# Small Satellite Tools for High-Resolution Infrared Fire Monitoring
## 3 The FireBIRD Main Payload
### Bi-spectral Sensors of TET-1 and BIROS  
Mechanical flaps are mounted in front of the MIR and TIR lenses of the bi-spectral sensor, which close the lens for the radiometric on-board calibration of these cameras as shown on the left for the TIR camera in Figure 1. During Earth imaging, the flap is open as shown on the right in Figure 1 for the MIR camera. Comments:  
1. The symbolic illustration of the flap positions in front of the MIR and TIR lenses chosen for Figure 1, however, does not correspond to the principal mode of their operation, which is characterized by both flaps being open or closed at the same time. 2. Tables and images from various original sources are used in this section, in some of which different abbreviations are used for the thermal and mid-IR wavelength ranges, such as: * LW or LWIR (Long-Wave IR) for thermal IR (TIR); and * MW or MWIR (Mid-Wave IR) for mid-IR (MIR). The detectors of the bi-spectral IR sensors of BIRD, TET-1, and BIROS are cooled Cadmium Mercury Telluride (CdHgTe) detector line arrays whose lines have an identical geometrical structure. In these detectors, two 512-element-long lines are arranged close to parallel to each other on a CdHgTe semiconductor chip in such a way that the centers of the detector elements are shifted by exactly half the distance between the elements (in the line direction). These so-called staggered line arrays allow for an improvement of the geometrical resolution. The function of staggered line arrays is well explained and illustrated in [7] on the basis of airborne IR data, and it is described in physical-mathematical terms in [9]. Figure 2: Working principle of a push-broom camera. The detector line A is located on the focal plane B of the camera lens C. The arrangement A–B–C moves on a carrier (aircraft, satellite) in the direction of the arrow over the ground, where a ground pixel D is represented. The pixel dwell time corresponds to the time interval in which a distance is flown over and is equal to the edge length of a pixel (in the direction of movement). A data sheet for the MIR and TIR detectors used in BIRD, TET-1, and BIROS is shown in Table 3 as a copy of a document (BIRD IR DETECTOR PROPOSAL) supplied in 1996 by the British company GEC Marconi for the BIRD project. Figure 3 shows 2 \(\times\) 5 elements of the staggered structure of the CdHgTe line array detectors used in the BIRD Hot Spot Recognition System (HSRS), which are also used in the IR sensors of TET-1 and BIROS [6]. Figure 3: Staggered structure of the BIRD MIR and TIR detectors, which are also used in the IR sensors of TET-1 and BIROS (only 2 \(\times\) 5 elements of a total of 2 \(\times\) 512 elements along with the read-out circuit of one detector element are shown here) [6]. \begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{1}{c}{**Detector**} \\ \hline Detector operation temperature & 80 K \\ \hline Number of elements & 1024 \\ \hline Element Material & CdHgTe, photovoltaic diode \\ \hline Multiplexer circuitry & CMOS \\ \hline Element stagger, line to line & 15 \(\upmu\)m \\ \hline Array format & 512 \(\times\) 2 elements \\ \hline Element pitch & 30 \(\upmu\)m \\ \hline Spacing between lines & 30 \(\upmu\)m \\ \hline Window & Germanium (AR coated) \\ \hline Field of view & f/2.0 \\ \hline Filter cut-on wavelength & MW: 3.4 \(\upmu\)m; LW: 8.5 \(\upmu\)m \\ \hline Filter cut-off wavelength & MW: 4.2 \(\upmu\)m; LW: 9.3 \(\upmu\)m \\ \hline Number of outputs & 4 \\ \hline Dynamic range & 1.6 V \\ \hline Power consumption & 55 mW \\ \hline Output impedance & 55 \(\Omega\) \\ \hline Integration capacitors & 1.1, 2, 2.2, 2.4, 3.5 pF \\ \hline \multicolumn{1}{c}{**Performance**} \\ \hline NEAT (median) & upper limit MW: 45 mK for 4 ms stare time: LW: 40 \\  & mK for 1.5 ms stare time. \\ \hline Number of dead elements & 10 maximum \\ \hline Number of defective elements & 50 maximum \\ \hline Signal uniformity & \(\pm\) 25\% of central value \\ \hline Pixel rate & 5.0 MHz max. per output \\ \hline \hline \end{tabular}
\end{table}