PASSIVE AND ACTIVE SENSORS
Remote sensors are the instruments which detect various objects on the earth's surface by measuring electromagnetic energy reflected or emitted from them. The sensors are mounted on the platforms discussed above. Different sensors record different wavelengths bands of electromagnetic energy coming from the earth's surface. As for example, anordinary camera is the most familiar type of remote sensor which uses visible portion of electromagnetic radiation.
Classification of Sensors
Remote sensors can be classified in different ways as follows.
1. On the Basis of Source of Energy Used: On the basis of source of energy used by the sensors, they can be classified into two types - Active sensors and Passive sensors.
2.3.1ACTIVESENSORS: Active sensors use their own source of energy and earth surface is illuminated by this energy.
Thenapartofthis energy is reflected back which is received by the sensor to gather information about the earth's surface (Fig.3).
When photographic camera uses its flash, it actsasan active sensor. Radar and laser altimeterare active sensors. Radar is composed of a transmitter and a receiver. The transmitter emits a wave, which strikes objects and is then reflected or echoed back to the receiver. The properties of an active sensor are: 1) It uses both transmitter and receiver units to produce imagery, hence it requires high energy levels. 2) It mostly works in microwave regions of EMR spectrum, which can penetrate clouds and is not affected by rain. 3) It is an all weather, day-night system and independent of solar radiation. 4)The RADAR signal does not detect colour information or temperature information, but it can detect the roughness, slope and electrical conductivity of the objects under study.
2.3.2. PASSIVE SENSORS: Passive sensors do not have their own source of energy. The earth surface is illuminated by sun/solar energy. The reflected solar energy from theearthsurfaceortheemittedelectromagneticenergybytheearthsurfaceitself is received by the sensor (Fig.3). Photographic camera is a passive sensor when it is used in sunlight, without using its flash. The properties of a passive sensor are:
1)It is relatively simple both mechanically and electrically and it does not have high power requirement.
2) The wavebands, where natural remittance or reflected levels are low ,high detector sensitivities and wider adiation collection apertures are necessary to obtain areasonable signal level. Therefore, most passive sensors are relatively wide band systems. 3) It depends upon good weather conditions.
2. On the Basis of Function of Sensors: On the basis of function of sensors, they are divided into two main types - Framing System and Scanning System.
a. Framing system: Inframing system, two dimensional images are formed at one single instant. Here, alensisused together the light which is passed through various filters and then focused on a flat photo sensitive target. In ordinary camera, the target is film emulsion, where asinvidicon camera, thetargetis electrically charged plate.
b. Scanning System: In scanning system, asingle detector/anumberofdetectors with specific field of view, is used which sweeps across a scene in a series of parallel inesand collect data for continuous cellsto produce an image. Multi Spectral Scanner, Microwave Radiometer, Microwave Radar, Optical Scanners are few examples of scanning system sensors.
3. On the Basis of Technical Components of the System: The sensors can be classified into three categories on the basis of technical components of the system and the capability of the detection. Theseare:1)Multispectralimagingsensorsystems,2)Thermalremote sensing systems, and 3) Microwave radar sensing systems. The multispectral or multiband imaging systems may use conventional type camers or may use a combination of both cameras and scanners for various bands of electromagnetic energy. As for example, Return Beam Vidicon (RBV) sensor of Landsatuses both photographic and Scanning systems, which is similar to an ordinary TV camera. The thermal system uses radiometers, photometers, spectrometers, thermometers to detect the temperature changes where microwave sensing systems use the antenna arrays for collecting and detecting the energy from the terrain elements.