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ASCI 511 Concepts and Foundations of Remote Sensing

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(Virtual Science Centre) 5. OVERVIEW OF REMOTE SENSING. Remote Sensing of the Earth ... (Virtual Science Centre) 7. Energy Interactions with Earth Surface Features ... – PowerPoint PPT presentation

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Title: ASCI 511 Concepts and Foundations of Remote Sensing


1
ASCI 511 Concepts and Foundations of Remote
Sensing
  • Dr. Walter Goedecke
  • Winter 2005

2
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

3
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)
8
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)
10
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

11
The Electromagnetic Spectrum
(The Wave Nature of Light)
12
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

14
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)
16
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

17
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)
18
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)
19
Infrared Sensing from Space
  • IR sensors at longer wavelengths measure the
    thermal infrared radiation from the Earth some
    examples are
  • Land
  • Sea
  • Humans

(Everett)
20
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

21
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

22
Thermal Radiation Principles (Continued)
Blackbody Radiation
(Everett IR)
23
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.

24
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

25
Thermal Radiation Principles (Continued)
(Blackbody)
26
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

27
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

28
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

29
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

30
Blackbody, Wien, and Stefan-Boltzmann Summary
(Atmospheric Radiation)
31
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)
34
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

35
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

36
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.

38
Atmospheric Effects
(Remote Sensing tutorial, from Sabins (Remote
Sensing Principles and Interpretation, 1987)
39
Energy Interactions in the Atmosphere Scattering
  • 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

40
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

41
Atmospheric Opaqueness
(The Wave Nature of Light)
42
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
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