Remote Sensing - a tool for environmental observation

Figure 115 colour formation on ir colour film

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Figure 1.15 Colour formation on IR colour film (Lillesand & Kiefer. 1994).
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21 Figure 1.14 A three dimensional space is required to described colour. Figure 1.16 A prism is used to split white light into its individual components.
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22 Figure 1.17 Illustration of how a) wavelength (hue), b) intensity and C) saturation of colour interact and mix colours in d).
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23
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24 Chapter 2: Multi-spectral Scanning 2.1 Introduction A multi-spectral scanner is an airborne or space borne remote sensing system that simultaneous- ly acquires images of the same scene at different wavelengths. The field of view (FOV) is the solid angle through which an instrument is sensitive to radiation. The sensors of a multi-spectral scanner are normally working in specific parts of the spectral range from 0.35 up to 14 μm. These specific parts of the spectrum in which remote sensing observation are made, are called bands or channels. The reflected radiation (0.3-2.5 μm) is usually detected by silicon photodiodes that are placed in the correct geometric position behind the prism. The emitted thermal infrared radiation is normally detected by photon detectors, manufactured from mercury doped germanium (Ge:Hg) which is sensitive to wavelengths from 3 to 14 μm, or they are manufactured from indium antimonide (InSb) which is sensitive for the spectral range from 3 to 5 μm, or they are made from mercury cadmium telluride (MCT or HgCdTe) which is sensitive from 8 to 14 μm. The electric signal produced by these photon detectors is theoretically proportional to the radiation energy in the selected spectral band. However, calibration of the signal is always necessary. The number of bands or channels varies largely from system to system. Two important advantages of multi-spectral scanning are: - Objects at the surface of the earth have varying reflection behaviour through the optical spectrum, they can be recognized and/or identified more easily using several spectral bands than using just one band. - A large number of objects do not reflect radiation very well in the visible part of the spectrum. Remote sensing observations outside the visible wavelengths or in combination with observations in the visible spectrum produce a much more contrasting image, which is helpful to identify objects or to determine their condition. 2.2 Meteorological Satellites and their Sensors NOAA-AVHRR The first satellite for remote sensing observations was launched in 1960 and was called TIROS: Television and Infrared Observation Satellite. It was developed for meteorological purposes by the National Oceanic and Atmospheric Administration (NOAA) of the USA. This early weather satellite sent down rather coarse views of cloud patterns. NOAA refined the sensors and the orbits of TIROS and has continued and will continue to launch weather satellites. One of their most famous series of satellites among environmentalists is the NOAA-AVHRR (NOAA- AVHRR 12 was launched on 14 may 1991). AVHRR stands for Advanced Very High Resolution Radiometer and was also built for meteorological observation. It has a near-polar, sun-synchronous orbit at an altitude of approximately 830 k. Due to its large field of view, its
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