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Layover
Layover
occurs when
the incidence
angle (q) is
smaller than
the foreslope
(a+)
i.e., q < a+.
This
distortion
cannot be
corrected!
Radar Shadowing
Radar shadowing can be useful for interpreting
geomorphological features
Radar Backscatter
Power received
= Power per unit area at target
x
Effective scattering area of the target
Spreading loss of reradiated signal
Effective receiving area of antenna
x
x
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 :
 
o

A
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

Radar Backscattering
Radar Backscattering
Depends on the properties of the target:
– roughness
– dielectric constant
Depends on characteristics of the radar:
– depression angle
– frequency/wavelength
– polarization
Electrical (E) and magnetic field (B) are orthogonal to each other
Direction of each field is perpendicular to the direction of wave
propagation.
Polarization
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
Radar
Polarization
a.
Ka - band, HH polarization
loo k d irection
b.
Ka - band, HV polarization
N
• 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)
Rayleigh Criterion for Roughness
• A surface is considered smooth at or below a height, h, if:
h
l
8sin g
[ cm ]
h = the vertical relief (average height of surface irregularities)
l = the radar wavelength (measured in cm)
g = the depression angle
Nile River
Sudan
Space
Shuttle
ColorInfrared
Photograph
SIR-C Color Composite:
• Red = C-band HV
• Green = L-band HV
• Blue = L-band HH
C-band, l= 6cm
L-band, l= 24cm
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”
Dielectric Constant
dielectric constant of liquid water is 80; dry soil is 2-4.
Radar frequency and backscatter
• Depth of radar
penetration
through the
vegetation
canopy varies
directly with l
Types of Active
Microwave Surface
and Volume
Scattering that Take
Place in a
Hypothetical Pine
Forest Stand
Response of A Pine Forest Stand to X-, C- and L-band Microwave Energy
SIR-C/X-SAR
Images of a
Portion of
Rondonia, Brazil,
Obtained on
April 10, 1994
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
constructive interference
destructive interference
Waves of
microwave energy
can have
constructive
interference or
destructive
interference
depending on how
they match up
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
It’s analogous to stereo-photogrammetry except
that it uses the phase information
Geometric
Relationship
Between Two SAR
Systems Used for
Interferometry to
Extract Topographic
Information
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
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 Earth’s surface (60o N to 54o S)
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
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
Radar Sensors
•
•
•
•
•
•
•
•
•
Geosat
Seasat
Shuttle Imaging Radar (SIR-A, SIR-B, SIR-C)
ERS-1, ERS-2
JERS-1
Radarsat
JPL AirSAR
TRMM
SRTM