The diagrams below illustrate some of
the common terms used to describe the geometry of a radar image. Most
important are the "look angle", the angle
at which the radar pulse hits the surface, and the interval between
pulses.
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| Radar images show surfaces that
bounce back the radar pulse (called "radar backscatter"), the bright
areas are strong reflectors (such as buildings in an urban area, or a
bolder field in the desert), while dark parts of the image represent
surfaces that reflect very little or no energy (such as a frozen lake
or oil film on the ocean). The amount of
backscatter varies as a function of incidence angle, surface roughness
and soil moisture (depending on the wavelength).
A smooth surface acts like a mirror when the angle is small, but with angles greater than 20 degrees the amount of backscatter sharply declines because the signal bounces off the surface away from the antenna. The opposite is true for a rough surface. At steep angles (incidence angle less than 20 degrees), most of the emitted pulse is scattered in random directions so that the total backscatter measured by the antenna is lower than from a smooth surface at the same angle. By changing the incidence angle and comparing how the backscatter changes with different angles it is possible to map different types of surfaces (such as smooth vs. rough lava). |
Backscatter for different incidence angles. |
Corner reflectors amplify backscatter, particularly in urban areas, "double bounce" "triple bounce". |
 Oil film from natural oil seeps off the coast from UCSB. (shorter wavelength) |
 Strong response from metal, see the oil rigs? (longer wavelength) |
 Open pit Copper mine. |
Angkor, Cambodia
Jungle Temple
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Using different combinations of wavelength and incidence angle, the characteristics of the recorded backscatter can be compared and interpreted. Acquiring a detailed dataset with the necessary calibration field work and measurements is an ordeal to plan and execute but with a good dataset radar imagery can reveal characteristics of the landslide that visible and infrared imagery cannot.
The assumption is that different vegetation types (e.g., desert, grasslands, forests or frozen tundra) will all have different backscatter signatures. In addition, the basic reflectivity of the soil, called the "dielectric constant" will change depending on the amount of water and organic matter that the soil contains. Dry soil has a low dielectric constant, so that little radar energy will be reflected. Saturated soil will have a high dielectric and will be a strong reflector. Moist and partially frozen soils will have intermediate values.
Short wavelength radar (3 cm)
will be reflected from the tops of trees. Long wavelength radar (24 cm)
data will normally go down to the ground and be reflected back.
Intermediate wavelengths (6 cm) will sometimes experience multiple
scattering events between the canopy, the branches and the ground. By
acquiring multiple wavelength, multi directional radar imagery of a
forested area it is possible to discern information about the canopy
structure and estimate above ground boimass. |
