Remote Sensing - a tool for environmental observation

2 the rotating mirror powered by an electric motor

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A telescope directs the radiation onto the rotating mirror. 2. The rotating mirror (powered by an electric motor) reflects the radiation passing through the telescope into the optics (the mirror is set at an angle of 45 ° which allows it to scan the terrain from side to side). 3. The optics focus the radiation into a narrow beam. 4. A refracting grid splits the narrow beam into its reflected and emitted components. The emitted radiation goes directly to the thermal infrared detectors. 5. A prism is placed in the path of the reflected radiation to divide it into its spectral components. 6. The detectors sense the radiation, then the signal is amplified and passed in electronic form to a device to store the data (tape unit, hard disk etc.). Most multi-spectral scanning systems scan multiple lines at a time (e.g. 6 lines for the Landsat MSS and 16 lines for the Landsat TM). Figure 1.12 Scanning systems for acquiring remote sensing images.
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18 Figure 1.13 Diagrammatic representation of a multi-spectral scanner (Curran, 1985). The construction and the technical performance of a remote sensing system determines whether objects can be detected and/or identified in an image. Detectability is the ability of an imaging system to record the presence or absence of an object. Recognizability is the ability to identify an object, the latter requires a skilled interpreter. Hence, the design of a remote sensing system is a complex task and a balance has to be found between the following points: - the spectral resolution, the band width or spectral regions of the sensors aboard the system; - the radiometric resolution or intensity resolution, the range of reflectance values that must be registered and the number of grey levels that must be distinguished. - photometric accuracy, the sensitivity of the sensors should be the same (or at least comparable) in all wavelengths; - spatial resolution, the pixel size (pixel = acronym for picture element) should match with the size of the objects to be detected. - geometric accuracy, the image should have the least possible systematic and stochastic geometric distortions. - temporal resolution, the frequency of image acquisition should match with the temporal changes of the objects of interest. Apart from the above-mentioned considerations, the following points are important: the position of the atmospheric windows and the spectral region in which the source of electromagnetic
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19 energy emits. 1.7 Colour technology The first reconnaissance or the interpretation of remote sensing images is most often performed with the human eye. Apart from grey tones, texture and patterns, colour and colour patterns play an important role in the interpretation. Therefore, it is important to have some knowledge of colour, mixing of colours and colour film technology. The average human eye can discriminate many more shades of colour than it can tones of grey. Hence, the use of colour increases largely the interpretability of images.
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