Lecture 4
LAWS
OF RADIATION
The propagation of EME follows
certain physical laws. All objects with temperatures above absolute zero have
temperature and emit energy. The amount of energy and the wavelength at which
it is emitted depend on the temperature of the object. As the temperature of
the object increases, the total amount of energy emitted also increases, and
the wavelength of maximum emission becomes shorter.
Stefan-Boltzman
Law
The law states that the total radiation emitted
from a black’ body is proportional to the fourth power of the absolute temperature. It defined the
relationship between total emitted radiation and temperature.
M = σT4
Where, M is energy of the body; σ is
Stefan-Boltzman’s constant. 5.67 x 10-8 w m-4; T is the
absolute temperature of the body.
This law states that hot bodies emit more energy
per unit area than the cool bodies
Wein’s
Law
The wein’s law states that, the dominant
wavelength or Wavelength at which a blackbody radiation reaches a maximum (M5,)
is related to its temperature.
λ max = a/T
Where, ‘a’ is a constant with value of 2898 µ in
°K; T is the absolute temperature of the blackbody in °K.
Thus, for a blackbody, the wavelength at which
the maximum spectral radiation existence occurs varies inversely with its
absolute temperature. The wavelength of maximum remittance shifts to shorter wavelength.
‘Kirchoff’s
Law
Kirchoff’s law states that the ratio of emitted
radiation to absorbed radiation is same for all blackbodies at the same
temperature. This law forms the basis for definition of emisssivity (E), as the
ratio between the emittance of a given object (M) and that a black body (Mb) at
the same temperature
Emittance of a body M
Emissivity = ------------------------------- =
-----------
Emittance of a black body Mb
The emissivity of a true blackbody is one and
that of a perfect reflector (white body) is zero.
Blackbodies and white bodies are concepts in the
laboratory under ideal conditions. In nature all objects have emisssivity that
falls between zero and one are Gray bodies. For these bodies emissivity is a
measure of their effectiveness as radiation of EM.
Planck’s
Law
The spectral existence i.e. the total energy
radiated in all directions by unit area in unit time in a spectral band for a
blackbody is given by Planck’s law.
Q=hv
Where, h is Planck’s constant (6.6256 x 10-34
J/sec); V is frequency
The spectral existance of a blackbody is not the
same at all wavelengths. The spectral existance is low for very short and very
long wavelength. The law indicates that a blackbody at higher temperature emits
more radiation than a blackbody at low temperature at all wavelengths.
In remote sensing we are interested in the
following wavelength ranges.
1. Visible
0.4 0.7 µm
a) Blue 0.4
~ 0.5 µm.
b) Green 0.5
0.6 µm
c) Red 0.6
0.7 µm
2. Infrared 0.7
30.0 µm
(a) Near infrared (NIR) 0.7 - 1.3 µm
b) Middle infrared‘(MIR) 1.3 3.0 µm
c) Far infrared (FIR) 3.0 30 µm
(thermal infrared)
3. Microwaves
1 mm 1m
Most common sensing systems operate in one or
several of the visible, IR or microwave portions of the spectrum.
In the infrared region, only the thermal infrared
is directly related to the sensation of heat and near and middle infrared energy is not.
REMOTE
SENSING MEASUREMENTS
In remote-sensing we measure the
intensities of reflected and emitted radiations from target surfaces or
objects. Due to characteristics interactions of different wave lengths of
radiation with different materials we get characteristic variations in the measurement.
Three important types of variations which form the basis of information about
the object are:
1. Spectral variation: These are changes in the
intensity of reflected/emitted radiations with wave length.
2. Spatial variation: These are changes in the
intensity of reflected or emitted radiation with location due to variation in
material composition or surface topography of the target.
3. Temporal variation: These are changes in the
intensity of reflected or emitted radiations with time due to dynamic
characteristics of target surface eg. Vegetation cover.
In order to derive information from the objects
we have to measure these variations and relate them to the processes of known objects or phenomena.
INTERACTION
OF EMR WITH ATMOSPHERE
Electromagnetic radiation (EMR) while travelling
from the source to surface of the
earth and then from there to the sensors on-board satellite comes in contact
with the atmospheric constituents and interacts with them. The atmospheric
constituents like dust particles, smoke particles and gases affect the incoming
radiation. Hence, in the atmosphere, the interactions
are caused mainly by scattering (Fig.3.1), absorption and refraction.
Scattering
Scattering is the redirection of EME in different
directions. It occurs in presence of large dust particles and gas molecules in
the atmosphere. The 'effect of scattering is to redirect the incoming radiation
back to space as well as towards earth’s surface (Fig.3.2). There are three
types of scattering depending on the size of particles in relation to
wavelength.
1. Rayleigh scattering
2. Mie scattering
3. Non-selective scattering
l. Rayleigh Scattering
It occurs when particles are very small compared
to the wavelength of radiation. These particles could be particles such as dust
particles, nitrogen and oxygen molecules. Rayleigh scattering causes shorter
wavelengths energy to be scattered more than longer wavelengths. It is the
dominant scattering in upper atmosphere. The blue colour of the sky and red and
orange colours at sunrise and sunset are due to Rayleigh scattering.
2. Mie Scattering
Mie scattering occurs when the atmospheric
particles are about the same size of the wavelength of the radiation. These
particles include dust, pollen, and smoke and water droplet. Mie scattering
occurs mostly in the lower atmosphere (0-5 km) where larger particles are more
abundant. It influences a broad range
of wavelength in and near visible region.
3. Non-selective Scattering
Non-selective scattering occurs when the particle
sizes are larger thin the wavelength of radiation. The particles may be dust
and water droplets. This scattering does not depend on the wavelength of the
radiation. This type of scattering causes fog and clouds to appear whitish
appearance of sky.
Absorption
This phenomenon occurs when the atmospheric
constituents absorb energy passing through the atmosphere. The gases like ozone (03), carbon dioxide (C02) and
water vapour (H20) absorb radiation in the atmosphere. Ozone absorbs UV
radiation, C02 absorbs radiation in the FIR portion of the spectrum and water vapour absorbs the
incoming IR and microwave radiation.
Transmission
or Atmospheric Window
Some radiations which are neither absorbed nor
scattered are transmitted through the atmosphere. The transparency of
atmosphere to such radiations is known as atmospheric window. Atmospheric
windows are the regions in the EMS for which
the atmosphere is transparent. i.e. these wavelengths are easily transmitted through the atmosphere. These are useful
regions for remote sensing purposes. The major atmospheric windows available
for remote sensing are given in Table: 3.1.
Refraction
Refraction is the bending of light rays at the
surface of interaction. When the light enter into a different medium, it
changes its direction or bends at the atmosphere as light passes through
atmospheric layer of varying clarity, humidity and temperature. These
variations influence the density of atmospheric layers, hence bending of light
occurs when it changes the medium of higher density to lower density.
INTERACTlON
OF EMR WITH EARTH SURFACE
The wave length of EMR that is useful for
valuable in environmental remote sensing are
1 Reflected radiation in Visible, NIR, MIR and
micro wave bands.
2. Emitted radiation in MIR and thermal IR wave
bands.
These are three main components of remotely
sensed scenes such as vegetation, soil and water. The processes involved in the interaction of EMR with earth’s
surface are reflection, scattering and transmission.
Radiation that is not absorbed or scattered in
the atmosphere reaches and interacts with the earth’s surface. According to the
law of conservation of energy, energy cannot be created nor be destroyed; it
can be converted or transformed to another form. Thus, the portion of incident
radiation is reflected by the surface, transmitted in to the surface or
absorbed by the surface (Fig.3.3). Different features on the earth’s surface
have different values of spectral reflectance, absorbance arid transmittance on
the basis of which they can be identified. The sum of each proportion of the
components is unity but the
magnitude of each component depends on the nature of the surface and hence
different.
Iλ = R λ +T λ +A λ
Iλ = R λ +T λ +A λ
Where I λ is incident radiation; R
λ is reflected radiation; T λ is transmitted radiation; A
λ is absorbed radiation
If the magnitude of the spectral radiance i.e.
reflected, absorbed or transmitted is vary different for different surfaces on
the earth surface than we can
identified those features on the basis of their spectral properties.
Of all the interactions in the reflective regions
“surface reflections” are the most useful in remote sensing applications.
It can be seen from the table that major
principal windows lie in visible, infrared and micro wave regions.
Reflection
Reflection occurs when radiation is redirected
from a nontransparent surface. Reflection depends on the roughness or
smoothness of the surface, in relation to wavelength of radiation. According to
Rayleigh criterion, if the surface height variations are less then λ8, the
surface height is considered to be smooth otherwise it is rough. Accordingly
there are two types of reflections;
(i) Specular and (ii) Diffuse reflection
·
Specular reflection: If the surface is smooth relative to the
Wavelength, Specular reflection occurs which follow the law of reflection. They
occur with surfaces such as mirror, metal and a calm water body. This type of
reflections is undesirable in
remote sensing.
·
Diffuse reflection: Diffuse reflection occurs when the surface iS' rough
in relation to wavelength. In these reflections energy is reflected almost uniformly
in all directions. Diffuse reflections
are useful in remote sensing. In nature mixed reflections occurs most
frequently.
Absorption
and Transmission
Absorption occurs when the object absorbs the
radiation. Transmission occurs when the radiation passes through an object or
target.
RESOLUTIONS
Resolution of a system refers to its ability to
record and display fine details. The images are described in terms of its scale
as well as in terms of its resolution. In remote sensing we need three
different types of information such as spatial, spectral and radiometric
(intensity) information. Accordingly the sensor system varies in principles of
detection and construction. The types
of sensor systems used to acquire different information.
Types
of Resolution
In remote sensing there are four types of
resolution
1. Spatial resolution
2. Spectral resolution
3. Radiometric resolution and
4. Temporal resolution
Spatial
Resolution
Spatial resolution refers to the size of the
smallest possible feature that can be detected. It is depended on the IFOV of
the sensor. In many of the remote sensors, a small elemental area is observed
at a time and such a field of view of the sensor is called the Instantaneous
Field of View (IFOV). However, it should be noted that though the spatial
resolution has a bearing on the IFOV, it does not entirely depend only on IFOV.
There are various other factors such as satellite altitude, the relative motion
between IFOV and the ground during the ‘dwell time’ (the time for which sensors
looks over the elemental area), sampling frequency of the measurement,
characteristics of all the subsystems of the sensing system, which contribute
significantly to the overall spatial resolution of the system. Spatial
resolution decides the smallest size of the observable picture element or pixel
(under a given state of the art of detector technology). spatial resolution of
remote sensing sensors is given in terms of the pixel size dimension. A pixel
can be a square or rectangular shape.
Spectral
Resolution
The radiation reaching the remote sensor from the
earth’s Surface cover the entire electromagnetic spectrum. The spectral resolve
the energy received in a given spectral bandwidth to characterize different
constituents of earth’s surface. Thus the spectral resolution is defined by the
spectral bandwidth of the filter and the sensitiveness of the detector. Thus, for example, onboard the Land
sat satellite; the multispectral scanner system had the capability to resolve
the earth’s surface features at 80 m spatial resolution using four spectral
bands viz, 0.5-0.6 µm, 0.6-0.7 µm, 0.7-0.8 µm, and 0.8-1.1 µm. The last band
has a bandwidth of 0.3 µm as opposed to 0.1 µm of the rest of the bands. As the
incoming solar radiation in this year infrared spectral region is small as
compared to other bands, one had to increase the bandwidth three times in order
to maintain the spatial resolution at 80 m as well as the given the signal to
noise ratio requirement. On the other hand if the spectral bandwidth of the
fourth band is maintained at 0.1 pm, the spatial resolution would be much
larger than 80 m in order to have the same signal to noise ratio.
The Thematic mapper (TM) of the Land sat
satellite has seven spectral bands viz., 0.45-0.52 µm, 0.52-0.60 µm, 0.630.69
µm, 0.76-0.90 µm, 1.55-1.75µm, 10.4-12.5 µm and 2.082.35 µm. With the
upgradation of technology that was used in the multi-spectral scanner design in
the earlier satellites, the spatial resolution could be increased to 30 m even
with the reduction in spectral bandwidth in the visible' and reflected infrared
region of electromagnetic spectrum, i.e. the TM bands 1 to 5 and band 7. In
case of TM thermal band (10.4 to 12.5 µm), the energy had to be integrated over
16 times larger area (i.e. 120 m) as well as over a bandwidth of 2.1 µm to
provide an acceptable signal to noise ratio. This is because energy emitted by
earth is small, the average surface temperature of earth being only 300 °K in
comparison to the sun with its surface temperature of 6000 °K even when the
differences in the distances are accounted for.
The Linear Imaging Self Scanning Sensor (LISS)
onboard Indian Remote sensing Satellites (IRS-lA, 1B) has four spectral bands,
viz., 0.45-0.52 µm, 0.52-0.59 µm,
0.62-0.68 µm and 0.77-0.86 µm.
In LISS system an array of 2048 element charge
coupled devices (CCDs) is provided for each spectral band so that a separate
detector collects signal from each pixel instead of through scanning mirror as
in Land sat, to see different pixels along a scan line. Due to satellite
velocity, the time, available to scan a line of say about 185 km (swath width
of Land sat) is fixed, and this is shared by Viewing say 11 pixels in the scan
line. In L188, 3 whole line is swept like the pushing broom on railway platform
during cleaning operation where in each detector corresponding to a pixel gets
the whole of the time available for scan
line as dwell time (time during Which signal is integrated) and this improves
the quality of signal from the pixel and also minimizes the geometric
distortions caused due to non-uniform motion of mirror as in Land sat. More
number of narrow spectral bands gives rise to greater ability to discriminate
various features of the earth’s surface. Table 5.1 gives the sensor details and
utility characteristics of various sensing systems onboard Land sat, IRS and
the French SPOT satellites. Table 5.2 gives description of IRS-lC and Spot-4
satellites.
Radiometric
Resolution
The ability to distinguish line variations in the
radiance values of the different objects is characterized by the radiometric
resolution.
In remote sensing, the reflected radiation from
different objects generates an electrical signal (say, voltage) as output from
the detector. This analogue voltage is digitized resulting into a digital
number corresponding to the elemental area of the ground scene or pixel. The
number of levels into which the output signal can be divided is dictated by the
availability of data bandwidth and the signal to noise ratio. This is similar
to the number of grey shades that can be seen in a black and white I
photograph. -For example, the multi-spectral scanner onboard the Land sat
satellites has a radiometric resolution of 1/64 in all the four spectral bands
it uses. It means that 64 different values of radiance can be detected on the
imagery obtained through the Land sat multi-spectral scanner. On the other
hand, the thematic mapper flown on the Land sat 4 and 5 satellites had a radiometric
resolution of 1/256 for all the seven bands in which it works. For comparison,
the LISS I and II (Linear imaging self Scanner) on board the Indian Remote
Sensing Satellite, IRS-l had a radiometric resolution of 1/128.
For radars operating in the microwave range, the
radiometric resolution is given in terms of decibels representing the minimum signal level that can be
detected with an acceptable signal to noise ratio. Decibel is one tenth of a
bit, which is the logarithm of the ratio of the signal strength to a reference
value. As an example, typical synthetic aperture radar has a radiometric
resolution of about 1 to 2 decibels.
Temporal
Resolution
Temporal resolution is specific to space borne
sensors particularly to sun-synchronous satellites. These are polar orbiting
satellites having 9-16 hours rotational period and cross the equator at the
same local time (solar time) in each orbit. Such an orbit offers similar sun
illumination conditions for all observations taken over different geographical
locations along given latitude in sun-lit areas. By suitable selection of the
spacecraft altitude and the inclination angle of the orbit, the spacecraft can
be made to cover the same area on the earth at regular intervals For example,
the Land sat 1, 2 and 3 had an orbiting altitude of 918 km, inclination of
99.114° and the repetition cycle of 18 days. For Land sat 4 and 5 with an
altitude of 705 km and an inclination of 982°, the repetition cycle is 16 days.
The Indian Remote Sensing satellite (IRS-lA and 1B) at an altitude of 904 km,
with inclination of 99.02° and repeat cycle of 22 days. With proper placement
of two satellites in orbit the repetition cycle could be reduced to half, say
11 days in case of IRS observation system. With
such a repetitive coverage, a given area on earth can be observed at regular
intervals and dynamic features such as vegetation and water resources can be
very effectively studied and analyzed. This ability to have revisit over any
given area by remote sensor at regular interval is defined as temporal
resolution.
SCALE
Images can be described in terms of scale which
is determined by the effective focal length of the lens of the remote sensing
device, altitude of the platform and the magnification factor employed in
reproducing the image.
Generally there are three type of scales such as
small scale, intermediate scale
and large scale. The quantitative range of the scale are given below:
Small scale
> 1:500,000 1
cm: >5 km
Intermediate
1: 50,000 to 1: 500,000 1
cm= 0.5 to 5 km
Large scale
<1: 50,000 ' ' 1
cm: < 0.5 km
The large scale images provide more detailed
information than the small scale images.
The following scales are used at different
levels:
1 : 1,00,000 Intermediate
scale National level
1 : 2,50,000 Intermediate
scale State level
1 : 50,000
Intermediate
scale District level
<1 : 8,000 Large
scale Village level
Photo
Scale
Photo scale of the 'aerial or satellite imageries
.is computed as the ratio of the distance and the photo or map ((1) to actual
distance on the ground (D) between any two known locations.
S = d/D
For the photographs taken in the vertical (Nadir)
view, the photo scale is a
function of the focal length of the camera (f), the flying height of the platform (H)
and the magnification factor (M) i.e.
Image scale = Mf/H
In this type of scanner, the scan direction is
along the track (direction of flight) and hence the name along track scanner
(Fig.5.7). It is also called push broom scanner because the detectors are
analogous to the bristles of a push broom sweeping a path on the floor.
Development of charge-coupled device (CCD) has
contributed to the successful design of the along track scanner. In this the
sensor elements consist of an array of silicon photodiodes arranged in a line.
There are as many silicon photodiodes as there are ground resolution cells
(corresponding to IFOV) accommodated within the restricted FOV of the sensor
optics. Each silicon photodiode, in turn, is coupled to a tiny charge storage
cell in an array of integrated circuit MOS (metal oxide semiconductor) device
forming a charge coupled (1ng (CCD) (Fig.5.8). When light from a ground
resolution cell strikes a photodiomm it generates a small current proportional
to the intensity of light falling on it and the current charges the storage
cell placed behind the diode. The charged cells formepart of an electronic
shift register which can be activated to read out the charge stored in the
cells in a sequential fashion. The output signals are correlated with the shift
pulses, and digitized to reconstitute the image.
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