# Small Satellite Tools for High-Resolution Infrared Fire Monitoring
## 3 The FireBIRD Main Payload
The bi-spectral IR sensor and the VIS-NIR camera of the BIROS multispectral camera system are located on a common mechanical carrier console as shown in Figure 1. In the center of the camera system is the lens of the VIS-NIR camera in whose focal plane three Charge-Coupled Device (CCD) lines with their respective spectral filters are arranged. Behind this lens, three so-called Front End Electronic (FEE) units for the two visible spectral bands and the near-IR band of the VIS-NIR camera are clearly visible as a compact block. To the right of the VIS-NIR lens is the lens and the so-called Integrated Detector Cooler Assembly (IDCA) of the MIR band of the bi-spectral IR sensor. To the left of the VIS-NIR lens is the lens and IDCA of the TIR band of the bi-spectral IR sensor of this camera system. The two FEE units of the bi-spectral sensor are mounted on the side of the carrier console and connected to the respective IDCA via a cable connection (shown in red for the MIR camera in Figure 1). The three CCD line arrays of the TET-1 and BIROS VIS-NIR cameras have a spacing of 10.1 mm to each other, which--in combination with the focal length of 90.6 mm--results in an angle of 6.4 degrees between the forward- and nadir-looking lines and the nadir- and backward-looking lines. Since there are significant differences in the operation of the bi-spectral sensor compared with the VIS-NIR camera, it makes sense to describe them separately. Table 2 contains features and parameters of the multi-spectral camera system of TET-1 and BIROS. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Features and Parameters** & **Bi-Spectral IR Sensor** & **VIS-NIR Camera** \\ \hline Wavelength bands (\(\mu\)m) & MIR: 3.4–4.2; TIR: 8–5–9.3 & VIS: 0.5 \& 0.6; NIR: \(\sim\)0.8 \\ \hline Field of View (FOV) \({}^{*}\) & \(19^{\circ}\) & \(19.6^{\circ}\) \\ \hline \multirow{2}{*}{Detector type} & CdHgTe line arrays, cooled to 71 K for TIR and 80 K for MIR & Charge-Coupled Device (CCD) line array without cooling \\ \hline Detector element pitch (\(\mu\)m) & \(30\times 30\) & \(7\times 7\) \\ \hline Number of detector elements & \(2\times 512\) & \(3\times 5164\) \\ \hline Swath width (km) & 178 & 211 \\ \hline Linear pixel size on ground (m) & 356 & 42.4 \\ \hline Sampling step on ground (m) & 178 & 42.4 \\ \hline Pixel dwell time Tdw (ms) & 25.4 & 6.05 \\ \hline \hline \end{tabular} \({}^{*}\) By slewing the satellites up to \(+/-30^{\circ}\) to the right and left of the nadir, this field of view can be directed to selected fire zones or volcanoes within a field of regard of \(\sim\)700 km.  
\end{table} 
Table 2: Features and parameters of the multi-spectral camera system of TET-1 and BIROS. Figure 1: Perspective view of the sensor system of BIROS with the VIS-NIR camera in the center and the MIR and TIR sensors arranged to the right and left of it, respectively. The multi-spectral camera systems of the FireBIRD satellites, i.e., the main payloads of the TET-1 and the BIROS satellites, shown in Figure 2, work according to the push-broom principle: The longitudinal axes of the lines are perpendicular to the forward motion of the satellite, as shown in Figure 2; and  
* The complete image scanning of the Earth takes place in one acquisition swath if the detector lines are read out with the pixel dwell time Tdw = GSD/V0, where:
* the Ground Sampling Distance (GSD) is the sampling step width on the ground; and
* V0 is the projection of the velocity vector of the satellite on the Earth's surface, i.e., the velocity with which the satellite foot point moves over the Earth.