Remote Sensing - a tool for environmental observation

Consequently remote sensing observations are limited

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Consequently, remote sensing observations are limited to the parts of the atmosphere that transmit radiation (table 1.1). These parts of the spectrum are called atmospheric windows . The most important atmospheric windows for remote sensing are: Visible region: 0.4 - 0.7 μm Infrared 0.7 - 2.5 μm Thermal infrared 3.0 - 5.0 μm Thermal infrared 8.0 - 14.0 μm Microwave (radar) 0.01 - 1.0 m Figure 1.6 Atmospheric windows in the visible, infrared and microwave regions of the spectrum. Wavelengths bands of commonly used remote sensing systems are shown.
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11 Figure 1.7 Atmospheric absorption and atmospheric scattering of electromagnetic radiance by particles in the atmosphere. Scattering Atmospheric scattering results from interactions between radiation and gases and particles in the atmosphere (figure 1.7 and table 1.1). Two types of scattering are recognized : selective scattering and non-selective scattering . In non-selective scattering all wavelengths of light are equally scattered. Non-selective scattering is caused by particles (dust, clouds and fog) much larger than the energy wavelengths being sensed. Water droplets with a diameter of 5 to 100 μm cause for example non-selective scattering. In selective scattering, the shorter wavelengths of ultraviolet and blue are scattered more severely than the longer wavelengths. Selective scattering is caused by smoke, fumes and by gases such as nitrogen, oxygen and carbon dioxide. Examples of selective scattering are Rayleigh scatter and Mie scatter. Rayleigh scatter is the most common type of scattering and affects the short visible wavelengths. It results in haze. Rayleigh scatter is caused by particles in the atmosphere much smaller than the wavelength of radiation (< 0.1 λ ). A blue sky is a result of Rayleigh scatter. Various types of atmosphere and their degree of Rayleigh scatter are shown in figure 1.8. Mie scatter is caused by particles in the atmosphere with the same size as the wavelength of radiation e.g. water vapour and dust.
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12 Figure 1.8 Relative scatter as a function of wavelength for various levels of atmospheric haze. Table 1.1 Types of atmospheric scattering in order of importance (Curran, 1985).
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13 1.5 Interactions mechanisms with the earth surface Environmental researchers using remote sensing techniques are mainly interested in the interaction mechanisms between radiation and the objects at the surface of the earth (except of course when the atmosphere and its constituents are the research topic e.g. for ozone or CFK concentration or aerosol studies). Atmospheric interference makes things only difficult for people interested in land applications of remote sensing. Electromagnetic energy that encounters matter, whether solid, liquid or gas, is called incident radiation. Interactions with matter can change the following properties of the incident radiation: - intensity; - direction; - wavelength; - polarization; - phase.
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