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

Remote sensing system

A typical remote sensing system consists

of the following sub-systems:

(a) scene

(b) sensor

(c) processing (ground) segment

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.

Interactions EMR interaction in Atmosphere

Atmospheric interaction consists of the following types:

Atmospheric AbsorptionEnergy 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.

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

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.

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.

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

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

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.

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.

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.

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.

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.

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

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.

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.

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

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.

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.

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).

Scattering dependence on wavelength and particle size

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.