# Small Satellite Tools for High-Resolution Infrared Fire Monitoring
## 3 The FireBIRD Main Payload
### Bi-spectral Sensors of TET-1 and BIROS
#### 3.1.2 Spatio-Temporal Sampling of the FireBIRD Bi-Spectral IR Sensors  
The MIR and TIR line arrays of the two sensor heads are read out at a rate corresponding to the pixel dwell time Tdw = 25.4 ms, as shown in Table 2. Thus, a scanning step width of 178 m is achieved in the direction of flight, although the edge length of a pixel on the ground is twice as large, at 356 m. By offsetting the two \"staggered linear arrays\" (see Figure 3, term: \"element stagger, line to line\") arranged on the focal plane of both the MIR and the TIR spectral bands by 15 \(\upmu\)m, a scanning step size of \(\sim\)178 m is also achieved perpendicular to the direction of flight, although the edge length of a pixel on the ground of 356 m is also twice as large in this direction.  
A special feature of BIROS's bi-spectral sensors is that the exposure times (or integration times) of their CdHgTe detectors can be:  
* varied via ground station command; and
* adjusted in real time to the signal strength.  
The latter means that during the used sampling time of 25.4 ms, the following processes take place automatically in the electronics close to the detector in the MIR and TIR sensor heads:  
1. If all 2 \(\times\) 512 signal levels of the respective staggered line array are sufficiently far away from the saturation level at the commanded integration time (T int-com), which occurs in the first quarter of the sampling period of 25.4 ms, then no second exposure takes place within this sampling period.
2. If, however, even one of the 2 \(\times\) 512 detecter element signal levels of the respective staggered line array is within the range of the saturation level at the commanded integration time Tint-com, which is triggered in the first quarter of the scanning period of 25.4 ms, a further exposure is carried out in the second half of the sampling period with an integration time shortened by a factor of 30 compared with T int-com. For the elements of the detector lines where the signal level was in the saturation range during the exposure with integration time T int-com, the digitized pixel samples are appended (at the end) to the data from the first sampling with T int-com.  
The data frame of the read-out digitized data of the detector lines is structured in such a way that it can accommodate the larger number of samples occurring in case (b) with exact assignment to the respective detector element.  
This \"intelligent\" procedure used in the bi-spectral IR sensor of BIROS for the significant extension of the radiometric dynamic range by means of two successive exposures with different integration times during one sampling cycle has already been (i) successfully tested in the Advanced BIRD Airborne Simulator (ABAS) [6], (ii) used in the Hot Spot Recognition System (HSRS) of BIRD, and (iii) used in the bi-spectral sensor of TET-1. It is a unique selling point in IR sensor development for the detection and evaluation of high-temperature events. This feature permits us to achieve saturation temperatures of pixels in the MIR band of 650 K (+ 377 \({}^{\circ}\)C) and in the TIR band of \(\sim\) 600 K (\(\sim\)327 \({}^{\circ}\)C), and it allows for a 20% occupation of the IR pixels of BIROS (with an area of 356 m \(\times\) 356 m = 126,736 m\({}^{2}\) = 12.7 Ha) by a HTE with a temperature of 1000 K (+727 \({}^{\circ}\)C). In other words, a very extensive fire with a temperature of 1000 K filling 0.2 \(\times\) 12.7 Ha = 2.54 Ha in a BIROS IR pixel can be registered without saturation and subsequently energetically evaluated.