Institute for Optical Sensor Systems in Berlin-Adlershof. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **HSRS** & **WAOSS-B** \\ \hline \multirow{2}{*}{Spectral bands} & MIR: 3.4–4.2 μm (nadir) & NIR: 0.84–0.90 μm \\  & TIR: 8.5–9.3 μm (nadir) & (nadir and off-nadir) \\  & RED: 0.60–0.67 μm (off-nadir) \\ \hline Focal length & 46.39 mm & 21.65 mm \\ \hline Field of view (FOV) & 19\({}^{\circ}\) & 50\({}^{\circ}\) \\ \hline f-number & 2.0 & 2.8 \\ \hline Detector type & CdHgTe arrays & CCD lines \\ \hline Detector cooling & Stirling, 80–100 K & Passive, 293 K \\ \hline Detector element size & 30 μm \(\times\) 30 μm & 7 μm \(\times\) 7 μm \\ \hline Pixel number & 2 \(\times\) 512 staggered & 2880 \\ \hline Quantization & 14 bit (for each exposure) & 11 bit \\ \hline Ground pixel size & 370 μm & 185 μm \\ \hline Sampling step & 185 μm & 185 μm \\ \hline Swath width & 190 km & 533 km \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Main characteristics of the BIRD sensor system.