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Remote Sensing - a tool for environmental observation

Ground resolution cell(or pixel are determined by the

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Unformatted text preview: ground resolution cell (or pixel) are determined by the detector instantaneous field of view IFOV (the field of view of the detector at one moment) and the altitude of the scanning system. The along-track scanner does not use a mirror, instead it has an individual detector for each ground resolution cell across the ground swath. The side-scanning system is mostly an active system, which provide its own energy source, and is used for radar or microwave systems. A more extensive description of scanners can be found in Lillesand and Kiefer (1994) or Sabins (1987). Multi-spectral scanner The previous described systems record basically a single image that represents a single spectral band (one single portion of the entire spectrum). For many remote sensing applications, it is necessary to record a scene with multi-spectral images (multiple images showing different parts 17 of the spectrum). The basic principle of a multi-spectral scanner is the same as for one cross- track scanners (figure 1.13): 1. 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. 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....
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