Remote Sensing - a tool for environmental observation

Figure 1 a system for electromagnetic remote sensing

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Figure 1: A system for electromagnetic Remote Sensing of the Earth including communication facilities 1.1 A Remote Sensing System A remote sensing system using electromagnetic energy consists basically of four components: 1. A source of electromagnetic energy , this is most often the sun’s reflected energy by the surface of the studied object or the emitted heat by the earth itself, or it is an artificial man- made source of energy such as microwave radar.
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5 2. Atmospheric interaction, electromagnetic energy passing through the atmosphere (sun to earth’s surface and earth’s surface to the sensor) is distorted, absorbed and scattered. 3. Earth’s surface interaction , the intensity and characteristics of electromagnetic radiation reflected from or emitted by the earth’s surface is a function of the characteristics of the objects at the surface and a function of wavelength. 4. The sensor, a man-made sensor is used to register emitted or reflected electromagnetic energy (camera, radiometer, CCD). The characteristics and the quality of the sensor deter- mine how well the radiation can be recorded. This schematic representation of a Remote Sensing system should be kept in mind and will be helpful to understand the later discussions of the performance of certain remote sensing systems. The source of energy utilized for remote sensing observation is used to divide remote sensing systems into passive and active systems: A passive remote sensing system observes the radiation reflected by the object of interest. The energy source of such a system is most often the sun. Examples of passive remote sensing systems are the sensors aboard the Landsat satellites and the SPOT satellites. Active remote sensing systems combine the energy source and the sensor. The artificial energy source emits electromagnetic energy in specific wavelengths towards the object and the sensor registers the reflected radiation. Especially radar systems, such as the Sideways Looking Airborne Radar (SLAR) and SAR (Synthetic Aperture Radar) are most often active systems. Another well-known example of an active remote sensing system is a photo camera with a flash. Figure 1.2 shows the components of an active and passive remote sensing system. 1.2 The electromagnetic spectrum The link between the components of a remote sensing system is electromagnetic energy. Energy is usually transferred from one point to another by conduction, convection and radiation. In remote sensing we are primarily concerned with energy transfer by means of radiation: electromagnetic energy refers to all energy that moves with the velocity of light in a harmonic wave pattern. The wave concept explains how electromagnetic energy propagates or moves. However, this energy can only be detected as it interacts with matter. In this interaction, electromagnetic energy behaves as though it consists of many individual bodies (photons) that have particle-like properties such as energy and momentum (Sabins, 1987). When light bends or refracts as it goes from one medium into another it behaves like a wave. When a radiometer measures the intensity of light, the photons produces an electric signal proportional to the number of photons.
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