Remote sensing is the science of acquiringinformation about the Earths surface withoutactually being in contact with it.• Remote sensing is the observation andmeasurement of objects from adistance, i.e. instruments or recorders arenot in direct contact with objects underinvestigation.What is Remote Sensing?
How Remote Sensing WorksWhat is Remote Sensing?
Energy recorded by remote sensing systems undergoesfundamental interactions that should be understood toproperly interpret the remotely sensed data.For example, if the energy being remotely sensed comes fromthe Sun, the energy:• is radiated by atomic particles at the source (the Sun),• propagates through the vacuum of space at the speed oflight,• interacts with the Earths atmosphere,• interacts with the Earths surface,• interacts with the Earths atmosphere once again, and• finally reaches the remote sensor where it interacts withvarious optical systems, filters, emulsions, or detectors.ELECTRO MAGNETIC RADIATION
Electromagnetic RadiationAn electrical field (E)A magnetic field (M) oriented at right angles to theelectrical field.Energy Source or Illumination (A)
A body surface can1. absorb incident radiation,2. reflect incident radiation,a) as a mirrorb) with spherical symmetry (Lambert)3. transmit incident radiation,4. emit radiation.The characteristics are a function of thewavelength of the radiation.LAW OF EMR
WavelengthThe wavelength is the length of one wave cycleElectromagnetic Radiation
FrequencyFrequency refers to the number of cycles of awave passing a fixed point per unit of time.
Radiation and the AtmosphereThe energy travels from its source to the target, it will come in contact with &interact with the atmosphere it passes through. This interaction may take place asecond time as the energy travels from the target to the sensor. All these interactionoccurred by the following ways- Interaction with the atmosphere:• Scattering• Absorption. Interaction with target/surface:• Absorption.• Transmission.• Reflection. Fig: Interaction with the Atmosphere. Fig: Interaction with the Atmosphere
Before radiation used forremote sensing reaches theEarths surface it has to travelthrough some distance of theEarths atmosphere. Particlesand gases in the atmospherecan affect the incoming lightand radiation. These effectsare caused by themechanisms ofScatteringAbsorption.EMR: Interaction with the Atmosphere
ScatteringScattering occurs when particles or large gas molecules presentin the atmosphere interact with and cause the electromagneticradiation to be redirected from its original path.How much scattering takes place depends on several factorsincluding the wavelength of the radiation, the abundance ofparticles or gases, and the distance the radiation travels throughthe atmosphere. There are three (3) types of scattering which takeplace1.Rayleigh Scattering.2.Mie Scattering.3.Non-selective Scattering.
Mie ScatteringMie scattering occurs when the particles are just about the same size as the wavelengthof the radiation.Dust, pollen, smoke and water vapor are common causes of Mie scattering which tendsto affect longer wavelengths than those affected by Rayleigh scattering.Mie scattering occurs mostly in the lower portions of the atmosphere where largerparticles are more abundant, and dominates when cloud conditions are overcast.Non-selective Scattering This occurs when the particles are much larger than the wavelength of the radiation. Waterdroplets and large dust particles can cause this type of scattering. Nonselective scattering gets its name from the fact that all wavelengths arescattered about equally. This type of scattering causes fog and clouds to appear white to our eyes becauseblue, green, and red light are all scattered in approximately equal quantities(blue+green+red light = white light).The final scattering mechanism of importance is called Non-selective scattering.This occurs when the particles are much larger than the wavelength of the radiation.Water droplets & large dust particle can cause this type of scattering. This type ofscattering causes fog& clouds to appear white to our eyes.
At sunset, solar radiation musttraverse a longer path through theatmosphere. Viewing a setting sun, theenergy reaching the observer is largelydepleted of blue radiation, leavingmostly red wavelengths (Rayleigh).Dust/smoke adds additional scatteringwith a wavelength dependence thatincreases the red sky effect (Mie).RED SKY AT NIGHT
AbsorptionAbsorption is the other main mechanism at work whenelectromagnetic radiation interacts with the atmosphere.In contrast to scattering, this phenomenon causes molecules inthe atmosphere to absorb energy at various wavelengths. Ozone,carbon dioxide, and water vapor are the three main atmosphericconstituents which absorb radiation.Ozone serves to absorb the harmful (to most living things)ultraviolet radiation from the sun. Without this protective layer inthe atmosphere our skin would burn when exposed to sunlight.You may have heard carbon dioxide referred to as a greenhousegas. This is because it tends to absorb radiation strongly in the farinfrared portion of the spectrum - that area associated withthermal heating - which serves to trap this heat inside theatmosphere.
Absorption of EMR by atmosphereDifferent molecules absorb different wavelengths ofradiation:• O2 and O3 absorb almost all wavelengths shorter than300 nm.• Water (H2O) absorbs many wavelengths above 700 nm,but this depends on the amount of water vapor in theatmosphere.When a molecule absorbs a photon, it increases theenergy of the molecule. We can think of this as heatingthe atmosphere, but the atmosphere also cools byemitting radiation.
Electromagnetic Radiation & HealthElectromagnetic radiation can be classified into two types: ionizing radiation andnon-ionizing radiation, based on its capability of ionizing atoms and breakingchemical bonds. Ultraviolet and higher frequencies, such as X-rays or gamma raysare ionizing, and these pose their own special hazards: see radiation and radiationpoisoning. Non-ionizing radiation, discussed here, is associated with electrical andbiological hazards. Electrical Hazard. Fire Hazard. Biological Hazard.In many respects, remote sensing can be thought of as a reading process.Using various sensors, we remotely collect date that may be analyzed toobtain information about the objects, areas or phenomena beinginvestigated. In most cases the sensors are electromagnetic sensors eitherairborne or space borne for inventorying.
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