## 2 SITHON
### Imaging payload  
#### 2.3.1 Camera  
The integration of the camera was simplified due to existing design elements of the plane. For instance, the aircraft was equipped with a gyro-stabilized camera mount, on which the infrared camera body was mounted and fixed. The Thermovision R 570 camera used is a light-weight high performance uncooled forward looking infrared camera. It uses advanced uncooled Focal Plane Array (FPA) micro-bolometer technology and stores images and data to memory cards. The detector consists of a cavity with 77 760 microbolometers made of Si-elements coated with a thermal resistance layer. The microbolometers are thermal detectors and are operated without cooling. But besides the thermal radiance of the target, the radiation of internal camera components may be detected by the uncooled detector arrays, and this can decrease the measurement accuracy. To tackle this problem the inside case temperature is measured at several locations and these measurements are used in the automatic re-calibration of the camera at predefined time intervals. In addition, in order to avoid any convective heat transfer to the detector elements, the bolometers are placed in a vacuum chamber and their temperature is kept close to the ambient temperature using Peltier elements. Although the response time of bolometers is remarkably longer than quantum detectors, an image frequency of 60 Hz is achievable with this system. The absence of moving parts makes the operation of the Thermovision R 570 IR-camera of AGEMA more comfortable and safe. Bolometers are not selective with respect to the spectral range, therefore optical filters are used with this camera to optimise response at a spectral range of 7.5-13. The camera optical system is using lenses with a focal length (_f_) of 20 mm. The technical characteristics of Thermovision R570 infrared camera are summarized in Table 1. The camera has been calibrated in the laboratory to resolve fire intensities up to 500. For this range of temperatures discrimination was approximately 1.0  per digital count with a systematic shift (bias) of 6. This was considered effective for the purposes of SITHON project as the interest was focused on mapping relative temperature differences between points rather than estimating absolute temperatures. In order to recover precisely the camera's internal characteristics, that are the coordinates of the principal point _(x, y)_ and the principal distance _(c)_, the camera underwent an analytical 3-D calibration (bundle adjustment with self calibration), using the 3-D calibration test field of the Laboratory of Photogrammetry [shown in Figure 2(a)] belonging to the School of Rural & Surveying Engineering (National Technical University of Athens). The resulted interior camera orientation after extensive tests [an example is shown in Figure 2(b)], is reported in Table 1. In order to facilitate the image ortho-rectification calculations during the operations, the interior orientation parameters _x, y_p and camera constant \(c\), have been expressed in pixel units with respect to FPA dimensions (320x240 pixels). The best value for the calibrated camera constant \(c\) was estimated to be 763,871 pixels, which is equivalent to a focal length of 19,86 mm, given that the physical dimension of pixel is 26 \(\upmu\)m. This calibration procedure is repeated annually prior to fire season and camera deployment. The forward motion of the aircraft during camera operation may result in a blurring effect of the acquired image. The image blur parameter is defined as:  
\[blur=V\frac{f}{H}\Delta t\,\cos\theta \tag{1}\]  
where \(V\) is the velocity of the platform (aircraft), \(f\) is the camera focal length, \(H\) is the flight height above ground level, \(\Delta t\) is the time for exposure and \(\theta\) is the off nadir angle. The SITHON imaging system has been designed as to avoid blurring effects on the acquired images. Taking as example the height and aircraft velocity conditions during SITHON demonstration, mounted on a CESSNA 310Q