Layover

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Layover
Layover occurs when the incidence angle (q) is
smaller than the foreslope (a) i.e., q lt a.
This distortion cannot be corrected!
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Radar Shadowing
Radar shadowing can be useful for interpreting
geomorphological features
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Radar Backscatter
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Radar Backscatter Coefficient

• The efficiency the terrain to reflect the radar
  pulse is termed the radar cross-section, ?
• The radar cross-section per unit area, (A) is
  called the radar backscatter coefficient (?)
  and is computed as
• The radar backscatter coefficient determines the
  percentage of electro- magnetic energy reflected
  back to the radar from within a radar pixel
• This is similar to the reflectance in optical
  remote sensing

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Radar Backscattering
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Radar Backscattering

• Depends on the properties of the target
• roughness
• dielectric constant
• Depends on characteristics of the radar
• depression angle
• frequency/wavelength
• polarization

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• Electrical (E) and magnetic field (B) are
  orthogonal to each other
• Direction of each field is perpendicular to the
  direction of wave propagation.

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Polarization
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Polarization

• Plane polarized light can be either
• vertically polarized (E0 is perpendicular to the
  plane of incidence)
• horizontally polarized (E0 is parallel to the
  plane of incidence)
• Solar radiation is unpolarized (random) but can
  become polarized by reflection, scattering, etc.
• Lasers and radars produce polarized radiation

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Radar Polarization

• Cinder cone and basalt lava flow in north-central
  Arizona.
• Strong return in the HH polarized image and weak
  HV polarization indicates that the lava is not
  depolarizing the radar pulse (it is composed of
  large blocks with smooth faces)

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Rayleigh Criterion for Roughness

• A surface is considered smooth at or below a
  height, h, if
• 
  cm
• h the vertical relief (average height of
  surface irregularities)
• l the radar wavelength (measured in cm)
• g the depression angle

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Nile River Sudan
Space Shuttle Color-Infrared Photograph
SIR-C Color Composite Red C-band HV
Green L-band HV Blue L-band HH
C-band, ?? 6cm L-band, ?? 24cm
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Radar and the Dielectric Constant

• Dielectric constant depends on the type of
  material as well as its moisture state
• it is analogous to the refractive index of the
  material
• it is primarily a function of moisture content
• also depends on chemical properties such as
  salinity
• Dielectric constant is the ratio of the
  capacitance of a material to that of a vacuum.
  Also known as the relative permittivity

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Dielectric Constant
dielectric constant of liquid water is 80 dry
soil is 2-4.
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Radar frequency and backscatter

• Depth of radar penetration through the vegetation
  canopy varies directly with l

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Types of Active Microwave Surface and Volume
Scattering that Take Place in a Hypothetical Pine
Forest Stand
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Response of A Pine Forest Stand to X-, C- and
L-band Microwave Energy
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SIR-C/X-SAR Images of a Portion of Rondonia,
Brazil, Obtained on April 10, 1994
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Interferometric Synthetic Aperture Radar InSAR
RADAR records both the backscattered intensity as
well as the phase of the wave when it interacted
with the terrain
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constructive interference

• Waves of microwave energy can have constructive
  interference or destructive interference
  depending on how they match up

destructive interference
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With SAR interferometry, radar images of the same
area are acquired at two different times
(multiple pass) or from two side-by-side
synthetic aperture radars (single pass) The
interference pattern between the two phase images
gives information on height and
displacement Its analogous to
stereo-photogrammetry except that it uses the
phase information
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Geometric Relationship Between Two SAR Systems
Used for Interferometry to Extract Topographic
Information
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Range differences produce meter-scale accuracy
but phase differences produce millimeter-scale
accuracy The phase shift between the two images
corresponds to relative difference in range
between the two radars
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Shuttle Radar Topography Mission (SRTM)

• Used C and X-band interferometry to produce the
  first near-global topographic map of the Earth.
  February 2000 (10-day mission)
• Space Shuttle Endeavor 2 radar antennas, 60 m
  apart
• Covered 80 of Earths surface (60o N to 54o S)

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InSAR and velocity

• If you have exact repeat overpass then you can
  use phase shifts from one time to another to
  indicate ground movement
• This has been used to map glacier velocities,
  movement along faults, volcanic uplift

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Lambert Glacier, Antarctica
Interferometric Synthetic Aperture Radar
(InSAR) uses phase difference between two images
to determine glacier speed and direction
Image courtesy CSA, NASA, Ohio State Univ., JPL,
ASF
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Radar Sensors

• Geosat
• Seasat
• Shuttle Imaging Radar (SIR-A, SIR-B, SIR-C)
• ERS-1, ERS-2
• JERS-1
• Radarsat
• JPL AirSAR
• TRMM
• SRTM