1. Properties used in RS for
discrimination
The following four properties are used for interpretation of RS
information:
spectral : Wavelength or frequency, refractive or emissive
properties of objects during interaction of EMR
spatial : Viewing angle of sensor, shape and size
of the object, position, site, distribution, texture
temporal : Changes in time and position which affect spectral
and spatial properties
polarization: Object effects in relation to the polarization
conditions of the transmitter and receiver
2. Remote sensing system
A typical remote sensing system
consists
of the following sub-systems:
(a) scene
(b) sensor
(c) processing (ground) segment
3.
4. The following steps indicate how remotely sensed data gets
converted into useful information:
1. Source of EM energy (sun/self emission: transmitter onboard sensor).
2. Transmission of energy from the source to the surface of the earth and its
interaction with the atmosphere (absorption/scattering).
3. Interaction of EMR with the earth surface (reflection, absorption,
transmission) or re-emission/self emission.
4. Transmission of reflected/emitted energy from the surface to the remote
sensor through the intervening atmosphere.
5. Recording of EMR at the sensor and transmission of the recorded
information (sensor data output) to the ground.
6. Preprocessing, processing, analysis and interpretation of sensor data.
7. Integration of interpreted data with other data sources for deriving
management alternatives and applications.
5. Interactions
EMR interaction in Atmosphere
Atmospheric interaction consists of the following types:
Atmospheric Absorption
Energy is absorbed and re-radiated again in all directions,
usually over a different range of wavelengths. This is a case of
radiation-matter interactions, in which the quantification of
energy is important, so we will use the particle description of
EMR.
Atmospheric Scattering
Energy is lost by redirection away from the satellite's field of
view, but wavelength remains the same.
6. Interactions
Irrespective of source, all radiation detected by
remote sensors passes through some distance
(known as the path length) of atmosphere and
the net effect of the atmosphere varies with:
1.Differences in path length
2.Magnitude of the energy signal that is being
sensed
3.Atmospheric conditions present
4.Wavelengths involved
7. Ray-1: Photons which leave the surface and reach sensor without change.
This constitutes useful signal for remote sensing.
Ray-2: Photons which leave the land/sea surface heading in the direction of
the sensor but which are absorbed by interaction with the atmosphere en route.
Ray-3: Photons diverted out of sensor's field of view (FOV) by scattering as a
result of atmosphere interaction.
8. Ray-4: Photons of EM energy which are emitted by the atmosphere itself.
Ray-5: Photons of energy from the illuminating source (sun or active radar
source) which are scattered into the FOV of the sensor without touching the
surface (land/sea target).
Ray-6: Photons which have left the ground and carry information from an area
other than the ground FOV of the sensor, and which are deflected by the
atmospheric scattering into the FOV of the sensor.
9. Mechanism for absorption
The primary mechanism by which the atmosphere absorbs
radiation is through molecular absorption by gases. A photon,
or quantum, of energy is exchanged between a molecule or
atom of gas and the electromagnetic wave by following
arrangements:
– Electron transitions: It causes promotion of electrons to
higher energy orbital for absorption (lower energy orbital for
emission) of EMR in the visible portion of the spectrum.
– Vibration of triatomic molecules: It is induced by EMR
in the infrared portion of the electromagnetic spectrum.
– Rotation of diatomic molecules: EMR in the infrared
and microwave wavelengths excites rotational motion of the
molecules
10. This quantization results in an absorption spectrum for each
molecule that is composed of a narrow set of absorption peaks or
lines. This absorption spectrum represents wavelengths at which
the corresponding energy can be absorbed. Thus for fixed E, h,
and c, so one can easily identify λ for a given change in E
11. Atmospheric scattering
• Scattering in the atmosphere caused by particles such as
liquid water drops, smoke, haze, and dust. Particles can
absorb as well as scatter, but scattering by particles usually
dominates over absorption by particles.
• Scattering occurs when radiation is reflected or refracted by
particles in the atmosphere which may range from
molecules of the constituent gases to dust particles and
large water droplets.
• It is considered as a disturbance of the EM field by the
constituents in the atmosphere resulting in the change in the
direction and spectral distribution of energy in the beam.
12. Atmospheric scattering
• Scattered radiation, whether coming from the sun (down
welling) or reflected from the earth surface (upwelling), is
not attenuated but rather redirected. This redirection is
wavelength-dependent. Pure scattering is said to occur
in the absence of all absorption; there is no loss of
energy- only redirection of energy.
• It must be remembered that while molecular absorption
removes energy as it passes through the atmosphere
and re-radiates uniformly in all directions at a different
wavelength, scattering changes the direction of
propagation only, not the wavelength.
13. Properties of scattering
• Strongly directionally dependent.
• Dependent on the polarization of the EMR.
• Dependent on the wavelength of the EMR:
shorter wavelengths scatter more.
• Strong dependence on the size of the scattering
particles relative to the wavelength of the EMR.
• Dependent on the density of the scattering
particles: multiscatter.
14. If the particles are sparse, EMR is scattered once. The scatter
primarily changes the angle of propagation, removing
(attenuating) energy from the beam of radiation.
15. If the particle density is high, EMR is scattered repeatedly.
This can both add and remove energy from the beam of
radiation, or result in isotropic radiance.
16. Three main types of scattering:
• Rayleigh or Molecular scattering, when λ >> d
• Mie scattering, when λ ~ d
• Isotropic or nonselective scattering, when λ << d
Water droplets causes such scatter.
Scatter all visible and near- to mid –IR wavelengths equally.
Consequently this scatter is non-selective with respect to
wave lengths
Where λ=wavelength and d =particle diameter
17. Rayleigh or Molecular scattering
• The magnitude of energy scattered is smaller than that
absorbed by the molecules .Effect is inversely
proportional to fourth power of wavelength. So shorter
wavelengths are scattered more than longer ones.
• Occurs when EMR wavelength is much larger than
particle size., e.g. scattering of visible light (0.4 µm < λ <
0.8 µm) by pure gas molecules (of the order of 10-4 µm)
in a clear atmosphere.
• Oxygen and Nitrogen molecules are behind this type of
scattering.
18. Rayleigh scattering results in:
Blue sky
Radiation in the shorter blue wavelengths is
scattered towards the ground much more
strongly than radiation in the red wavelengths.
Red during sunset
As the sun approaches the horizon and its rays
follow a longer path through the atmosphere, the
shorter wavelength radiation is scattered,
leaving only the radiation in the longer
wavelengths, red and orange to reach our eyes.
19. Mie Scattering
• It is an intermediate case when the particle size is comparable
to the radiation wavelength, i.e. λ ~ d and manifests itself as a
general deterioration of multispectral images across the optical
spectrum under conditions of heavy atmospheric haze.
• Energy scattered is roughly inversely proportional to λ.
• Water vapor and dust particles cause this type of scattering.
• Significant in slightly overcast atmospheric conditions
• Handling this case is mathematically complex.
• The incident light is scattered mainly in the forward direction.
0
20. Isotropic or non-selective scattering
• Radiation is scattered equally in all directions and
occurs when λ<< d. The total amount of scattering
is independent of wavelength and causes uniform
attenuation at all wavelengths.
• Occurs for visible wavelengths in clouds and thick
fog where water droplets have radii = 5-10 mm.
This is why they are white.
• Whitish appearance of sky under heavy haze
condition is due to non-selective scattering.
21. Isotropic or non-selective scattering
• Total effect of large-particle scattering is the sum of the
contributions from three processes involved in the interaction of
the radiation with the particle:
Reflection from the surface of the particle with no penetration,
Passage of the radiation through the particle with or without
internal reflections,
Refraction at the edge of the particle.
• Non-selective scattering usually occurs when the atmosphere is
heavily dust-laden and results in a severe attenuation of the
received data. However, the occurrence of the scattering
mechanism frequently is a clue to the existence of large
particulate matter in the atmosphere above the scene of
interest, and this in itself is sometimes useful data.
22. Different types of scattering
• Rayleigh Scattering : important above 4.5 km in the pure
atmosphere which is dry and clean and scatters equally in
forward and backward directions
• Mie Scattering: important below 4.5 km, where there are
sufficient numbers of large particles: dust, haze, water vapor.
More scatter occurs in the forward direction.
• Non-selective scattering : becomes important in the lower
atmosphere when there are numbers of even larger particles.
Isotropic scatter (direction independent or equal in all
directions).
24. Significance of scattering in RS
Energy is directed outside the field of view (FOV) of the
sensor:
Large FOV
Some scattered radiation will be accepted, enhancing the
signal being received by the sensor
Small FOV
Virtually all scattered radiations will be rejected producing an
apparent attenuation or dimming of the image. In both cases,
scattering degrades image quality and adversely affects RS
observations in two ways:
(a) Reduces image contrast
(b) Changes spectral signature of ground object being sensed
by sensor.
