Remote Sensing System, Earth, and Electromagnetic Wave Length - Lecture Note - Completely Remote Sensing tutorial, GPS, and GIS - facegis.com
Remote Sensing System, Earth, and Electromagnetic Wave Length

Visual System

passive remote sensing Passive Remote Sensing: The eyes passively senses the radiation reflected or emitted from the object. The sensing system depends on an external source of illumination.

The human visual system is an example of a remote sensing system in the general sense. The sensors in this example are the two types of photosensitive cells, known as the cones and the rods, at the retina of the eyes. The cones are responsible for colour vision. There are three types of cones, each being sensitive to one of the red, green, and blue regions of the visible spectrum. Thus, it is not coincidental that the modern computer display monitors make use of the same three primary colours to generate a multitude of colours for displaying colour images. The cones are insensitive under low light illumination condition, when their jobs are taken over by the rods. The rods are sensitive only to the total light intensity. Hence, everything appears in shades of grey when there is insufficient light.

As the objects/events being observed are located far away from the eyes, the information needs a carrier to travel from the object to the eyes. In this case, the information carrier is the visible light, a part of the electromagnetic spectrum. The objects reflect/scatter the ambient light falling onto them. Part of the scattered light is intercepted by the eyes, forming an image on the retina after passing through the optical system of the eyes. The signals generated at the retina are carried via the nerve fibres to the brain, the central processing unit (CPU) of the visual system. These signals are processed and interpreted at the brain, with the aid of previous experiences.

When operating in this mode, the visual system is an example of a "Passive Remote Sensing" system which depends on an external source of energy to operate. We all know that this system won't work in darkness. However, we can still see at night if we provide our own source of illumination by carrying a flashlight and shining the beam towards the object we want to observe. In this case, we are performing "Active Remote Sensing", by supplying our own source of energy for illuminating the objects.

passive remote sensing Active Remote Sensing: The sensing system provides its own source of illumination.

 

The Planet Earth

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The planet Earth is the third planet in the solar system located at a mean distance of about 1.50 x 108 km from the sun, with a mass of 5.97 x 1024 kg. Descriptions of the shape of the earth have evolved from the flat-earth model, spherical model to the currently accepted ellipsoidal model derived from accurate ground surveying and satellite measurements. A number of reference ellipsoids have been defined for use in identifying the three dimensional coordinates (i.e. position in space) of a point on or above the earth surface for the purpose of surveying, mapping and navigation. The reference ellipsoid in the World Geodetic System 1984 (WGS-84) commonly used in satellite Global Positioning System (GPS) has the following parameters:

  • Equatorial Radius = 6378.1370 km
  • Polar Radius = 6356.7523 km

The earth's crust is the outermost layer of the earth's land surface. About 29.1% of the earth's crust area is above sea level. The rest is covered by water. A layer of gaseous atmosphere envelopes the earth's surface.

 

The Earth's Atmosphere

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The earth's surface is covered by a layer of atmosphere consisting of a mixture of gases and other solid and liquid particles. The gaseous materials extend to several hundred kilometers in altitude, though there is no well defined boundary for the upper limit of the atmosphere. The first 80 km of the atmosphere contains more than 99% of the total mass of the earth's atmosphere.

Vertical Structure of the Atmosphere

atmosphere vertical profile

The vertical profile of the atmosphere is divided into four layers: troposphere, stratosphere, mesosphere and thermosphere. The tops of these layers are known as the tropopause, stratopause, mesopause and thermopause, respectively.

  • Troposphere: This layer is characterized by a decrease in temperature with respect to height, at a rate of about 6.5şC per kilometer, up to a height of about 10 km. All the weather activities (water vapour, clouds, precipitation) are confined to this layer. A layer of aerosol particles normally exists near to the earth surface. The aerosol concentration decreases nearly exponentially with height, with a characteristic height of about 2 km.

  • Stratosphere: The temperature at the lower 20 km of the stratosphere is approximately constant, after which the temperature increases with height, up to an altitude of about 50 km. Ozone exists mainly at the stratopause. The troposphere and the stratosphere together account for more than 99% of the total mass of the atmosphere.

  • Mesosphere: The temperature decreases in this layer from an altitude of about 50 km to 85 km.

  • Thermosphere: This layer extends from about 85 km upward to several hundred kilometers. The temperature may range from 500 K to 2000 K. The gases exist mainly in the form of thin plasma, i.e. they are ionized due to bombardment by solar ultraviolet radiation and energetic cosmic rays.

The term upper atmosphere usually refers to the region of the atmosphere above the troposphere.

Many remote sensing satellites follow the near polar sun-synchronous orbits at a height around 800 km, which is well above the thermopause.

Atmospheric Constituents

The atmosphere consists of the following components:

  • Permanent Gases: They are gases present in nearly constant concentration, with little spatial variation. About 78% by volume of the atmosphere is nitrogen while the life-sustaining oxygen occupies 21%. The remaining one percent consists of the inert gases, carbon dioxide and other gases.

  • Gases with Variable Concentration: The concentration of these gases may vary greatly over space and time. They consist of water vapour, ozone, nitrogeneous and sulphurous compounds.

  • Solid and liquid particulates: Other than the gases, the atmosphere also contains solid and liquid particles such as aerosols, water droplets and ice crystals. These particles may congregate to form clouds and haze.

 

Electromagnetic Waves

electromagnetic wave

Electromagnetic waves are energy transported through space in the form of periodic disturbances of electric and magnetic fields. All electromagnetic waves travel through space at the same speed, c = 2.99792458 x 108 m/s, commonly known as the speed of light. An electromagnetic wave is characterized by a frequency and a wavelength. These two quantities are related to the speed of light by the equation,

speed of light = frequency x wavelength

The frequency (and hence, the wavelength) of an electromagnetic wave depends on its source. There is a wide range of frequency encountered in our physical world, ranging from the low frequency of the electric waves generated by the power transmission lines to the very high frequency of the gamma rays originating from the atomic nuclei. This wide frequency range of electromagnetic waves constitute the Electromagnetic Spectrum.

The Electromagnetic Spectrum

Electromagnetic Spectrum

The electromagnetic spectrum can be divided into several wavelength (frequency) regions, among which only a narrow band from about 400 to 700 nm is visible to the human eyes. Note that there is no sharp boundary between these regions. The boundaries shown in the above figures are approximate and there are overlaps between two adjacent regions.

Wavelength units: 1 mm = 1000 µm; 1 µm = 1000 nm.

  • Radio Waves: 10 cm to 10 km wavelength.
  • Microwaves: 1 mm to 1 m wavelength. The microwaves are further divided into different frequency (wavelength) bands: (1 GHz = 109 Hz)
    • P band: 0.3 - 1 GHz (30 - 100 cm)
    • L band: 1 - 2 GHz (15 - 30 cm)
    • S band: 2 - 4 GHz (7.5 - 15 cm)
    • C band: 4 - 8 GHz (3.8 - 7.5 cm)
    • X band: 8 - 12.5 GHz (2.4 - 3.8 cm)
    • Ku band: 12.5 - 18 GHz (1.7 - 2.4 cm)
    • K band: 18 - 26.5 GHz (1.1 - 1.7 cm)
    • Ka band: 26.5 - 40 GHz (0.75 - 1.1 cm)
  • Infrared: 0.7 to 300 µm wavelength. This region is further divided into the following bands:
    • Near Infrared (NIR): 0.7 to 1.5 µm.
    • Short Wavelength Infrared (SWIR): 1.5 to 3 µm.
    • Mid Wavelength Infrared (MWIR): 3 to 8 µm.
    • Long Wanelength Infrared (LWIR): 8 to 15 µm.
    • Far Infrared (FIR): longer than 15 µm.

    The NIR and SWIR are also known as the Reflected Infrared, referring to the main infrared component of the solar radiation reflected from the earth's surface. The MWIR and LWIR are the Thermal Infrared.

  • Visible Light: This narrow band of electromagnetic radiation extends from about 400 nm (violet) to about 700 nm (red). The various colour components of the visible spectrum fall roughly within the following wavelength regions:
    • Red: 610 - 700 nm
    • Orange: 590 - 610 nm
    • Yellow: 570 - 590 nm
    • Green: 500 - 570 nm
    • Blue: 450 - 500 nm
    • Indigo: 430 - 450 nm
    • Violet: 400 - 430 nm
  • Ultraviolet: 3 to 400 nm
  • X-Rays and Gamma Rays

Photons

According to quantum physics, the energy of an electromagnetic wave is quantized, i.e. it can only exist in discrete amount. The basic unit of energy for an electromagnetic wave is called a photon. The energy E of a photon is proportional to the wave frequency f,

E = h f

where the constant of proportionality h is the Planck's Constant,

h = 6.626 x 10-34 J s.

 

Scattering of Electromagnetic Radiation

Scattering of electromagnetic radiation is caused by the interaction of radiation with matter resulting in the reradiation of part of the energy to other directions not along the path of the incidint radiation. Scattering effectively removes energy from the incident beam. Unlike absorption, this energy is not lost, but is redistributed to other directions.

Both the gaseous and aerosol components of the atmosphere cause scattering in the atmosphere.

Scattering by gaseous molecules

The law of scattering by air molecules was discovered by Rayleigh in 1871, and hence this scattering is named Rayleigh Scattering. Rayleigh scattering occurs when the size of the particle responsible for the scattering event is much smaller than the wavelength of the radiation. The scattered light intensity is inversely proportional to the fourth power of the wavelength. Hence, blue light is scattered more than red light. This phenomenon explains why the sky is blue and why the setting sun is red.

The scattered light intensity in Rayleigh scattering for unpolarized light is proportional to (1 + cos2 s) where s is the scattering angle, i.e. the angle between the directions of the incident and scattered rays.

Scattering by Aerosols

Scattering by aerosol particles depends on the shapes, sizes and the materials of the particles. If the size of the particle is similar to or larger than the radiation wavelength, the scattering is named Mie Scattering. The scattering intensity and its angular distribution may be calculated numerically for a spherical particle. However, for irregular particles, the calculation can become very complicated.

In general, the scattered radiation in Mie scattering is mainly confined within a small angle about the forward direction. The radiation is said to be very strongly forward scattered.

 

Absorption by Gaseous Molecules

The energy of a gaseous molecule can exist in various forms:

  • Translational Energy: Energy due to translational motion of the centre of mass of the molecule. The average translational kinetic energy of a molecule is equal to kT/2 where k is the Boltzmann's constant and T is the absolute temperature of the gas.

  • Rotational Energy: Energy due to rotation of the molecule about an axis through its centre of mass.

  • Vibrational Energy: Energy due to vibration of the component atoms of a molecule about their equilibrium positions. This vibration is associated with stretching of chemical bonds between the atoms.

  • Electronic Energy: Energy due to the energy states of the electrons of the molecule.

The last three forms are quantized, i.e. the energy can change only in discrete amount, known as the transitional energy. A photon of electromagnetic radiation can be absorbed by a molecule when its frequency matches one of the available transitional energies.

Ultraviolet Absorption

Absorption of ultraviolet (UV) in the atmosphere is chiefly due to electronic transitions of the atomic and molecular oxygen and nitrogen. Due to the ultraviolet absorption, some of the oxygen and nitrogen molecules in the upper atmosphere undergo photochemical dissociation to become atomic oxygen and nitrogen. These atoms play an important role in the absorption of solar ultraviolet radiation in the thermosphere. The photochemical dissociation of oxygen is also responsible for the formation of the ozone layer in the stratosphere.

Ozone Layers

Ozone in the stratosphere absorbs about 99% of the harmful solar UV radiation shorter than 320 nm. It is formed in three-body collisions of atomic oxygen (O) with molecular oxygen (O2) in the presence of a third atom or molecule. The ozone molecules also undergo photochemical dissociation to atomic O and molecular O2. When the formation and dissociation processes are in equilibrium, ozone exists at a constant concentration level. However, existence of certain atoms (such as atomic chlorine) will catalyse the dissociation of O3 back to O2 and the ozone concentration will decrease.

It has been observed by measurement from space platforms that the ozone layers are depleting over time, causing a small increase in solar ultraviolet radiation reaching the earth. In recent years, increasing use of the flurocarbon compounds in aerosol sprays and refrigerant results in the release of atomic chlorine into the upper atmosphere due to photochemical dissociation of the fluorocarbon compounds, contributing to the depletion of the ozone layers.

Visible Region

There is little absorption of the electromagnetic radiation in the visible part of the spectrum.

Infrared Absorption

The absorption in the infrared (IR) region is mainly due to rotational and vibrational transitions of the molecules. The main atmospheric constituents responsible for infrared absorption are water vapour (H2O) and carbon dioxide (CO2) molecules. The water and carbon dioxide molecules have absorption bands centred at the wavelengths from near to long wave infrared (0.7 to 15 µm).

In the far infrared region, most of the radiation is absorbed by the atmosphere.

Microwave Region

The atmosphere is practically transparent to the microwave radiation.

 

Effects of Atmosphere

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When electromagnetic radiation travels through the atmosphere, it may be absorbed or scattered by the constituent particles of the atmosphere. Molecular absorption converts the radiation energy into excitation energy of the molecules. Scattering redistributes the energy of the incident beam to all directions. The overall effect is the removal of energy from the incident radiation. The various effects of absorption and scattering are outlined in the following sections.

Atmospheric Transmission Windows

Each type of molecule has its own set of absorption bands in various parts of the electromagnetic spectrum. As a result, only the wavelength regions outside the main absorption bands of the atmospheric gases can be used for remote sensing. These regions are known as the Atmospheric Transmission Windows.

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The wavelength bands used in remote sensing systems are usually designed to fall within these windows to minimize the atmospheric absorption effects. These windows are found in the visible, near-infrared, certain bands in thermal infrared and the microwave regions.

Effects of Atmospheric Absorption on Remote Sensing Images

Atmospheric absorption affects mainly the visible and infrared bands. Optical remote sensing depends on solar radiation as the source of illumination. Absorption reduces the solar radiance within the absorption bands of the atmospheric gases. The reflected radiance is also attenuated after passing through the atmosphere. This attenuation is wavelength dependent. Hence, atmospheric absorption will alter the apparent spectral signature of the target being observed.

Effects of Atmospheric Scattering on Remote Sensing Images

Atmospheric scatterring is important only in the visible and near infrared regions. Scattering of radiation by the constituent gases and aerosols in the atmosphere causes degradation of the remotely sensed images. Most noticeably, the solar radiation scattered by the atmosphere towards the sensor without first reaching the ground produces a hazy appearance of the image. This effect is particularly severe in the blue end of the visible spectrum due to the stronger Rayleigh Scattering for shorter wavelength radiation.

Furthermore, the light from a target outside the field of view of the sensor may be scattered into the field of view of the sensor. This effect is known as the adjacency effect. Near to the boundary between two regions of different brightness, the adjacency effect results in an increase in the apparent brightness of the darker region while the apparent brightness of the brighter region is reduced. Scattering also produces blurring of the targets in remotely sensed images due to spreading of the reflected radiation by scattering, resulting in a reduced resolution image.

 

Source : http://www.crisp.nus.edu.sg