Topic 1 : DATA ACQUISITION : SOURCE OF ENERGY

Remote sensing is the measurement of the acquisition of data about the Earth’s surface without

contact with it. This is done by sensing and recording reflected or emitted electromagnetic radiation.

Remote sensing involves analyzing and applying that information. The process involves the following

elements (Fig. 6-1., Ravi P. Gupta ,1991):

Energy source - the first requirement for remote sensing is an energy source which provides

electromagnetic energy.

Radiation and the atmosphere – 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 (active remote sensing) as the energy travels from the target to the

sensor.

Interaction with the target - 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.

Recording of energy by the sensor - after the energy has been reflected by, or emitted from

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

electromagnetic radiation.

Transmission, reception, and processing - 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).

Interpretation and analysis - the processed image is interpreted, visually and/or digitally, to

extract information about the target.

Application - 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.

Fig. 6-1. The physical elements of remote sensing process: energy source, radiation and the

atmosphere, interaction with the target, sensing, analysis and application6.2.1 Electromagnetic radiation

The energy to use in remote sensing is in the form of electromagnetic radiation. For understanding

remote sensing we need to understand two important characteristics of electromagnetic radiation,

which are basic to wave theory. These are the wavelength and frequency (Fig. 6-2.). The wavelength is

the distance between successive wave crests. Wavelength is usually represented by lambda (λ) and

measured in meters or some factor of meters such as nanometers (nm, 10-9m) or micrometers (μm, 10-

6m). 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) which is cycle per second. Frequency and wavelength are

inversely proportional: the higher the frequency, the shorter the wavelength.

Fig. 6-2. Electromagnetic wave, λ-wavelength. Source:

http://www.landmap.ac.uk/ipc/ccrs/chapter1/chapter1_2_e.html

The electromagnetic spectrum

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 ((Fig. 6-3.).

The visible wavelengths cover a range from approximately 0.4 to 0.7 μm. The light which human

eyes can detect is part of the visible spectrum. This is the only portion of the spectrum we can

associate with the concept of colors. The primary colors of the light are blue, green and red. Other

colors can be made by combining them in various proportions.

The infrared (IR) part of the electromagnetic spectrum covers the range from roughly 0.7 μm to 1

mm. The infrared region can be divided into two categories based on their radiation properties - the

reflected IR, and the emitted or thermal IR. Physical processes that are relevant for this range are

similar to those for visible light. The reflected IR covers wavelengths from 0.7 μm to 5.0 μm. It can be

divided into near- and mid parts. 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 wavelengths from approximately 3.0 μm to 100 μm. The Earth

emits most strongly in approximately 10 μm (Ravi P. Gupta,1991).

Fig. 6-3. Types of energy level changes associated with different part of electromagnetic spectrum

The microwave covers region from about 1 mm to 1 m. This covers the longest wavelengths used for

remote sensing. The shorter wavelengths have properties similar to the thermal infrared region while

the longer wavelengths approach the wavelengths used for radio broadcasts.6.2.2 Interaction with the Atmosphere

Particles and gases in the atmosphere have effect on remote sensing data and on spectral band

selection. These effects are caused by the mechanisms of scattering and absorption ((Fig. 6-4.).

Scattering occurs when radiation is reflected or refracted by particles or large gas molecules present in

the atmosphere. Redirection of the electromagnetic radiation depends on several factors including the

wavelength of the radiation, the size of particles or gases, and the distance the radiation travels

through the atmosphere.

Fig. 6-4. Interaction with the Atmosphere: scattering, absorption

Absorption is the other main mechanism when electromagnetic radiation interacts with the

atmosphere and molecules of the atmosphere to absorb energy at various wavelengths. Ozone, carbon

dioxide, and water vapour are the three main atmospheric constituents which absorb radiation. These

gases absorb electromagnetic energy in very specific regions of the spectrum. There are some regions

of the spectrum where radiation is passed through the atmosphere with relatively little attenuation and

are useful to remote sensing. Those regions are called atmospheric windows.6.2.3 Interaction with the target

Radiation passed through the atmosphere interact with the Earth's surface. There are three forms of

interaction: absorption, transmission and reflection. In remote sensing, we are most interested in

measuring the radiation reflected from targets. We refer to two types of reflection, which represent the

two extreme ends of the way in which energy is reflected from a target: specular and diffuse

reflection. The interaction with the surface depends on the wavelength of the energy and the

material and condition of the surface feature. Different materials reflect and absorb differently the

electromagnetic spectrum. The reflectance spectra of a material is a plot of the fraction of radiation

reflected as a function of the incident wavelength and serves as a unique signature for the material.

The following graph ((Fig. 6-5.) shows the typical reflectance spectra of five materials: clear water,

turbid water, two types of soil (dry and wet), and vegetation (Lillesand T. at al, 2008,

The reflectance of clear water is generally low. The reflectance is maximum at the blue part of the

spectrum and decreases as wavelength increases. Turbid water has some sediment suspension

which increases the reflectance in the red part of the spectrum. Soil reflectance dependents on its

physical and chemical properties. Most important factors determining reflectance are the following:

organic matter, moisture content of soil, parent rock, existing colored compounds. In the example

shown, the reflectance increases monotonically with increasing wavelength. If moisture content of soil

increases, there is a decrease in reflectance. The reflectance of vegetation is low in both the blue and

red regions of the visible spectrum, due to absorption by chlorophyll for photosynthesis. It has a peak

at the green region which gives rise to the green color of vegetation. In the near infrared (NIR) region,

the reflectance is much higher than that in the visible band due to the cellular structure in the leaves.

In the mid infrared there are more water absorption regions.

Fig. 6-5. Reflectance of common natural objects: water, soil and vegetation

Topic 2 : PROPAGATION THROUGH ATMOSPHEREwe have discussed the principal sources of electromagnetic energy, the propagation of this energythrough the atmosphere, and the interaction of this energy with earth surface features. Combined, these factors result i n en erg y "signals" from which we wish to extract information. We now consider the procedures by which these signals are detected, recorded and interpreted. Th e d et ect i o n of electromagnetic energy can be performed either photographically or electronically. The processo f p h o tography uses chemical reaction on the surface of a light sensitive film to detect energy variations within a scene. Photographic systems offer many advantages: they are relatively simple and inexpensive and provide a high degree of spatial detail and geometric integrity. Electronic sensors generate an electrical signal that corresponds to t h e en ergy variations in the original scene. A familiar example of an electronic sensor is a television camera. Al t h o u gh considerably more complex and expensive than photographic systems, electronic sensors offer the advantages of a broader spectral range of sensitivity, improved calibration potential, and the ability to electronically transmit image data. Anoother mode of electronic sensor is recording with the help of charge coupled device which is used to convert electrical signal to digital signal. By developing a photograph, we obtain a record of its detected

signals. Thus, the film acts as both the detecting and recording medium. Electronic sensor signals are generally recorded onto magnetic tape. Subsequently, the signals may be converted to an image form by photographing a TV-like screen display of the data, or by using a specialized film recorder. In these cases, photographic film is used only as a recording medium. We can s ee that the data interpretation aspects of remote sensing can involve analysis of pictorial (image) and/or numerical data. Visual interpretation of pictorial image data has long been the workhorse of remote sensing.. Visualtechniques make use of the excellent ability of the human mind to qualitatively evaluate spatial patterns in a scene. Th e ability to make subjective judgments based on selective scene elements is essential in many interpretation efforts. Visual interpretation techniques have certain disadvantages, however, in that they may require extensive t raining and are labour intensive. In addition, spectral characteristics are not always fully evaluated in visual interpretation efforts. This is partly because of the limited ability of the eye to discern tonal values on an image and the difficulty for an interpreter to simultaneously analyze numerous spectral images. In applications where spectral p a t t e rns are highly informative, it is therefore preferable to analyse numerical, rather than pictorial, image data. In t h i s case, the image is described by a matrix of numerical brightness values covering the scene. These values may be analyzed by quantitative procedures employing a computer, which is referred to as digital interpretation.The use of computer assisted analysis techniques permits the spectral patterns in remote sensing data to be more fully exami n ed . Digital interpretation is assisted by the image processing techniques such as image enhancement, i n formation extraction etc. It also permits the data analysis process to be largely automated, providing cost ad vantages over visual interpretation techniques. However, just as humans are limited in their ability to interpret spectral patterns, computers are limited in their ability to evaluate spatial patterns. Therefore, visual and numerical t ech n i q u es are complementary in nature, and consideration must be given to which approach (or combination of approaches) best fits a particular application.

Topic 3.1 : ATMOSPHERE WINDOW

Some wavelengths cannot be used in remote sensing because our atmosphere absorbs essentially all the photons at these wavelengths that are produced by the sun. In particular, the molecules of water, carbon dioxide, oxygen, and ozone in our atmosphere block solar radiation. The wavelength ranges in which the atmosphere is transparent are called atmospheric windows. Remote sensing projects must be conducted in wavelengths that occur within atmospheric windows. Outside of these windows, there is simply no radiation from the sun to detect--the atmosphere has blocked it.

The figure above shows the percentage of light transmitted at various wavelengths from the near ultraviolet to the far infrared, and the sources of atmospheric opacity are also given. You can see that there is plenty of atmospheric transmission of radiation at 0.5 microns, 2.5 microns, and 3.5 microns, but in contrast there is a great deal of atmospheric absorption at 2.0, 3.0, and about 7.0 microns. Both passive and active remote sensing technologies do best if they operate within the atmospheric windows.

There...I think you have reviewed all the essential physics you need to start chatting with remote sensing people without embarrasing yourself by saying something really foolish. Thrill them! 

Topic 3.2 : ATMOSPHERE SCATTERING

Scattering: The redirection of EM energy by the suspended particles in the air.

Different particle sizes will have different effects on the EM energy propagation.

dp <<  Rayleigh scattering Sr

dp = Mie scattering Sm

dp >>  Non-selective scattering Sn

The atmosphere can be divided into a number of well marked horizontal layers on the basis of temperature.

Details check NCERT....

Topic 4 : ELECTROMAGNETIC RADIATION

Topic 5 : ELECTROMAGNETIC SPECTRUMThe 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.

Violet: 0.4 - 0.446 µmBlue: 0.446 - 0.500 µm

Green: 0.500 - 0.578 µmYellow: 0.578 - 0.592 µmOrange: 0.592 - 0.620 µm

Red: 0.620 - 0.7 µm

Details check NCERT....

Topic 6 : RADIATION LAWRadiation principles are important for the understanding of thermal radiation which is emitted by any object depending on its temperature and also its material properties. Efficiencies of absorbance and emission of radiation are material properties which need consideration here and their dependence on each other is given by Kirchhoff’s law.

The temperature dependence of emitted radiation follows the Stefan-Boltzmann law. Radiation is emitted in the form of electromagnetic waves, their intensity being a function of the wavelength. The emission maximum of thermal radiation is explained by Wien’s displacement law, while the shape of the entire emission spectrum is given by Planck’s law.

Kirchhoff’s law

The emission efficiency ε denotes the efficiency of an object to emit thermal radiation, a quantity varying between 0 (no emission at all) and 1 (highest possible emission). The absorption efficiency α of an object denotes its efficiency to absorb incident radiation. It is defined as

α = absorbed radiation/incident radiation

and varies between 0 and 1, whereby 1 corresponds to total absorption and 0 to total reflection or transmission.

Kirchhoff’s law, found in 1859, states:

α=ε,

the efficiencies of absorption and emission of an object are the same. Therefore, objects which absorb all incident radiation (α=1) have highest thermal emission efficiency (ε=1). They are denoted as black body emitters, where the term black indicates that there is no reflected radiation. Objects which absorb a fraction of the incident radiation only (α<1) are denoted asgrey body emitters.

Absorption and emission efficiencies are wavelength dependent in case of aselective emitter. High or low absorption efficiencies of an object in different spectral ranges go along with high or low emission efficiencies in the same spectral ranges. Therefore, a generalised Kirchhoff'’s law can be written as:

αλ=ελ

The Earth in visible light (left) and in the thermal infrared (right) seen by Meteosat in 2004.Source: Beckel 2007

Stefan-Boltzmann law

This law theoretically found by Josef Stefan in 1879 and experimentally confirmed by Ludwig Boltzmann in 1884 explains the temperature dependence of the intensity of thermal radiation emitted by an object. It increases strongly with increasing absolute temperature T given in Kelvin (K). The radiant exitance M, which is the radiative power emitted by the surface of an object and given in units of W/(m2) is:

M=ε σ T4

with the Stefan-Boltzmann constant σ=5.7·10-8 W/(m2K). For example, increasing the absolute temperature of an object by a factor of two results in a 16 times stronger emission of thermal radiation.

This temperature change is also connected with a changing emission spectrum, which is explained by Planck’s law (see on the following page).

The Planck radiation law

In 1900, Max Planck introduced the concept of quantised properties of light, i.e., photons. This became the starting point of quantum theory. In the framework of classical physics and on the basis of electromagnetic waves it had not been possible to understand the physics of thermal radiation. Using the concept of photons, Planck derived an equation which describes the intensity and spectral shape of thermal radiation of an ideal black body emitter. Omitting some physical constants, it reads:

Mλ ∝ λ-5 / {exp(hc/λkT) -1)}

where λ ist the emission wavelength and T is the absolute temperature. It ish=6.68·10-34 Ws2 Planck’s constant, c the velocity of light, and k=1.38·10-23 Ws/K Boltzmann’s constant.

Obviously, emission spectra of ideal black bodies depend on their temperature only, and material properties do not interfer. Planck’s law therefore holds with solids, liquids and gases if their absorbance is α=1 (i.e., they absorb all incident radiation). Examples of Planck curves are shown in the graph on the right. Emission spectra of grey body emitters can be calculated using their spectral emission efficiency, i.e., ελMλ.

Wien's displacement law

Already in 1893 Wilhelm Wien derived an equation which allows to calculate the wavelength of maximum intensity λmax of a black body emission spectrum as a function of the temperature T:

λmax T = const.

where the value of the constant is 0.30 cm K. Hence, high temperatures correspond to maxima at low wavelengths and vice versa.

Emission as a function of wavelengths of objects having different absolute temperatures.Source: ESA Eduspace with modifications

The sun with its high surface temperature of about 6.000 K emits visible light having a maximum at around λmax=0.5 µm. The Earth with an ambient temperature of 300 K emits predominantly in the mid infrared with a maximum around 10 µm; this spectral range is called the thermal infrared.