Shumbhankar Chatterjee (TE-Elecs-VESIT)
Hear the word remote sensing and the first thing that comes to our head is remote control. Well, the functioning of the remote control and the process of remote sensing do have some similarities. What we do with the remote control is that we operate some device without physically come in contact with that device. In the same way remote sensing is the art and science of deriving information about an object from measurements made at a distance from the object without coming in physical contact with it.
Remote sensing actually deals with the inventory, monitoring and assessment of natural resources through the analysis of data obtained by observations from a remote platform. The observations are synoptic, provide repetitive coverage of large areas and the data is quantifiable. In this context, any force field-gravity, magnetic or electromagnetic could be used for remote sensing, covering various disciplines from astronomy to laboratory testing of materials. However, it is currently being used for identification of earthly features.
Electromagnetic energy is used for remote sensing. Every object reflects/scatters a portion of the electromagnetic energy incident on it depending on the physical properties. In addition, objects emit radiation depending on their temperature and emissivity. The reflectance/eminence of any object at different wavelength follows a pattern which is characteristic of that object known as spectral signature'. Proper interpretation of the spectral signal leads to the identification of the object.
If the observation is made based on the electromagnetic radiation from the sum or the self-emitted radiance, then it is called as Passive Remote Sensing. It is also possible to produce electromagnetic radiation of a specific wavelength or band of wavelengths to illuminate the terrain. The interaction of this radiation can be studied by the sensing the scattered radiance from the target. This is called Active Remote Sensing.
The atmosphere is opaque in most of the regions. In some regions of the electromagnetic spectrum, however it is transparent Remote sensing uses these transparent regions know as windows to avoid the effects of absorption of radiation. The most useful regions are:
The visible (0.4 to 3 Micro M)
The reflected IR (0.7 to 3 Micro M)
The thermal IR (8 to 14 Micro M)
These microwave radiations can penetrate even through clouds. Thus, pictures of the ground can be taken even in rainy season.
The remote sensing system basically consists of a sensor to collect the radiation and platform on which the sensor can be mounted.
The sensors are of 2 types:
1.The active sensors which have a source of energy to illuminate the target.
2. Passive sensors, which use the sun as a source.
For platforms we have many options like aircrafts, balloons, satellites etc.
If aircrafts are used the resolution will be very good but it will prove to be very expensive and the area covered is also very limited.
The main problem with balloons is that they are uncontrollable. Due to the above limitations, satellites are generally used as platforms. The satellites used for remote sensing are polar orbiting at an altitude of about 1000-KM.
The information received by the sensor is suitably manipulated and the transported back to the earth. It may be telemetered as in the case of unmanned spacecraft or brought back through films, magnetic tapes etc. The raw data acquired from satellite has radiometric and geometric distortions. This is corrected by processing the data on computer systems. The corrected data is then put in the required format. The different levels of processing are:
Raw data: No processing is done here and the relevant scene is extracted and put on the media. It is generally available only as digital product.
Partially processed data: It is radiometrically corrected but geometrically uncorrected. This type of data is useful for stereo products.
Standard data: It is radiometrically and geometrically corrected data. Normally all full scene, quadrant data is supplied as standard data.
After the data is processed it can be uses for various purpose and in various fields like seismic geomorphology monitoring of water resources, land use mapping of coastal regulation zone (CRZ) etc.
Thus we have discussed the basics of remote sensing, sensors, platforms and the levels of processing of data.