ASCI 511 Concepts and Foundations of Remote Sensing

1
ASCI 511 Concepts and Foundations of Remote
Sensing

• Dr. Walter Goedecke
• Winter 2005

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Topics

• Overview of Remote Sensing
• Electromagnetic Energy, Photons, and the Spectrum
• Visible Wavelengths
• Infrared Sensing
• Thermal Radiation
• Radiation from Real Materials
• Blackbody Radiation
• Microwave Remote Sensing

• Solar Irradiation
• Atmospheric Effects
• Earths Surface Effects
• Interaction of Thermal Radiation with Terrain
  Elements
• Rayleigh Scattering
• Mie Scattering
• Remote Sensing Images

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OVERVIEW OF REMOTE SENSING

• We perceive our surrounding world through our
  five senses
• Sight and hearing do not require close contact
  between sensors and externals
• Thus, our eyes and ears are remote sensors
• We perform remote sensing essentially all of the
  time

(Virtual Science Centre)
4
OVERVIEW OF REMOTE SENSING Remote Sensing from
Afar

• Sensor not in direct contact with objects or
  events being observed
• Information needs a carrier
• Electromagnetic radiation is normally used as
  information carrier
• The output of a remote sensing system is usually
  an image representing the observed scene

(Virtual Science Centre)
5
OVERVIEW OF REMOTE SENSING Remote Sensing of the
Earth

• Information acquired by airborne platforms, such
  as
• Aircraft
• Balloons
• Information acquired by spaceborne platforms,
  such as
• Satellites
• Space Shuttle
• Information pertains to all areas of interest,
  such as
• Land
• Oceans
• Atmosphere

(Virtual Science Centre)
6
OVERVIEW OF REMOTE SENSING Remote Sensing from
Space

• Some practical applications are
• Weather observing
• Mapping and cataloging
• Early warning
• Media coverage
• Extensions of astronomical capabilities, such as
  
• Earthbound telescopes
• Spacecraft carrying visible light sensors
• Addition of radio wave, infrared, ultraviolet,
  x-ray, and gamma ray sensors

(Virtual Science Centre)
7
Energy Interactions with Earth Surface Features

• Solar radiation detected in visible and near
  infrared wavelengths (VNIR)
• Energy reflected/scattered from the Earth
• Resemblance of photographs from camera high in
  space
• Different materials (water, soil, etc.) reflect
  VNIR in different ways
• Each has its own spectral
• reflectance signature

(Virtual Science Centre)
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Electromagnetic Energy
(Remote Sensing Tutorial)
9
Electromagnetic Waves

• Electromagnetic waves are energy transported
  through space in form of periodic, sinusoidal
  disturbances of electric and magnetic fields.
• They travel through space at c 299,792,458
  meters/second (exact) in free space.
• The waves are characterized by frequency and
  wavelength, related by

• c ??
• where
• c speed of light (3108 m/sec)
• ? frequency
• ? wavelength, usually in ?m 10-6 meters, or
  nm 10-9 meters

(Wave Nature of Light)
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Energy Sources and Radiation Principles

• Radio waves through gamma rays are all
  electromagnetic (EM) waves
• Differ only in wavelength
• Length of light--measured crest to crest--is .4
  to .7 microns, where a micron is one-millionth of
  a meter
• Ultraviolet, x-rays, and gamma rays are shorter,
  while infrared, microwaves, television, and radio
  waves are longer.
• AM radio ranges up to miles in wavelength
• Visible light is one form of electromagnetic
  energy, which includes
• Radio waves
• Infrared, or radiant heat
• Ultraviolet rays
• X-rays

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The Electromagnetic Spectrum
(The Wave Nature of Light)
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Photons and Waves

• Photons travel as EM waves having two components
  that oscillate as sine waves at right angles
• One consisting of varying electric field
• Other consisting of varying magnetic field
• Both have same amplitudes that reach maxima and
  minima at same time
• Can travel through vacuum (bend when change
  mediums)

(Remote Sensing Tutorial)
13
Photons cont.

• A photon is quantized energy
• Photons can have different discrete energy values
• The energy of a quantum is given by Planck's
  equation

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Photons and Waves concluded

• Photons of shorter wavelengths (?), or higher
  frequency waves (? or f), are more energetic than
  those of longer wavelengths, or lower frequencies
• An x-ray photon is more energetic than a light
  photon
• Long wavelength radiation can only measure
  distances and objects on the order of the
  wavelength
• An micrometer wavelength infrared light will
  resolve better than decimeter wavelength radio
  waves

(Remote Sensing Tutorial)
15
Electromagnetic Spectrum in Perspective
(Atmospheric Radiation)
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Electromagnetic Waves(Concluded)

• The Sun at 11,000 F emits most energy in the
  visible spectrum
• A human body at 98.6 F emits longer IR
  wavelengths
• Objects reflect EM waves from other sources
• Green leafs reflect green light
• Red flower reflects red light
• IR or ultraviolet also reflected, but cant be
  seen
• Imaging devices which are sensitive to other
  wavelengths are employed
• x-rays pass through body and create shadowgram of
  the interior
• Can identify anomalies such as a broken bone
• Can use film sensitive to thermal infrared
• Create image by heat given off
• Employ thermal devices such as radiometers and
  scanners

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Visible Light Bands

• This narrow band of electromagnetic radiation
  extends from about 400 nm (violet) to about 700
  nm (red).
• The various color 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

(Virtual Science Centre)
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Infrared Bands

• Infrared is from 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 Wavelength Infrared (LWIR) 8 to 15 µm.
• Far Infrared (FIR) longer than 15 µm.
• The NIR and SWIR bands are also known as
  reflected infrared, referring to the main
  infrared component of the solar radiation
  reflected from the earth's surface.
• The MWIR and LWIR are known as thermal infrared

(Virtual Science Centre)
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Infrared Sensing from Space

• IR sensors at longer wavelengths measure the
  thermal infrared radiation from the Earth some
  examples are
• Land
• Sea
• Humans

(Everett)
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Thermal Radiation Principles

• Kinetic temperature
• Measuring device placed in contact with or
  immersed in body
• Average translational energy of molecules
• Radiant temperature
• Radiation of energy as a function of temperature
• Emitted energy is external manifestation of
  energy state
• Used for determining radiant temp of earth
  surface features
• Basis for thermal scanning

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Thermal Radiation Principles

• Any object having temperature greater than
  absolute zero emits electromagnetic radiation
• Intensity and spectral composition a function of
  material type involved and temperature of object
• High temperature ? Short wavelengths
• Lower temperature ? Longer wavelengths
• Blackbody radiation
• Hypothetical, ideal radiator that totally absorbs
  and reemits all energy incident upon it
• Actual objects only approach this ideal

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Thermal Radiation Principles (Continued)
Blackbody Radiation
(Everett IR)
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Other Pertinent Equations (Concluded)

• The dominant wavelength the one at which a
  blackbody radiation curve reaches maximum -- is
  related to temperature by (Wilhelm) Wiens
  displacement law
• ?m A / T
• where
• ?m wavelength of maximum spectral radiance, ?m
• A 2898 ?m K
• T temperature K
• The energy peak shifts toward shorter wavelengths
  with increased temperature
• An example is when a piece of iron changes color
  from red, to orange, to yellow, and then to white
  when heated at higher temperatures.

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Thermal Radiation Principles (Continued)

• The Stefan-Boltzmann law tells how much energy an
  object radiates
• The total radiant exitance from a body surface
  given by the area under curve

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Thermal Radiation Principles (Continued)
(Blackbody)
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Thermal Radiation Principles (Concluded)

• From previous slide, total radiant exitance for
  blackbody varies as fourth power of absolute
  temperature
• Remote measurement of radiant exitance M from a
  surface can be used to infer temperature of
  surface
• This indirect approach to temperature measurement
  used in thermal scanning
• Radiant exitance M measured over discrete
  wavelength range and used to find radiant
  temperature of radiating surface

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Radiation from Real Materials

• All real materials emit only a fraction of the
  energy emitted by a blackbody at the equivalent
  temperature
• Emitting ability of real material as compared to
  a blackbody is called a materials emissivity, ?

• ? ranges between 0 and 1
• Emissivity can vary with wavelength, viewing
  angle, and somewhat with temperature
• A graybody has an emissivity less than 1 but is
  constant at all wavelengths
• A selective radiator has an emissivity that
  varies with wavelength

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Radiation from Real Materials(Concluded)

• Many materials radiate like blackbodies over
  certain wavelength intervals (see figure 5.13,
  page 329)
• Most surface features have peak energy emissions
  in the atmospheric window of 8 to 14 ??m (figure
  5.13, page 329)
• Most thermal sensing done in this region
• However, emissivities of different objects vary
  greatly in this range
• But, for a given material type, emissivity often
  considered constant in this range when broadband
  sensors being used
• Thus, materials often treated as graybodies
• Table 5.2, page 330 provides typical emissivities
• As objects are heated above ambient temperatures,
  emissive radiation peaks shift to to shorter
  wavelengths
• For forest fire mapping, 3 to 5 ?m range may be
  used

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Temperature Examples

• Temperature of sun 5700 Kelvin
• ? sun 2900 / 5700 0.51 ?m
• Suns maximum emission - middle of the visible
  spectrum
• Human body temperature 98.6 F 37 C 310 K
  
• ? body 2900 / 310 9.4 ?m
• Human body emits in the thermal infrared region

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Blackbody, Wien, and Stefan-Boltzmann Summary
(Atmospheric Radiation)
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Microwave Remote Sensing

• Can be passive or active
• Active systems emit pulses of microwave radiation
  to illuminate images of the Earths surfaces
• Images can be acquired day or night
• Wavelengths can penetrate clouds
• Synthetic aperture radar (SAR) provides high
  resolution images of the Earth

(Virtual Science Centre)
32
Microwave Bands

• Microwaves are from 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)

(Virtual Science Centre)
33
Passive Microwave

• Principle underlying passive microwave radiation
  implicit in accompanying curves
• Curves have similar shapes, but intensity
  increases with temperature
• Important point is that there is radiation
  from thermal bodies even at longer
  ?s--albeit at less intensity
• Intensity much weaker but still detectable
  and not attenuated much by atmosphere
• Passive microwave detectors generally operate
  in .15 to 30 cm range

(Remote Sensing Tutorial)
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Passive Microwave Sensing

• These systems do not supply their own
  illumination but sense naturally available
  microwave energy within their field of view
• Operate in much the same manner as thermal
  sensing, but employ antennas rather than photon
  detection elements
• Operate in the low energy tail of the 300 K
  blackbody radiation curve
• Passive microwave signals generally made up of a
  number of source components
• Emitted
• Reflected
• Transmitted
• Signal extremely weak, complex to process
• Particularly effective in detecting soil moisture
  and temperature

• Components of signal
• emitted from object
• emitted from atmosphere
• reflected from surface
• transmitted from subsurface

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Solar Irradiation

• Optical remote sensing depends on sun as sole
  source of illumination
• Spectrum above atmosphere modeled as black body
  radiation spectrum
• Sun has source temperature of 59000 K with peak
  irradiation of 500 nm (0.5 ?m) wavelength

(Virtual Science Centre)

• Ground and space-based sensors used to measure
  suns irradiance
• Significant from .25 to 3 ?m

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Effects of the Atmosphere

• Atmospheric composition causes wavelength
  dependent absorption and scattering
• Atmospheric effects degrade the quality of images
• Some atmospheric effects are correctable before
  an image is analyzed and interpreted

(Virtual Science Centre)
37
Atmospheric Effects

• Atmosphere has substantial effect on the
  intensity and spectral composition of the energy
  recorded by thermal system
• It influences the selection of optimal spectral
  bands to measure thermal energy signals by
• Absorption
• Scatter
• Emission
• Sensing of solids and liquids in two atmospheric
  windows possible.

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Atmospheric Effects
(Remote Sensing tutorial, from Sabins (Remote
Sensing Principles and Interpretation, 1987)
39
Energy Interactions in the AtmosphereScattering

• Unpredictable diffusion of radiation by particles
  in the atmosphere
• Rayleigh scatter is common when radiation
  interacts with atmospheric molecules
• Inversely proportional to fourth power of
  wavelength
• Much stronger tendency for short wavelengths to
  scatter
• Blue sky is result of Rayleigh scatter
• Absence of scatter would result in black sky
• Shorter--blue-- wavelengths scattered more
  dominantly than others
• Causes bluish-gray haze to image
• Can eliminate by filtering short wavelengths
• Mie scatter due in large part to water vapor and
  dust
• Atmospheric particle diameters about same as
  sensed wavelengths
• Nonselective scatter particle diameters much
  larger than sensed ?
• Nonselective to ? fog and clouds appear white

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Energy Interactions in the Atmosphere Absorption

• Effective loss of energy to atmospheric
  constituents
• Most efficient absorbers water vapor, carbon
  dioxide, and ozone
• Absorption takes place in specific wavelength
  bands
• Concept of atmospheric windows
• Visible range coincides with both an atmospheric
  window and the peak level of energy from the sun
• Emitted heat energy from the Earth is sensed
  through windows at 3 to 5 µm and 8 to 14 µm with
  thermal scanners
• Multispectral scanners sense simultaneously
  through narrow ranges in the visible and thermal
  spectral regions
• Radar and passive microwave operate in the 1mm to
  1m region

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Atmospheric Opaqueness
(The Wave Nature of Light)
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Atmospheric Transmittance
(Remote Sensing Tutorial)
43
Energy Interactions with Earth Surface Features

• Solar radiation detected in visible and near
  infrared wavelengths (VNIR)
• Energy reflected/scattered from the Earth
• Resemblance of photographs from camera high in
  space
• Different materials (water, soil, etc.) reflect
  VNIR in different ways
• Each has its own spectral reflectance signature

(Virtual Science Centre)
44
Energy Interactions with Earth Surface Features

• Electromagnetic (EM) energy which is incident on
  the Earths surface is reflected, absorbed,
  and/or transmitted
• The principle of conservation gives EI(?)
  ER(?) EA(?) ET(?)

• where
• EI(?) incident energy
• ER(?) reflected energy
• EA(?) absorbed energy
• ET(?) transmitted energy
• Proportion of energy reflected, absorbed, and
  transmitted is dependent on Earth features, such
  as
• Material type
• Conditions

(Remote Sensing Tutorial)

• Differences permit us to distinguish different
  features on an image

45
Energy Interactions with Earth Surface Features -
Continued

• Reflective properties of materials are dependent
  on wavelength
• Two features may be indistinguishable in one
  spectral band but be very different in another
• Spectral differences result in color in the
  visible band
• Blue at a shorter wavelength
• Red at a longer wavelength
• Since reflectance is very important to many
  remote sensing systems, the energy balance
  equation is a useful tool
• ER(?) EI(?) EA(?) ET(?)
• In addition to its material content, the manner
  in which an object reflects energy is also
  important, such as
• Specular reflections
• Diffuse reflections

46
Interaction of Thermal Radiation with Terrain
Elements

• Interest in thermal sensing is the radiation
  emitted from terrain features
• Energy radiated is usually a function of the
  energy incident on a feature
• Energy incident can be absorbed, reflected, or
  transmitted, as given by
• EI EA ER ET
• where
• I energy incident on surface of terrain element
• A component of incident energy that is absorbed
• R component of incident energy that is
  reflected
• T component if incident energy that is
  transmitted

47
Interaction of Thermal Radiation with Terrain
Elements (Continued)

• Division by EI normalizes to unity, yielding
• Ratios are convenient to further describe thermal
  energy interactions
• where
• ?(?) absorptance of terrain element
• ?(?) reflectance of terrain element
• ?(?) transmittance of terrain element
• Furthermore, as expected, the sum is unity, thus
  defining the interrelationship among terrains
  absorbing, reflecting, and transmitting
  properties

48
Interaction of Thermal Radiation with Terrain
Elements (Continued)Kirchoff Radiation Law

• Kirchoff radiation law states that the spectral
  emissivity of object equals its spectral
  absorptance
• Thus good absorbers are good emitters
• In the previous equation absorptance can be
  replaced with emissivity
• While Kirchoffs law is based on thermal
  equilibrium, the relation holds true for most
  sensing conditions

49
Interaction of Thermal Radiation with Terrain
Elements (Continued)Emissivity

• In most remote sensing applications, objects that
  are dealt with are opaque to thermal radiation,
  or the transmittance is negligible
• Thus, we have
• This means that the lower an objects
  reflectance, the higher its emissivity
• As an example, water has nearly negligible
  reflectance in the thermal spectrum, and
  therefore its emissivity is nearly unity

50
Interaction of Thermal Radiation with Terrain
Elements (Continued)Emissivity

• Emissivity of an object is important when
  measuring radiant temperatures
• The Stefan-Boltzmann (S-B) law applied to
  blackbody radiators is
• M ?T4
• When this is extended to real materials, this
  becomes
• M ??T4
• Above equation describes the interrelationship
  between the measured signal a sensor sees, M, and
  the parameters of temperature and emissivity
• Because of emissivity differences, Earth surface
  features can have the same temperature, but
  completely different radiant exitances

51
Interaction of Thermal Radiation with Terrain
Elements (Continued)Emissivity

• The output from a thermal sensor is a measure of
  sensed objects radiant temperature
• For a blackbody, objects radiant temp equals its
  kinetic temperature
• For real objects, there is an emissivity factor
• Above equation states that radiant temperature
  will always be less than the kinetic temperature
  for real bodies
• A final point is that thermal sensors detect
  radiation from the surface, approximately first
  50 ?m, of ground objects

52
Rayleigh Scattering and the Blue Sky Effect

• Selective scattering (or Rayleigh scattering)
  occurs when certain particles are more effective
  at scattering particular wavelengths of light
• Air molecules (oxygen and nitrogen) are small in
  size and more effective at scattering shorter
  wavelengths of light (blue and violet)
• Selective scattering by air molecules are
  responsible for producing blue skies on a clear
  day

(Image/Text/Data from the University of Illinois
WW2010 Project)
53
Mie Scattering

• This is scattering of all wavelengths
• An example is a large particle whose diameter
  ?, such as a cloud drop, scatters all light
• This is why clouds are white

(Atmospheric Radiation)
54
Remote Sensing Images

• Remote sensing images often in the form of
  digital images (pixels)
• Image processing can be used to enhance an image
• Correct
• Restore
• Segmentation and classification used to delineate
  areas into thematic classes

(Virtual Science Centre)
55
Supplemental References

• The Wave Nature of Light (Michael Blaber),
  http//wine1.sb.fsu.edu/chm1045/notes/Struct/Wave/
  Struct01.htm
• Global Positioning System, Peter H. Dana, The
  Geographers Craft Project, Dept of Geography,
  University of Colorado at Boulder,
  http//www.colorado.edu/geography/gcraft/notes/gps
  /gps_f.htm
• The Wave Nature of Light (Michael Blaber),
  http//wine1.sb.fsu.edu/chm1045/notes/Struct/Wave/
  Struct01.htm
• The Virtual Science Centre Project on Remote
  Sensing, http//www.sci-ctr.edu.sg/ssc/publication
  /remotesense/rms1.htm
• Spectral Reflectance, http//geog.hkbu.edu.hk/GEOG
  3610/Lect-06/sld011.htm
• Everett Infrared and Electro-optic Technology,
  http//www.everettinfrared.com/detectors.htm
• Remote Sensor Tutorials, http//rst.gsfc.nasa.gov
• SAR Imagery Sandia Laboratories,
  http//www.sandia.gov/images/estancia.html
• Scattering of Light, http//ww2010.atmos.uiuc.edu/
  (Gh)/wwhlpr/scattering.rxml?hret/guides/mtr/opt/a
  ir/crp.rxml
• Atmospheric Radiation, http//www.public.iastate.e
  du/sege/radiation.html