Radar (Microwave) Remote Sensing
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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.
 
Incidence Angle & Swath Width - Click to Enlarge
Incidence Angle, Swath Width & Pulse Interval - Click to Enlarge


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).

Scattering, Incidence Angle - Click to Enlarge
Backscatter for different incidence angles.
Corner Reflectors - Click to Enlarge
Corner reflectors amplify backscatter, particularly in urban areas,  "double bounce" "triple bounce".
Oil Spills SIR-B SB Channel - Click to Enlarge
Oil film from natural oil seeps off the coast from UCSB. (shorter wavelength)
Oil Islands SIR-C SB Channel - Click to Enlarge
Strong response from metal, see the oil rigs? (longer wavelength)
Worlds Largest Copper Mine - Brazil
Open pit Copper mine.

Angkor, Cambodia
Jungle Temple
 

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.

Combinations of Incidence Angles and Backscatter - Click to Enlarge.EMR - Horizontal and Vertical Polarizations - Click to Enlarge.Horizontal (H) and Vertical (V) Polarizations

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.

Water and the Complex Dialectric - Click to Enlarge.3 Components of Backscatter by Trees - Click to Enlarge

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.
Foreshortening and Layover - Click to Enlarge.Foreshortening and Layover - Click to Enlarge.
SAR transmits pulses of microwaves and measures the strength and the time delay of the energy that is scattered back to the antenna.

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