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Remote sensing is a way of collecting and analysing data to get information about an object without the instrument used to collect the data being in direct contact with the object. Remote sensing can involve visual light, infrared, radio waves or any other electromagnetic wave.
Remote sensing can be either passive sensing or active sensing.
Passive sensing involves the use of pure receivers, with no transmitters involved. The radiation sought by these receivers occurs naturally. Of interest are radiation peaks indicating the presence of specific chemicals, or the absence of certain frequencies indicating the absorption of the frequency signals by atmospheric gases. The strength or absence of signals at particular frequencies is used to determine whether specific gases (moisture and pollutants being obvious examples) are present and if so, in what quantity and at what location. A variety of environmental information can be sensed in this manner.
Active sensing, on the other hand, involves both transmitters and receivers. The uses of active sensing vary from measuring the characteristics of the sea surface to determining the density of trees in the rain forest.
Sensing can be done from the earth surface, an airborn platform or a satellite (see EESS).
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