# Small Satellite Tools for High-Resolution Infrared Fire Monitoring
## 3 The FireBIRD Main Payload
### Bi-spectral Sensors of TET-1 and BIROS
#### 3.1.3 Radiometric On-Board Calibration of the FireBIRD Bi-Spectral IR Sensors  
The detection of high-temperature events (HTE) from space requires the use of infrared sensors, which measure the radiated power of HTEs in a reproducible and highly stable way. From this bi-spectrally measured radiated power, for example, the so-called effective temperatures can be derived [8]. In order to guarantee the stability of the measured values(i) under different \"environmental influences\" on the sensor system in space (e.g., different angles of solar incidence), and (ii) considering the properties of the detector system used, the measurement signals obtained from the bi-spectral sensors must be radiometrically cleaned from interfering signal components. For this purpose, the FireBIRD IR sensors must be radiometrically calibrated on board at certain time intervals. This is conducted by measuring reference radiation sources that are periodically swiveled into the field of view of the IR sensors. In the wavelength range of \(>\)3.5 \(\upmu\)m, blackbody radiators are used for this purpose. A special feature of IR small satellite missions is that the IR sensors with their cooling systems cannot be in operation continuously for power consumption reasons. This means that data acquisitions can be taken over selected target areas only, for which the detectors of the IR sensors have to be repeatedly cooled down and temperature-stabilized, as described above. In the CdHgTe detectors used in BIRD, TET-1, and BIROS, which were developed in the 1990s, both the signal offset and the sensitivity of the detector elements vary after each cooling process. These variations must be measured on-board radiometrically to avoid spurious artifacts in the processed image data products. Therefore, it is essential for IR sensors operated in space with cooled detectors, especially due to their high radiometric sensitivity, to apply a radiometric on-board calibration procedure, which radiometrically characterizes the sensor system after each cooling process of the detectors and at the same time also considers the current thermal state of the system. With the help of the flap arrangement used in front of the IR sensor heads for radiometric on-board calibration, as illustrated in Figure 1, both detector-specific changes and thermally different states of the overall system (optics, housing, etc.) can be considered. Each of these flaps is equipped with a heated blackbody radiator on its side that closes the objective. The temperature of these blackbodies can be varied via a surface-heating foil mounted on their rear sides. After the detectors have cooled down (see Figure 6), which takes place before each data take in a target area, the flaps are opened by means of stepper motors. The flaps close the optical inputs of the IR sensor heads after the end of each scene data acquisition (also to protect them from contamination), and the radiometric on-board calibration procedure is then performed, since the flaps (and the blackbodies mounted on them) are cooled down to a minimum temperature in the open state, thus enabling the radiometric correctable range of the sensor to have low temperatures. With the aid of the surface-heating foils on the rear sides of the blackbodies, the following procedures are conducted:  
* ramp-like increasing temperature curves are used for the blackbodies;
* continuous measurements of the blackbody radiation by the IR sensors; and
* temperature values of the temperature sensors integrated in the flap are recorded for each sampled measurement step of the IR sensor. These measured temperatures of both flap blackbodies are input variables for the radiometric calibration calculation. The output variables of this calibration procedure are the offset and sensitivity coefficients for each detector element of the MIR and TIR sensors. These coefficients, calculated for each detector element, are used to computationally convert the scene measurement data obtained as digitized raw signal levels into radiance values.