Remote Sensing - a tool for environmental observation

The same is true for crops soils and rock formation

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coniferous trees are much larger in the near-infrared than in the visible part of the spectrum. The same is true for crops, soils and rock formation in specific parts of the spectrum as will be discussed in the following chapters. Figure 1.11 Spectral reflectance curves for vegetation and a number of other objects.
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16 1.6 Sensors and scanners The most well-known type of remote sensing system is the photo camera. The reflected radiation is directly registered on film. However, photographic systems are limited to visible and near infrared spectral range 0.3 to 0.9 μm. Electronic detectors are required to measure radiation outside this range; they extend the potential range of sensing from approximately 0.3 to 14 μm. Sensors Sensors are tools for indirect image registration in contrast to the conventional camera. A sensor or detector is a device that receives electromagnetic radiation, converts it into a (electric) signal and presents it in a form suitable for obtaining environmental information (Curran, 1985). This suitable form is most often a digital format, the intensity of the radiation is expressed in the classes available in a byte or word. This largely facilitates computer processing of sensor- derived information. Some knowledge about the technical operation of a sensor and scanner is important because the airborne or space borne images are sometimes affected by the performance of the system. Patterns or lines in the images caused by the sensor should not be misinterpreted by the environmentalist. Scanners A scanner or a scanning system employs a detector with a narrow field of view which sweeps across the terrain, the parallel scan lines are combined together to produce an image. There are four common methods of scanning (figure 1.12): - cross-track scanner; - circular scanner; - along-track scanner; - side scanning system. The most widely used scanner is the cross-track scanning system. A faceted mirror is horizontally rotated by an electric motor. Constructing images by line scanning relies on the forward motion of the platform (satellite or aircraft) carrying the device. The dimensions of the 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
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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.
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  • Winter '12
  • JOHN
  • Remote Sensing, Electromagnetic spectrum, µm, Infrared

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