RS Compendium

8/8/2019 RS Compendium

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UNIT 1

REMOTE SENSING

y Definitionofremote sensing

Remote sensing is the small- or large-scale acquisition of information of an object or

phenomenon, by the use of either recording or real-time sensing device(s) that are wireless, or

not in physical or intimate contact with the object (such as by way ofaircraft, spacecraft,

satellite, buoy, or ship).

Earth observation or weather satellite collection platforms, ocean and atmospheric observing

weather buoy platforms, the monitoring of a parolee via an ultrasound identification system,

Magnetic Resonance Imaging (MRI), Positron Emission Tomography (PET), X-radiation (X-RAY)

and space probes are all examples of remote sensing.

y Components ofremote sensing

4 basic components of a remote sensing system

y energy sourcey transmission pathy targety sensor

1. Energy Source or Illumination (A) - the first requirement for remote sensing is to have an

energy source which illuminates or provides electromagnetic energy to the target of interest.

2. Radiation and the Atmosphere (B) - as the energy travels from its source to the target, it will

come in contact with and interact with the atmosphere it passes through. This interaction may

take place a second time as the energy travels from the target to the sensor.

3. Interaction with the Target (C) - once the energy makes its way to the target through the

atmosphere, it interacts with the target depending on the properties of both the target and the

radiation.


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4. Recording of Energy by the Sensor (D) - after the energy has been scattered by, or emitted

from the target, we require a sensor (remote - not in contact with the target) to collect and

record the electromagnetic radiation.

5. Transmission, Reception, and Processing (E) - the energy recorded by the sensor has to be

transmitted, often in electronic form, to a receiving and processing station where the data are

processed into an image (hardcopy and/or digital).

6. Interpretation and Analysis (F) - the processed image is interpreted, visually and/or digitally

or electronically, to extract information about the target which was illuminated.

7. Application (G) - the final element of the remote sensing process is achieved when we apply

the information we have been able to extract from the imagery about the target in order to

better understand it, reveal some new information, or assist in solving a particular problem.

y Active and Passive remote sensingThere are two main types of remote sensing: passive remote sensing and active remotesensing. Passive sensors detect natural radiation that is emitted or reflected by the object or

surrounding area being observed. Reflected sunlight is the most common source of radiation

measured by passive sensors. Examples of passive remote sensors include film photography,

infrared, charge-coupled devices, and radiometers. Active remote sensing, on the other hand,

emits energy in order to scan objects and areas whereupon a sensor then detects and measures

the radiation that is reflected or backscattered from the target. RADAR is an example of active


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remote sensing where the time delay between emission and return is measured, establishing

the location, height, speeds and direction of an object.

y Platforms aerial and spatial platformsAerial photography is the taking of photographs of the ground from an elevated position. Theterm usually refers to images in which the camera is not supported by a ground-based

structure.

Aerial photography is used in cartography (particularly in photogrammetricsurveys, which are often the

basis for topographic maps), land-use planning, archaeology, movie production, environmental studies,

surveillance, commercial advertising, conveyancing, and artistic projects.

The term 'spatial remote sensing' is used to describe the measurements taken from satellites (the

term 'airborne remote sensing' is used when such measurements are taken from aircraft).

Sensors may be placed on a ladder, scaffolding, tall building, cherry-picker, crane, etc. Aerial

platforms are primarily stable wing aircraft, although helicopters are occasionally used. Aircraft

are often used to collect very detailed images and facilitate the collection of data over virtually

any portion of the Earth's surface at any time.

In space, remote sensing is sometimes conducted from the space shuttle or, more commonly,

from satellites. Satellites are objects which revolve around another object - in this case, the

Earth. For example, the moon is a natural satellite, whereas man-made satellites include those

platforms launched for remote sensing, communication, and telemetry (location and

navigation) purposes. Because of their orbits, satellites permit repetitive coverage of the Earth's

surface on a continuing basis. Cost is often a significant factor in choosing among the various

platform options.

Advances in radio controlled models have made it possible for model aircraft to conduct low-

altitude aerial photography.

There are several advantages to using balloons or blimps as platforms for remote sensing data

collection. One of the most significant advantages is that balloons can be deployed quickly and

data collected immediately. Extensive operator training is not required. For instance, balloonscould be sent up quickly to monitor chemical spills or accidental releases. Balloons may also be

able to collect data that current helicopter or fixed wing cameras cannot. A balloon system

could be deployed to monitor a specific site for hours or days which would be logistically

difficult or impossible with current airborne imagery. Tethered balloons can be moved and

relocated easily, providing a more flexible method to collect data. For instance, tethered

balloons could be deployed on small boats in rivers to conduct water quality surveys over

several stream miles.


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y Synoptivity and repetivity

y Electromagnetic Radiation

Electromagnetic radiation (often abbreviated E-M radiation or EMR) is a phenomenon that

takes the form of self-propagating waves in a vacuum or in matter.

Electromagnetic radiation consists of:

Electrical Field (E) which varies in magnitude in a direction perpendicular to the direction in

which the radiation is traveling, and a

Magnetic

Field (M) oriented at right angles to the electrical field. Both these fields travelat thespeed of light (c).

Electromagnetic radiation is classified into several types according to the frequency of its wave;

these types include (in order of increasing frequency and decreasing wavelength): radio waves,

microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays. A

small and somewhat variable window of frequencies is sensed by the eyes of various

organisms; this is what is called the visible spectrum. The photon is the quantum of the

electromagnetic interaction and the basic "unit" of light and all other forms of electromagneticradiation and is also the force carrier for the electromagnetic force.


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Two characteristics of electromagnetic radiation are particularly important for understanding

remote sensing. These are the wavelength and frequency:

1. Thewavelength is the length of one wave cycle, which can be measured as the distance

between successive wave crests.

- Wavelength is usually represented by the Greek letter lambda (P).

- Wavelength is measured in metres (m) or some factor of metres such as nanometres (nm, 10-

9 metres), micrometres (mm, 10-6 metres) or centimetres (cm, 10-2 metres).

2. Frequency refers to the number of cycles of a wave passing a fixed point per unit of time.

- Frequency is normally measured in hertz (Hz), equivalent to one cycle per second, and various

multiples of hertz.

Wavelength and frequency are related by the following formula:

y The electromagnetic spectrum


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The Electromagnetic Spectrum ranges from the shorter wavelengths (including gamma and x-

rays) to the longer wavelengths (including microwaves and broadcast radio waves). There are

several regions of the electromagnetic spectrum which are useful for remote sensing.

The light which our eyes can detect is part of the visible spectrum (small) There is a lot of radiation around us which is "invisible" to our eyes, but can be detected

by other remote sensing instruments and used to our advantage

The visiblewavelengths cover a range from approximately 0.4 to 0.7 mm (violet to red)

Blue, green, and red are the primary colours or wavelengths of the visible spectrum. They are defined as

such because no single primary colour can be created from the other two, but all other colours can be

formed by combining blue, green, and red in various proportions. Although we see sunlight as a uniform

or homogeneous colour, it is actually composed of various wavelengths of radiation in primarily the

ultraviolet, visible and infrared portions of the spectrum


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For most purposes, the ultraviolet or UV portion of the spectrum has the shortest wavelengths which

are practical for remote sensing. This radiation is just beyond the violet portion of the visible

wavelengths, hence its name. Some Earth surface materials, primarily rocks and minerals, fluoresce or

emit visible light when illuminated by UV radiation.

The infrared (IR) region covers the wavelength range from approximately 0.7 mm to 100 mm - more

than 100 times as wide as the visible portion! The infrared region can be divided into two categories

based on their radiation properties - the reflected IR, and theemittedor thermal IR. Radiation in the

reflected IR region is used for remote sensing purposes in ways very similar to radiation in the visible

portion. The reflected IR covers wavelengths from approximately 0.7 mm to 3.0 mm. The thermal IR

region is quite different than the visible and reflected IR portions, as this energy is essentially the

radiation that is emitted from the Earth's surface in the form of heat. The thermal IR covers wavelengthsfrom approximately 3.0 mm to 100 mm. Thermal IR energy is more commonly known as

"heat".


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y Blackbody Radiationy black body is an idealized object that absorbs all electromagnetic radiation falling on it.

Blackbodies absorb and incandescently re-emit radiation in a characteristic, continuousspectrum. Because no light (visible electromagnetic radiation) is reflected or transmitted,the object appears black when it is cold. However, a black body emits a temperature-dependent spectrum of light. This thermal radiation from a black body is termed black-body radiation. In the blackbody spectrum, the shorter the wavelength, the higher the

frequency, and the higher frequency is related to the higher temperature. Thus, the colorof a hotter object is closer to the blue end of the spectrum and the color of a coolerobject is closer to the red.

y At room temperature, black bodies emit mostly infrared wavelengths, but as thetemperature increases past a few hundred degrees Celsius, black bodies start to emitvisible wavelengths, appearing red, orange, yellow, white, and blue with increasingtemperature. By the time an object is white, it is emitting substantial ultravioletradiation.

.

Plank's Radiation Law for Blackbodies gives the spectral radiance of an object as a function of its

temperature.

Planck's law describes the spectral radiance of electromagnetic radiation at all wavelengths

emitted in the normal direction from a black body in a cavity in thermodynamic equilibrium. As a

function of frequency and absolute temperature T


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y StefanBoltzmann lawTheStefanBoltzmann law, also known asStefan's law,states that the total energyradiated per unit surface area of a black body per unit time (known variously as theblack-body irradiance, energy flux density, radiant flux, or the emissive power), j*, isdirectly proportional to the fourth power of the black body's thermodynamic temperatureT (also called absolute temperature):

A more general case is of a grey body, the one that doesn't absorb or emit the fullamount of radiative flux. Instead, it radiates a portion of it, characterized by itsemissivity, :

According to the Stefan-Boltzmann law, a doubling oftemperature means an increase of

energy emitted at the surface increases by a factor of 16. This means that a star with twice

the temperature has 16 times the energy.

UNIT 2

EMR interaction with atmosphere and earth materials


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y Atmosphericcharacteristics

Four layers of the atmosphere:

1) Troposphere (0-15 km)2) Stratosphere (15-50 km)3) Mesosphere (50-90 km)

Thermosphere or Ionosphere (>80 km

The Troposphere

The troposphere is where all weather takes place; it is air moves as currents, up-and downdrafts, andwind. The air pressure at the top of the troposphere is only 10% of that at sea level. The boundary

between the troposphere and the stratosphere lies 6-17 km above Earth and is called thetropopause.

The Stratosphere andthe Ozone Layer

Above the troposphere is the stratosphere, where air flow is mostly horizontal. The thin ozone layer in

the upper stratosphere has a high concentration of ozone, a particularly reactive form of oxygen. This

layer is primarily responsible for absorbing the ultraviolet radiation from the Sun. (Jet Stream)

The Mesosphere and Ionosphere

Above the stratosphere is the mesosphere and above that is the ionosphere (or thermosphere),where many atoms are ionized (have gained or lost electrons so they have a net electrical

charge). The density of gas molecules in the ionosphere is very thin

aurora absorbs the most energetic solar radiation reflects radio waves.


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y Scattering of EMRElectromagnetic radiation that passes through the earth's atmosphere without being absorbedor scattered reaches the earth's surface to interact in different ways with different materials

constituting the surface.

There are three ways in which the total incident energy will interact with earth's surface

materials. These are

y Absorptiony Transmission, andy Reflection

Absorption (A) occurs when radiation (energy) is absorbed into the target while transmission(T) occurs when radiation passes through a target. Reflection (R) occurs when radiation

"bounces" off the target and is redirected.

The amount of energy absorbed, transmitted or reflected by a material will depend upon:

Wavelength of the energyMaterial constituting the surface, and

Condition of the feature.

In remote sensing, we are most interested in measuring the radiation reflected from targets.

Reflection from surfaces occurs in two ways:


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1. When the surface is smooth, we get a mirror-like or smooth reflection where all (oralmost all) of the incident energy is reflected in one direction. This is called Specular

Reflection and gives rise to images.2. When the surface is rough, the energy is reflected uniformly in almost all directions.

This is called Diffuse Reflection and does not give rise to images.

y Rayleigh scatteringRayleigh scattering occurs when particles are very small compared to the wavelength of the

radiation. These could be particles such as small specks of dust or nitrogen and oxygen molecules.

Rayleigh scattering causes shorter wavelengths of energy to be scattered much more than longer

wavelengths. Rayleigh scattering is the dominant scattering mechanism in the upper atmosphere. The

fact that the sky appears "blue during the day is because of this phenomenon. As sunlight passes

through the atmosphere, the shorter wavelengths (i.e. blue) of the visible spectrum are scattered more

than the other (longer) visible wavelengths. At sunrise and sunset the light has to travel farther through

the atmosphere than at midday and the scattering of the shorter wavelengths is more complete; this

leaves a greater proportion of the longer wavelengths to penetrate the atmosphere.


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y Mie scatteringMie scattering occurs when the particles are just about the same size as the wavelength of the

radiation. Dust, pollen, smoke and water vapour are common causes of Mie scattering which tends to

affect longer wavelengths than those affected by Rayleigh scattering. Mie scattering occurs mostly in the

lower portions of the atmosphere where larger particles are more abundant, and dominates when cloud

conditions are overcast.

y Nonselective scatteringThe final scattering mechanism of importance is called nonselective scattering. This occurs when

the particles are much larger than the wavelength of the radiation. Water droplets and large dust

particles can cause this type of scattering. Nonselective scattering gets its name from the fact that all

wavelengths are scattered about equally. This type of scattering causes fog and clouds to appear white

to our eyes because blue, green, and red light are all scattered in approximately equal quantities

(blue+green+red light = white light).


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y Raman scatteringSurface enhanced Raman scattering (SERS) is a technique whereby the molecule under investigation is

adsorbed on a metallic structure, typically made of silver. Raman signals are amplified through an

interaction between the pump light and the surface plasmons propagating in the metal. Gratings are better

suited to couple light into the surface plasmon modes owing to momentum conservation conditions. Opalline

structures are made of a dosed pack, fcc arrangement of silica spheres with a precise control over the pitchand hence the gratings' depth. Coupling into surface plasmon modes of these two-dimensional structures

should therefore, show a significant enhancement to specific vibrational lines.

y EMR interactionwithwater vapour andozoney

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