23: Basics of Remote Sensing 244 BASICS OF REMOTE SENSING Shibendu Shankar Ray Mahalanobis National Crop Forecast Centre, Department of Agriculture & Cooperation, Krishi Vistar Sadan, Pusa Campus, New Delhi – 110 012, INDIA From a general perspective, remote sensing is the science of acquiring and analyzing information about objects or phenomena from a distance (Jensen, 2000, Lillesand and Keifer, 1987). However, conventionally, Remote Sensing (RS) refers to the identification of earth features by detecting the characteristic electromagnetic radiation that is reflected/emitted by the earth surface. The sensors on-board various platforms detect the radiation received from the targets in different spectral regions. Compared to conventional monitoring from the ground, the advantages of satellite remote sensing are:  capability to achieve a synoptic view,  potential for fast survey,  capability of repetitive coverage to detect the changes  low cost involvement  higher accuracy  use of multispectral data for increased information,  inaccessible area coverage  all weather / day and night capability  simultaneous observations from a single platform at different resolutions / angles, spectral regions over land, atmosphere and oceans 1. COMPONENTS OF REMOTE SENSING Though the methods for collection, processing, and interpretation of remotely sensed data are very diverse, imaging systems have the following essential components (Panigrahy and Ray, 2006): 1.1 Energy Source or Illumination The first requirement for remote sensing is to have an energy source, which illuminates or provides electromagnetic energy to the target of interest. Sensors can be classified as passive or active, based on the energy source they are using. Sensors, which sense natural radiations, either emitted or reflected from the Earth, are called passive sensors. Most of the remote sensing sensors are passive in nature, which measure the solar radiation reflected from the target. On the other hand, the sensors which produce their own electromagnetic radiation, are called active sensors (e.g. LIDAR, RADAR). 1.2 Interaction with the Target