# Small Satellite Tools for High-Resolution Infrared Fire Monitoring
## 3 The FireBIRD Main Payload
### VIS-NIR Push-Broom Camera of the BIROS Main Payload  
Satellite sensors for the monitoring and evaluation of HTEs must have, in addition to the MIR and TIR bands, at least two spectral bands in the reflective wavelength range, namely:  
* a band in the visual range (VIS), at 0.56-0.72 \(\upmu\)m, in order to distinguish fire from solar reflections by pixel-wise ratio formation of the coregistered signals of the VIS band and MIR band, and to eliminate the latter as false alarms; and
* a near-infrared (NIR) band, at 0.79-0.93 \(\upmu\)m, to create a cloud mask in daytime images to observe fires in cloud-free areas.  
For remote sensing of normal-temperature phenomena (NTP) and for burned area determination, an additional green band at 0.46-056 \(\upmu\)m is useful. Therefore, the VIS-NIR camera of the main BIROS payload, incidentally to the VIS-NIR camera of TET-1, is equipped with these three VIS-NIR spectral bands in the reflective wavelength range.  
Figure 7: BIROS IR images of a normal-temperature scene around Mexico City on 5 May 2017: the image fragments from the BIROS TIR band in the top row and the image fragments from the BIROS MIR band in the bottom row; in both cases, the raw image fragments are shown on the left, the image fragments after the linear radiometric correction are shown in the middle, and the image fragments after the (residual) strip correction are shown on the right.  
Structure and Function of the BIROS VIS-NIR Push-Broom Camera  
In the VIS-NIR camera head, as can be seen in the center of Figures 4 and 5 shown in the previous subsection on the bi-spectral IR sensors, three Charge-Coupled Device (CCD)-type linear array detectors are arranged as a hybrid focal unit on the focal plane of the entrance lens of this camera. In front of the Thomson TH 7808 B-type CCD line arrays are mounted their respective spectral filters.  
Due to the mounting orientation of the VIS-NIR camera head between the MIR and TIR sensor heads in the multi-spectral camera system of the BIROS payload segment, the longitudinal axes of the three CCD lines are perpendicular to the direction of movement of the satellite track over the Earth during data acquisition in orbit, as illustrated in Figure 2 (also shown in the previous subsection) for a scene scan according to the \"push-broom\" principle.  
The linear ground sampling distance (GSD) of a push-broom camera in the nadir area is determined by the following simple relation:  
\[\text{GSD}=\text{p}\times\text{(h/f)} \tag{1}\]  
where:  
* p is the distance between two detector elements (pitch);
* f is the focal length of the entrance lens; and
* h is the orbit height of the satellite above the Earth.  
The pixel dwell time is calculated according to the formula:  
\[\text{Tdw}=\text{GSD}/\text{V0} \tag{2}\]  
where V0 is the projection of the satellite's velocity vector onto the Earth's surface, i.e., the velocity at which the satellite footprint moves over the Earth's surface. Referring to the GSD of the BIROS VIS-NIR camera of 42 m (see Table 2) and a V0 of 7 \(\times\) 10\({}^{3}\) m/s, the pixel dwell time for all three CCD lines of the BIROS VIS-NIR camera is:  
\[\text{Tdw}=\text{g}/\text{V0}=42\text{ m}/7\times 10^{3}\text{ m}/\text{s}=6\text{ ms} \tag{3}\]  
The VIS-NIR CCD lines are read out at a rate of 6 ms so that no imaging gaps occur during the push-broom scene sampling. The exposure or integration time tint of the VIS-NIR camera is only half of the Tdw with tint = 2.7 ms to keep the pixel smearing in the flight direction low. The three lines (green, red, and NIR) are arranged parallel to each other on a hybrid focal plane at a distance between two lines of d = 10.1 mm. This results in an angle of arc tan d/f = 6.4\({}^{\circ}\) for the lines looking slightly forward and backward (with the green and the NIR spectral filter, respectively) to the line--with the red spectral filter--looking in the nadir direction.