Remote Sensing : Definition, Classification, Methods

Remote Sensing Definition

Remote sensing is broadly defined as science and information about objects, area of phenomenon from distance without being in physical contact with them. Remote Sensing is the process of obtaining information about an object, terrain or phenomenon through the analysis of data acquired by a sensor device without actual physical contact with that object, terrain or phenomenon.
In the present context, the definition of remote sensing is restricted to mean the process of acquiring information about any object without physically contacting it in any way regardless of whether the observer is immediately adjacent to the object or millions of miles away.
Human eye is perhaps the most familiar example of a remote sensing system. In fact sight, smell and hearing are all rudimentary forms of remote sensing.
However, the terms remote sensing is restricted to methods that employ electromagnetic energy (such as heat, microwave) as means of detecting and measuring target characteristic. Air craft and satellites are the common platforms used for remote sensing.
Collection of data is usually carried out by highly sophisticated sensor (i.e. camera, multispectral scanner, radar etc.) The information carrier or communication link is the electromagnetic energy. Remote sensing data basically consists of wave length intensity information by collecting the electromagnetic radiation leaving the object at specific wavelength and measuring its intensity.
Photo interpretation can be considered as the primitive form of remote sensing. Most of the modern remote sensing methods make use of the reflected infrared bands, thermal infrared band and microwave portion of the electromagnetic spectrum.
The sensors operating from a convenient platform emit and reflect electro-magnetic energy from the features of the earth’s surface to acquire necessary data in pictorial or in digital form. The data thus censored is examined by various viewing and interpreting devices to compile maps, tables and computer files.
They will be merged with reference data and geological information system so that the final information becomes user friendly to facilitate decision making.

Classification of Remote Sensing:

Remote sensing is broadly classified into two categories. They are Passive remote sensing and Active remote sensing.

Passive Remote Sensing : It is used as an existing source of EM energy and records the energy that is naturally radiated and or reflected from the objects.

Ex: Sun rays
Line scanners are passive systems that employ a rotating system to scan successive strips of ground along the track of the aircraft or satellite.
Active Remove Sensing : It uses its own source of EM energy, which is directed towards the object and return energy is measured. This is the essence of RADAR (Radio Detection and Ranging System)

Idealized Remote Sensing System

An idealized remote sensing system consists of the following stage :
  • Energy source.
  • Propagation of energy through atmosphere.
  • Energy interaction with earth’s surface features.
  • Air borne/space borne sensors receiving the reflected and emitted energy.
  • Transmission of data to earth station and generation of data produce.
  • Multiple – data users.

Basic Principles of Remote Sensing : (a)

Electromagnetic Energy :

It is a form of energy that moves with the velocity of light (3 x 108 m/ sec) in a harmonic pattern consisting of sinusoidal wave, equally and repetitively spaced in time. It has two fields : (i) Electrical field and (ii) Magnetic field, both being orthogonal to each other.

The sun is the most obvious source of electro-magnetic radiation for Remote Sensing. However all matter at temperatures above the absolute zero (-273°C) continually emits electro-magnetic radiation. The total energy emitted by an object varies as the fourth power of the absolute temperature of its surface.
An ideal radiator is that which totally absorbs and re-emits all energy incident on it. It is also called a black body. It is a hypothetical one. Electro-magnetic energy radiates in many form such as visible light, radio waves, ultra-violet rays, x-rays etc.
There are three theories available to explain the radiation viz.
  • Wave theory.
  • Quantum theory or Particle theory.
  • Weins Displacement Law.

1. Wave Theory :

Electro-magnetic energy is assumed to consist of photons which have particle like properties such as energy and momentum. They move with the speed of light describing simple harmonic motion i.e sinusoidal waves. If ‘C’ is the velocity, ‘λ’ is the wave length i.e, distance between successive peaks of the wave and ‘f’ is the frequency i.e, the no. of cycles done per unit time, wave theory slates that
c = f . λ
Since velocity of light is constant equal to 3 x 108 m/ sec, frequency and wave length will be inversely proportional to one another. Frequency is measured in Heertz, Kilo Heertz etc.
  • 1 Hertz (Hz) = 1 cycle/second
  • 1 Kilo Hertz (kHz) = 10³ Hz
  • 1 Mega Hertz (MHz) = 106 Hz
  • 1 Nano Hertz (NHz) = 109 Hz
  • 1 Giga Hertz (GHz) = 1012 Hz

2. Quantum Theory or Particle Theory

According to this theory, Electro-magnetic energy consists of photons or quanta whose energy is proportional to its frequency.
Q = h.f 
Q = energy of quantum in Joules ,
h = Planck’s constant 6.626 x 1034 Js
f = frequenc in Hz
But from equation of wave theory, frequency is inversely proportional to wave length. Hence it will be obvious that energy levels will be inversely proportional to the wave lengths. Hence lower levels of energy correspond to longer wave lengths and vice versa. Thus if the surface features of the earth emanate longer wave lengths, it will be more difficult to read their radiation that in the case of short wave length radiations.

3. Wien’s Displacement Law

The dominant wave length or the wave length at which a black body radiation curve reaches a maximum is related to its temperature by wein’s displacement law. , =
λm = wave length or maximum spectral radiant existence (μm)
A = 2898 μm k
T = Temperature (°k)

Electromagnetic Spectrum:

Although visible light is the most obvious manifestation of EM radiation, other forms also exist. EM radiation can be produced at a range of wave lengths and can be categorized according to its position into discrete regions which is generally referred to electro-magnetic spectrum.
The electromagnetic spectrum is the continuum of energy that ranges from meters to nano-meters in wave length travels at the speed of light and propagates through a vacuum like the outer space (Sribine 1986)
The electro-magnetic spectrum has a very wide range of wave lengths ranging from as small as 10-11 m and less in the case of gamma rays going upto radio wave, having more than 0.1 m passing along x-rays, ultra violet rays, visible portion, infra red rays and micro waves.
The visible portion of the spectrum is an extremely small band of wave length from 0.4 to 0.7 micrometer. In this portion the maximum radiation occurs at 0.5 micrometer. This wave length is most useful in photogrammetry.
The various regions of wave lengths of electro-magnetic spectrum are shown in fig. To provide a realistic view, instead of wave lengths, their logarithms are taken on the horizontal axis.
remote sensing
Most of the sensing systems operate in one of the several visible, infrared or microwave portions of the electro-magnetic spectrum. In below figure the expanded portion of the spectrum useful in Remote Sensing is shown.
regions of electromagnetic spectrum usefull in remote sensing
In the 0.7 to 3 micro-meters wave length region, sensing can be made using infra-red sensitive films. In the invisible portion of 3 to 5 micro-meters wave length of thermal infra-red band the radiated energy cannot be sensed by photographic emulsion.
Special sensor devices like crystal detectors have to be used to detect the signals. The region of wave lengths from 1 mm to 300 mm are used in Radars. Remote sensing systems which supply their own sources of energy i.e, man-made sources to illuminate the features of the earth interested in sensing are known as ‘Active systems’ of Remote Sensing. Eg, Radars, camera with flash bulbs.
The kinds of Remote sensing systems which make use of naturally available energy only are called ‘Passive systems’. Eg: Camera flash bulbs using only sun-light, the portions of the spectrum which have high transmission of electro-magnetic radiations are known as ‘atmospheric windows’. Such wave lengths produce good images in Remote sensing.
The maximum spectral radiant existence the earth’s surface features occurs at a wave length of 9.7 micrometers in the thermal infra-red portion. In this portion, thermal infra-red systems can function round the clock.

Method of Remote Sensing

The currently used methods of remote sensing are as follows :
  • Photogrammetry and Aerial Photogrammetry including interpretation Aerial Photography.
  • Thermal and Multispectral Scanning.
  • Microwave Sensing
  • Earth Resource Satellites .

Photogrammetry :

This method will enable in obtaining reliable measurements and maps of earth’s features. This method is mostly used in preparing topographic sheets besides application in forestry, geography, geology, planning, soil science etc.

Thermal and Multi Spectral Scanning :

In this method, sensing is done in the thermal infr- red band of 3-14 micrometers wavelength and radiant temperatures are measured by the sensors. One multi-spectral scanning, a wider range of the spectrum from the photographic to the thermal band can be made use of.

Microwave Sensing :

This method of sensing enables securing very valuable environmental and mineral resources information using the microwave band of the spectrum. These sensors are capable of working in adverse weather conditions such as haze, light rain, snow, clouds, smoke etc.
Radar is an active microwave sensor. While microwave radiometer is a passive sensor. LIDAR (Light Detection and Ranging) uses laser light.

Earth Resources Satellites :

This is the latest technique used to study the earth from space. Sensors are borne in the space to assist in the weather and crop forecast, mineral exploration, detection of pollution, commercial fishing etc. LANDSAT’s of the USA and INSAT series of India are examples of this kind of remote sensing techniques.

Applications of Remote Sensing

Remote Sensing pervaded almost all types of modern human activity. Its applications are not restricted to surveying and engineering but found in various fields.
  • Environmental Applications : for weather prediction, pollution control and management, profiling the atmospheric conditions like pressure, temperature, content of water vapor, measurement cf wind velocity etc.
  • Mineral Exploration : Locating and detailing mineral wealth and providing basic geological data.
  • Agricultural Applications : for assessing land use and land cover, forestry of monitor the extent and type of vegetation cover, its state of health, mapping soil types, forecasting crop yield, erosion of soil etc.
  • Applications in Disaster Control and Management : for detection of earthquakes, land slides, volcanic eruptions, floods and assessing the extent of damage suffered due to these causes etc.
  • Archaeological Applications : for recognizing pre-historic sites of civilization etc.
  • Military Application : to monitor movement of vehicles, military formation and assessing the terrain.
  • Hydrological Application : for assessing water resources, forecasting run-off etc.

Remote Sensing In India

India is not lagging behind the leading countries of the developed world in taking giant steps forward in Remote Sensing. The ISRO, the Indian Space Research Organization, launched numerous satellites from 1975 till now, the latest being the PSL V-C14 (Prithvi Satellite Launching Vehicle) on September 23, 2009 and the Prithvi-II Satellite in October 2009.
The other satellites worthy of mention are the IRS series, Bhaskara Series, Rohini Series etc. All these provided very valuable information enabled modernizing  the communication and broad casting network too.

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