Unit 2 Ce333 PDF
Unit 2 Ce333 PDF
Unit 2 Ce333 PDF
Atmospheric characteristics
The main part of the radiance measured from high flying aircraft or satellite s
form multiple scattering in the atmosphere. Therefore, the remaining signal is
interpreted in terms of suspensions only after a careful correction for the
spherical contribution. For this reason the varying optical parameters of
atmosphere enter the radioactive transfer calculations' (Fischer J, 1989).
Before we study the effects of solar radiation and atmospheric properties, we
shall consider the mass quantities which determine the spectral upward
radiance. The source of the short wave radiation field in. atmosphere is the
Sun emitting' in a broad spectral range.
The extraterrestrial irradiance at the top of the atmosphere, the solar constant,
depends, on the black body emission of the Sun's Photosphere and on
scattering and,absorption process in the Sun's chromosphere.
Important-Fraunhofer lines caused by the strong absorption in the Sun's
SCATTERING OF EMR
Atmospheric Interactions with Electromagnetic Radiation
All electromagnetic radiation detected, by a remote sensor has to pass through
the atmosphere twice, before and after its interaction with earth's atmosphere.
This passage will alter the speed, frequency, intensity, spectral distr1bution, and
direction of the radiation. As a result atmospheric scattering and absorption
occur (Curran, 1988) these effects are most severe in visible and infrared
wavelengths, the range between crucial in remote sensing.
During the transmission of energy through the atmosphere light interacts with the
Gases and particulate matter in a process called atmospheric scattering, The two
major processes in scattering are selective scattering and non-selective scattering.
Rayleigh, Mie and Raman scattering are of selective type.
Non selective scattering is independent of wavelength. It is produced by particles
whose radii exceed 10mm, such as, water droplets and ice fragments present the
clouds. This type of scattering reduces the contrast of the image. While passing
through the atmosphere, electromagnetic radiation is scattered and absorbed by
gasses and particulates.
Besides the major gaseous components like molecular nitrogen and oxygen, other
constituents like water vapor methane, hydrogen, helium and nitrogen compounds
play an important role in modifying the incident radiation and reflected radiation.
This causes a reduction in the image contrast and introduces radiometric errors.
Regions of the electromagnetic spectrum in which the atmosphere is transparent
are called atmospheric windows.
The image contrast and introduces radiometric errors. Regions of the
electromagnetic spectrum in which the atmosphere is transparent are called
atmospheric windows.
The atmosphere is practically transparent in the visible region of the
electromagnetic spectrum and therefore many of the satellite based remote
sensing sensors are designed to collect data in this region.
Some of the commonly used atmospheric windows are 0.38 - 0.72 Jl /m ~.
(visit), 0.72 -3.00 Film (near infrared and middle infrc0.72 -3.00 film (near
infrared and middle infra red) and 8.00 -: 4.00 J/m (thermal infrared). Fig.
2.10. Shows relative scatter a
TYPES OF ATMOSPHERIC SCATTER IN ORDER OF IMPORTANCE
Type of Size of Type of Scatter particles Effect of scatter
scatter effective effective on visible And
atmospheric atmospheric near visible
particles wavelength
Rayleigh Smaller than Gas Molecule 'absorbs
high energy Affects short
wavelength molecules
radiation and visible
of radiation Re-emits. Skylight. wavelengths,
scatter is inversely resulting in haze
Proportional to in photography,
fourth power and blue skies.
of wavelength..
Mie Same size as spherical
the particles Affects all visible
Physical scattering
wavelengths
wavelength fumes and under over cast skies
of radiation dust
Non Larger than Water Affects all visible
wavelengths
selective wavelength droplets and
Physical scattering by equally resulting
of radiation dust clouds and fog white fog and
clouds
The phase functions of aerosols are nearly wavelength independent within the
visible and near infrared. For the radiative transfer calculations the scattering
functions are estimated by Mie theory.
The range of atmospheric turbidity values used to study the effects of aerosol
scattering on the measured spectral radiances correspond to horizontal
visibilities at the surface between 6 and 88 km.
Atmospheric constituents like water vapour, carbon dioxide, and ozone are the
primary absorbers of electromagnetic energy. The regions of the electromagnetic
spectrum in which atmospheric absorption is low are called atmospheric
'windows' and it b through these 'windows' that remote sensing of the earth's
surface is made.
The atmospheric windows are not totally free from atmospheric absorption as
gases and suspended particles in the atmosphere absorb certain amount of
radiation from ground objects, resulting in a decrease in radiation at the sensor.
These effects are variable in space and time and often necessitate the
atmospheric correction of remotely sensed data
EI = Incident energy
EA = Absorbed energy
This equation is called balance equation. From this mathematical equation, two
important points can be drawn as
ER (A) /E1 (A) = E1 (A) /E1(A) [(EA (A)/E1(A) +ET(A) /E1(A)]
According to principles of physics, it is known that
ER (A)/E1(A) ; EA(A) /E1(A) AND ET(A) /E1(A) are called reflectance , absorbance and
transmittance, and can be denoted as p(A.), (A.) and Y(A). Simply, it can be
understood, that the measure of-how much electromagnetic radiation is reflected
off a surface is called its reflectance.
The reflectance range lies between 0 and 1. A measure of 1.0 means that 100% of
the incident radiation is reflected off the surface, and a measure of '0' means that
0% is reflected. The reflectance characteristics are quantified by "spectral
reflectance, pA which is expressed as the following ratio.
PA = energy of wavelength A reflected from the object
energy of wavelength A incident upon the object
Since, almost all earth surface features are very .opaque in nature, the transmittance
Y(A.) can be neglected. According to Kirchoff's law of physics, the absorbance is taken
as Emissivity (). Therefore Eq. (2.12) becomes:
pa = 1 - (A)
Eq. (2.13) is the fundamental equation by which the conceptual design of remote
sensing technology is built. If ( (A) is a zero, then p(A.), that is, the reflectance
is one, which means, the total energy incident on the object is reflected and
recorded by sensing systems.
The classical example of this type of object is snow (white object). If (A) is
zero , then p(A ) is one then A = 0 indicating what ever the energy incident on
the object is completely absorbed by the that object therefore the reflectance
varies from 0 ( black body ) to 1 ( white body ).Secondly, from the wavelength
dependency of the energy components, it is evident that even within a given
feature type, the proportion of reflected, absorbed, and transmitted energies may
vary at different wavelengths.
Thus two features which are indistinguishable in one spectral range, may exhibit
a marked contrast in another wavelength band. Because many remote sensing
systems operate in the wavelength regions in which reflected energy
predominates, the reflectance properties of terrestrial features are very
important.
Spectral reflectance curves
A measure of 1.0 means the 100% of the incident radiation is reflected off the
surface, and a measure of 0 means that 0% is reflected.
The term complex means of electromagnetic radiation with the top layer of
sand grains on the beach, when an incident ray of electromagnetic radiation
strikes an air/grain interface, part of the ray is reflected and part at it is
transmitted into the sand grain.
For a sand grain with complex index of refraction N(A} = n(A)[1 - ik(A.)],
the spectral reflectance is expressed by Fresnel's equation (Jenkins and White
1957),
where,
r5 (A) = spectral reflectance of one reflecting grain (0 -1). RS (A ) :s. 1
m(A) = intrinsic spectral index of refraction of the grain
k(A) =. Intrinsic spectral index of absorption of the grain
g( , ) = generally non zero function of the angle of incidence () and angle of
observation ( ) with respect to the macroscopic surface
The average RS (A) for all the grains in the beach from which electromagnetic
radiation is reflected is defined as the volume reflectance of the beach, RS(A).
The total reflectance of the beach, RT(A) is the averaged sum of the spectral
and volume reflectance, as follows:
Note that when we use the terms transparent or opaque to explain optical
behavior, we must designate both a wavelength region and the material because
the complex index of refraction of any material is generally non-constant over a
large range of wavelength.