The technique of collecting information about an object from a distance, without making a physical contact with that object, is called remote sensing. Remote sensing is another area of application of space science which is accomplished by the use of remote-sensing satellites. The remote-sensing satellites are placed in sun-synchronous orbits around the earth. The orbits of remote sensing satellites are such that the satellite always passes over a particular area of the earth at approximately the same local time. In other words, it means that whenever a satellite passes over a particular area of the earth, then the position of the sun with respect to that area of the earth remains approximately the same.This arrangement enables the remote sensing satellite to take photographs of a particular area of the earth with nearly the same illumination every time it passes over that area.
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The most significant feature of remote-sensing satellite technology is that it makes possible the repeated survey of vast areas in a very short time even if the area is otherwise inaccessible. The photographs and other data collected by Indian remote sensing satellites, IRS-IA and IRS-IB, have been used for many practical purposes. The important applications (or uses) of remote-sensing satellites are : Groundwater surveys; Forest surveys; Preparing wasteland maps; Drought assessment; Estimation of crop yields; Detection of crop diseases; Survey for detecting coal, oil and ores; and Detection of potential fishing zones of the sea. The remote sensing satellites are also used for doing “spying work” for military purposes.