*Concepts of Remote Sensing, EMR, Electromagnetic Spectrum, Remote Sensing Systems, Interaction of EMR with matter, atmosphere

Unit I

*Definition of Remote SensingMost prominent technique of collecting information from a distance. The data collected from a distance is called remotely sensed data.Definitions:It is the science of acquiring information about the earths surface without actually being in physical contact with the surface.It can also be defined as the science or technology by which the characteristics of objects of interest can be identified, measured or analyzed without direct contact.It is the science and art of obtaining information about an object, area, or phenomena through the analysis of data acquired by a device that is not in contact with the object, area or phenomena under investigation.It is the science and art of collecting information about objects, area or phenomena from distance without being in physical contact with them.

*Best example of Remote Sensing is the human eye.The term remote sensing was first used in the US in the 1960s. With the introduction of Landsat I, the first observation satellite launched in 1972, remote sensing became widely used.Source of Remote Sensing data is the Electromagnetic radiation reflected or emitted from an object. The characteristics of objects can be determined using reflected or emitted EMR from the object.According to laws of nature, Each object has its own unique and different characteristics of reflection or emission, depending upon different environmental conditions. Every object will reflect out energy in some specific wavelength. This concept is applied in Remote Sensing to identify objects.Therefore, Remote Sensing is the technology to identify and understand the object or the environmental condition, through the uniqueness of the reflection or emission.Thus, basically Remote Sensing data consists of collecting information on that specific wavelength of EMR leaving the object and measuring its intensity.

Concepts of Remote Sensing

*Device to detect the EMR reflected or emitted from an object is a Sensor. Eg: Cameras, Scanners, Radar.Vehicle to carry the sensor is called a platform. Eg. Aircrafts or satellites.Remote Sensing includes studying aerial photography and satellite imagery to identify objects. Now-a-days, most natural resource mapping is done using remote sensing.Aerial photography has been used to produce topographic maps such as forest maps, geological maps, land use maps and soil maps; detailed city maps. (easy for upgradation; these are formed fast).Satellite-based systems can measure phenomena that change continuously over time and cover large and inaccessible areas.Remote Sensing is similar to normal study of photographs, with the only difference that large scale photographs and images are analyzed.

Concepts of Remote Sensing (Continued)

*Idealized Remote Sensing System13265471. Energy Source2. Propagation of energy through atmosphere3. Energy interaction with earths surface features4. Airborne/space borne sensors receiving reflected and emitted energy5. Transmission of data to earth station and generation of data produce6. Processing, interpretation and analysis7. Application by users

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*Electromagnetic RadiationSource of Remote Sensing data.Is a carrier of EM energy and has the characteristic of both wave motion and particle motion.Characteristics as Wave motion: EMR can be considered as a transverse wave with an electric field and a magnetic field, located at right angles to each other.It travels in a vacuum of free space with the speed of light, c ~ 3 x 108 m/s.It is characterized by the equation c =

Wavelength in mFrequency in Hz

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*Characteristic in Particle motion:Electro-magnetic can be treated as a photon or a light quantum. The energy E is expressed E = h where h = Planks constant. = Frequency in Hz.Electromagnetic Radiation (Continued)

*Principle of Remote SensingRemote Sensing employ EMR and to a great extent relies on the interaction of EMR with the matter. It refers to the sensing of EMR, which is reflected, scattered or emitted from the object.The main principle underlying Remote Sensing is All matter reflects, absorbs, penetrates and emits EMR in a unique way.Why does a leaf look green? Its because the chlorophyll in the leaf absorbs blue and red spectra and reflects the green spectrum.This unique characteristic of matter is called its Spectral Characteristics.All matters are composed of atoms and molecules with a particular composition. Therefore, all matter will emit or absorb EMR at a particular wavelength with respect to the inner state.

*From laws of nature, mainly 3 forms of interactions taking place based on which we are able to identify objects present on the surface of the earth. They are1. Absorption (A)2. Transmission (T)3. Reflection (R)What you are interested is only the final reflected energy from the matter after all the above processes.

The proportion of each of the above will vary depending on the wavelength of energy incident and the material, composition as well as the condition of the matter.Depending on that, matter will reflect EMR at a particular wavelength, which is thereby captured to help in identifying the matter from a set of several objects.Eg: How are you able to identify different coloured chalks.

Principle of Remote Sensing (Continued)

*Electromagnetic SpectrumAlthough Visible light is the most popular form of EMR, other forms also exist, that help in identifying objects that visible light cannot achieve.That many wave forms including the Visible light form part of what is known as Electromagnetic Spectrum.

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*Major regions of the electromagnetic spectrum.

Region NameWavelengthCommentsGamma Ray< 0.03 nanometersEntirely absorbed by the Earth's atmosphere and not available for remote sensing.X-ray0.03 to 30 nanometersEntirely absorbed by the Earth's atmosphere and not available for remote sensing.Ultraviolet0.03 to 0.4 micrometersWavelengths from 0.03 to 0.3 micrometers absorbed by ozone in the Earth's atmosphere.Photographic Ultraviolet0.3 to 0.4 micrometersAvailable for remote sensing the Earth. Can be imaged with photographic film.Visible0.4 to 0.7 micrometersAvailable for remote sensing the Earth. Can be imaged with photographic film.Infrared0.7 to 100 micrometersAvailable for remote sensing the Earth. Can be imaged with photographic film.Reflected Infrared0.7 to 3.0 micrometersAvailable for remote sensing the Earth. Near Infrared 0.7 to 0.9 micrometers. Can be imaged with photographic film.Thermal Infrared3.0 to 14 micrometersAvailable for remote sensing the Earth. This wavelength cannot be captured with photographic film. Instead, mechanical sensors are used to image this wavelength band.Microwave or Radar0.1 to 100 centimetersLonger wavelengths of this band can pass through clouds, fog, and rain. Images using this band can be made with sensors that actively emit microwaves.Radio> 100 centimetersNot normally used for remote sensing the Earth

*Electromagnetic Spectrum (Continued)EMR regions used in Remote sensing are UV (0.3 0.4m), visible light (0.4 0.7m), near infrared and thermal infrared (0.7 - 14m) and microwave (1mm 1m).

Wavelength regions and their application in Remote Sensing

RegionWave length (m)Principal Application(a) Visible RegionBlue0.45 0.52Soil and Vegetation discrimination, Coastal studiesGreen0.52 0.60Rock and soil discrimination, Water Turbidity studiesRed0.63 0.69Plant species discrimination(b)Infrared RegionNear IR0.76 2.35Vegetation discrimination, Delineation of Water features, Vegetation and Soil moisture content, Geological featuresThermal IR3 14Fires, Volcanoes studies, Thermal Sensing(c) Microwave1mm 1mDistribution of snow, Water vapour, Distribution of rain, rain type

*Types of Remote SensingBased on Type of energy sourcePassive Remote SensingSource is the sun.Makes use of sensors that detect the reflected or emitted EMR from natural sources.Active Remote SensingMakes use of sensors that detect reflected responses from objects that are irradiated from artificially generated energy source (Eg. Radar).Based on Wavelength regionsVisible and reflective IR Remote SensingSource is the Sun.Makes use of sensors that are sensitive to the visible and IR region of the EM spectrum.Depends on reflectance of objects on the ground surface, i.e., influenced by solar reflection than emission from the ground.Affected by atmospheric obstacles like dust, smoke, clouds etc.Cannot be used in the night.

*Thermal IR Remote SensingThe source of radiant energy used in Thermal IR remote sensing is the object itself.Under normal temperature conditions, every objects emits radiation.Thus, this remote sensing makes use of sensors sensitive to thermal energy, i.e., emission from grounds surface dominates.Can be used in the night.Microwave Remote SensingUsed to measure MW radiation emitted from objects.High penetration power, thus, Less affected by atmospheric obstacles, scattering, cloud etc.Very useful in studying studying snow, rain, soil moisture as it cannot penetrate them.It can help in determining the exact location of storm centres, measuring the speed of the storm, and notifying the public of areas of potentially severe weather.All weather remote sensing.Example: Precipitation radar, Doppler radar.

Types of Remote Sensing (Continued)

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*Interaction of EMR with matter :Spectral SignaturesAll objects on the surface of the earth have spectral signatures.Spectral signature is the distinctive set of distinguishable characteristics of the object.It is defined by quantifying the reflectance characteristics of earth surface features by measuring the portion of incident energy that is reflected. This is known as spectral reflectance, given by the expression:() in percentage = ER () / EI() [Radiation Budget equation: EI() = EA () + ER () + ET ()]All matter have their unique spectral reflectance. Based on this, objects are identified on a photograph or image and this forms the basis of Remote Sensing.

*Interaction of EMR with matter :Spectral SignaturesThe amount and properties of reflection is dependent on the wavelength of the incident energy and the material, condition of feature.Surface of the material plays a vital role.Two types of surface involved: Specular (Smooth) and Diffuse (Rough)

*Spectral Signatures: Spectral reflectance curvesThe spectral reflectances are presented graphically in spectral reflectance curves, showing the relationship of EM spectrum wavelength with the associated percent reflectance for any given material.

*Spectral Signatures (Continued)Vegetation has high reflectance in IR region.Reflectance of Water is stronger only in the visible region.Soil shows uniform reflectance throughout. However, the presence of moisture in soil will cause a decrease in its reflectance and thus, you can identify soil moisture content.

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*Even though Visible region is suitable for identify vegetation, to identify the content of chlorophyll present in the plant, Near infrared is more suitable for vegetation surveys and mapping because such a steep gradient at 0.7-0.9 m is produced only by vegetation. It can also be used to identify different types of plants.In using visible light, it will only enable us to identify that it is a plant among many other objects.

Spectral Signatures of Vegetation

*Spectral Signatures of Vegetation

*Spectral Signatures of Vegetation

*Interaction of EMR with atmosphereSolar radiation as it passes through the atmosphere is affected by two processesAbsorption ScatteringBy atmospheric molecules and aerosols.AbsorptionProcess by which radiant energy is absorbed and converted into other forms of energy.An absorption band is a range of wavelengths in the EM spectrum within which radiant energy is absorbed by a substance.Ozone, Carbon dioxide and water vapour : 3 main constituents that absorb radiations.Ozone absorbs ultraviolet; Carbon dioxide absorbs thermal infrared portion, Water vapour absorbs thermal infrared and microwave radiations.How radiations are absorbed is quantified to determine the quantum of these atmospheric constituents in the atmosphere.

*Thus, the atmospheric gases absorb EME in specific regions of the spectrum and this is identified using meteorological satellites that directly measure absorption phenomena. Those regions in the spectrum that are not influenced by atmospheric absorption is useful for remote sensing and is called atmospheric window.

Interaction of EMR with atmosphereDips in figure specifies absorption

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*Interaction of EMR with atmosphere

*Interaction of EMR with atmosphereScattering of radiationVery serious effect of atmosphereCaused by small or large gas molecules present in the atmosphere, which causes the EMR to be redirected from its original path.The following elements will influence the transmittance of the atmosphere. a. Atmospheric molecules (smaller size than wavelength): carbon dioxygen, ozone, nitrogen gas, and other molecules b. Aerosols (larger size than wavelength): water drops such as fog and haze, smog, dust and other particles with a bigger size Two classes of scattering: (i) Selective (ii) Non-selective

*Interaction of EMR with atmosphereSelective scattering: Two typesRayleigh scattering:Occurs when the effective diameter of the atmospheric molecules are many times smaller (< 0.1 times) than the wavelength of incident EMR.Amount of scattering is inversely proportional to the fourth power of wavelength of radiation.Occurs in the upper 4.5 km of the atmosphere.Responsible for the blue appearance of the sky.At sunrise and sunset, the sky looks red in colour because of this effect.Mie scattering:Takes place in the lower 4.5 km of the atmosphere, Where the size of the atmospheric molecules are larger than the size of the wavelength (0.1 to 10 times) and larger molecules are abundant.Affects longer wavelengths.

*Interaction of EMR with atmosphereNon-selective scatteringTakes place in the lowest parts of the atmosphere where particle sizes are very large (10 times more than wavelength).All wavelengths are scattered proportionately.Water droplets, ice crystals scatter all wavelengths.Thus, clouds and fog appear white.

*Some examples for your interestVisible region of the spectrumInfrared region of spectrum**Identify the variation in chlorophyll in the leaves of the tree

*Using Visible and IR region of the spectrum

*Using Visible and IR region of the spectrum

*Precipitation Radar

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