# Small Satellite Tools for High-Resolution Infrared Fire Monitoring
## 3 The FireBIRD Main Payload
### Bi-spectral Sensors of TET-1 and BIROS
#### 3.1.4 Radiometric Processor of the BIROS Bi-Spectral IR Sensor  
For availability and cost reasons, for TET-1 and BIROS, IR detector line arrays identical to those in BIRD were used. The sensitivities of the individual detector elements of these so-called \"staggered linear arrays\" of type E3437 (MIR) and E3438 (TIR) vary strongly because they correspond to the technological state of the art of the CdHgTe detector technology of the second half of the 1990s. Therefore, in order to obtain scientifically usable and high-resolution radiometric image data from the bi-spectral IR sensors, the raw scene data must be converted into radiance values using the detector-element-related offset and sensitivity coefficients, i.e., for each detector element of the staggered linear arrays used in the IR sensor, obtained from the radiometric calibration measurements:  
1. in the laboratory from external blackbody measurements; and
2. on board with blackbody measurements using the closed flaps. As before for TET-1, for BIROS an improved radiometric processor was implemented, which essentially performs the following tasks:  
1. Reading in and checking the consistency and completeness of the calibration data, the scene raw image data, and the \"House Keeping (HK)\" data;
2. Determining the detector-element-related offset and sensitivity coefficients from the on-board calibration measurements for the MIR and TIR CdHgTe lines;
3. Calculating radiance values from the in-flight scene measurements for each detector element, including those elements where the signal level at the exposure with integration time Tint-com is in the saturation range, and, therefore, a second exposure with a shortened integration time is automatically performed within the same sampling period, and then--by \"on-board data handling (OBDH)\" --appending these digitized pixel samples of the so-called hot pixels to the data from the first exposure (at the end);
4. Determining insensitive detector elements, so-called \"dead pixels\"; and
5. Detecting and correcting (residual) striping artifacts. The procedures described here, which run automatically in the radiometric processor of the BIROS bi-spectral sensors, were tested and further developed in the commissioning phase of BIROS (2016/2017) on numerous scenes with different mean temperatures. This applies in particular to the procedures for reducing (residual) striping structures, which were iteratively and successively improved by optimizing the operating points of the detector lines with the help of so-called \"software uploads\" during the commissioning phase of BIROS. Figure 7 shows BIROS IR image fragments of a normal-temperature scene around Mexico City from 5 May 2017, namely: (in the upper row) the image fragments from the BIROS TIR band; and (in the lower row) the image fragments from the BIROS MIR band. 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 including a reconstruction of the \"dead columns\" are shown on the right. The striping structures, illustrated in Figure 7, are caused, among other things, by strongly deviating properties and instabilities of certain pixels or even pixel clusters (noise, sensitivity, periodic and spontaneously occurring disturbing signals), which are caused by detector technology. Thus, in addition to the first radiometric processing step, namely the conversion of the digital raw image data into radiance values, the algorithmic recognition and reduction of these interfering structures constitute an important second step in the radiometric calibration process of the FireBIRD IR data. Compared with the TET-1 radiometric data correction, the radiometric processor of BIROS uses a further development of the so called \"striping correction\".