Introduction to Remote Sensing

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Introduction to Remote Sensing

• Lecture VEH1
• 8-26-03

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What is Remote Sensing?

• The acquisition of information about a target in
  the absence of physical contact
• Measure changes in fields
• Electromagnetic fields (spectroscopy)
• Acoustic fields (sonar)
• Potential fields (gravity)
• Describe as energy intensity vs. wavelength
• map into spatial or temporal variations
• use to interpret properties (e.g., chemistry,
  mineralogy, roughness) of the material

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Remote Sensing Systems

• Two types
• Passive detection of energy from natural
  (solar) illumination or emission
• Active detection of energy after illuminating
  material and observing returned energy
• Long history of spacecraft airborne sensors
• Landsat, TIMS, ASTER, SPOT, AVIRIS, AHI, SEBASS,
  SeaSat, SIR-A,-B,-C, TOPEX/Poseidon

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Energy Detection and Physics of EM Waves

• Interaction of EM energy (light) with material
  changes the energy
• Five basic types of interactions between light
  sources and materials, such that the light is
• Reflected illuminating energy is returned from a
  surface with an angle of reflection equal and
  opposite to the incidence angle characteristic
  of smooth (at scale of wavelength) surfaces
• Scattered illuminating energy is deflected in
  multiple directions characteristic of rough (at
  scale of wavelength) surfaces

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Incident light
Reflected
Scattered
Material
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5 basic types of interactions, cont.

• Transmitted (refracted) illuminating energy is
  passed through the material changes in spectrum
  are caused by change in density (velocity of
  incident wave) between material and surroundings
  (index of refraction)
• Absorbed energy is transformed (usually into
  longer wavelength heat)
• Emitted energy is released from the material
  (it's now the source)

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Incident light
Reflected
Scattered
Emitted
Material
Absorbed
Transmitted
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Energy Detection and Physics of EM Waves

• Energy recorded vs. wavelength controlled by
• Optical properties of the material
• n, index of refraction
• k, absorption coefficient R (n-1)2k2 /
  (n1)2 k2
• Physical properties of the material surface
• ratio of particle size to l (? grain size!)
• Geometric optics (r gtgt l), specular
• Rayleigh scattering (r ltlt l) intensity
  proportional to l-4
• Mie scattering (r l)
• roughness

r
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EM Spectrum

• EM waves vary in wavelength and frequency by
  c ln
• c speed of light (3 x 108 m/sec)? wavelength
  (so, if ????10 µm)? frequency (then ? 3 x
  1013 s-1)
• Relate the frequency (?) to energy by E hn
• h Planck constant (6.626 x 10-34 Jsec)

Long l
Short l
Low Frequency Low Energy
High Frequency High Energy
Note Frequency refers to of wave crests of
same l that pass by a point in 1 second
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Wavelength Ranges

• Gamma rays (lt 0.0001 µm), change in energy state
  of neutrons/protons information on elements
• X-rays (lt 0.01 µm) photons absorbed by the
  inner shell of electrons
• Ultraviolet (UV, lt 0.4 µm) photons
  emitted/absorbed by the outer shell of
  electrons information on transition metals
  (Fe2, Fe3, Cu2), chlorophyll

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Wavelength Ranges, cont.

• Visible (VIS, lt 0.67 µm) similar to UV, also
  sensitive to H2O, OH- information on
  mineralogy, vegetation
• Near infrared (NIR, lt 1.5 µm) similar to VIS
  information on mineralogy, vegetation
• Thermal infrared (TIR, lt 100 µm) photons
  interact with molecular vibrations (controlled by
  bond length, strength) information on
  mineralogy, surface T
• Microwave (0.1 cm 10 m) radar information
  on particle size, macro-roughness

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Interpreting Spectral Information

• Data can be complex function of composition,
  texture, wavelength
• Information content also affected by the
  collection device (sensor) e.g., sensitivity,
  /width of wavelength bands, spatial resolution
• Multispectral Several (usually few - dozen)
  broad, but discrete wavelengths in spectral
  region of interest
• Hyperspectral Many (usually gt100) narrow,
  discrete wavelengths in same spectral region of
  interest

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Interpreting Spectral Information

• Atmospheric gases also interact with light
• Can inhibit use of certain wavelength ranges
• Atmospheric windows are regions not blocked by
  atm. gases/dust - have high transmission and low
  absorption
• H2O, CO2, and O3 are primary VIS-TIR absorbers on
  Earth CO2 is primary at Mars (silicate dust too)
• Transmissivity measure of the fraction of
  radiation that passes through the atmosphere
  unattenuated (varies between 0 and 1)

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Atmospheric Windows
Absorptions primarily due to H2O, CO2, O3
Mostly Opaque due To H2O
Transmittance
Wavelength (µm)
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Interpreting Spectral Information

• Atmospheric influence, cont.
• Path length Distance traveled through the
  atmosphere by a photon
• Function of location of energy source, sensor
  type, wavelength
• Path radiance Energy contributed by interactions
  with atmosphere prior to detection at sensor
• Energy at sensor Esensor Rpath Rground
• Atmospheric correction (separation) algorithms
  remove/lessen contribution of Rpath to get at
  Rground

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Imaging Systems - Overview

• Images are made of up rows and columns of picture
  elements, or pixels
• Values recorded as digital numbers (DN)
• 8-bit (28) data ranges from DN 0 - 255
• 16-bit (216) data ranges from DN 0 - 65,535
• Value chosen (8- or 16-bit) depends on the
  precision required for parameter you are storing
• Display single wavelength data in grayscale
  (human eye only sees 30 levels of gray)
• Display three wavelengths in color, with
  wavelengths represented in red, green, and blue
  (RGB)

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Color Imaging
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Color Imaging
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Color Imaging
THEMIS VIS
Band 1 0.654 µm
Band 2 0. 540 µm
Band 3 0.425 µm
Bands 1,2,3 RGB
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Imaging Systems - Overview

• Spatial resolution
• Defined one of two ways
• Pixel size (area viewed on ground)
• Size of smallest resolvable feature (generally
  larger than pixel size)
• Determined by two parameters
• Height of sensor above ground
• Instantaneous field of view (IFOV) of sensor
• Pixel size H x IFOV
• If H 2 km, and IFOV 2.5 mrad, then pixel 5 m