Radar Images
Radar images are composed of pixels and each pixel represents the amount of backscatter from that area on the ground. As with passive microwave energy, the physical properties of objects on the Earth's surface determine the amount and characteristics of the backscatter returned to the sensor. Darker areas in the image represent low backscatter, brighter areas represent high backscatter. Radar images highlight the differences in surface roughness, geometry and moisture content of objects on the surface. The amount of backscatter reflected from a surface depends on the incidence angle of the energy, the wavelength and polariztion of microwave energy, the surface roughness and the electrical characteristic of the surface.
Surface Roughness
The surface “smoothness” or “roughness” in respect to radar depends can depend on the wavelength and incident angle of the microwave energy. A smooth surface or specular reflector will tend to reflect the microwave energy in one direction. Smooth surfaces tend to appear very dark in radar images because all of the backscatter is directed away from the sensor. A rough (lambertian or diffuse) surface will scatters radiation in all directions. Objects like buildings with right angles are corner reflectors. The right angles of corner reflectors cause the microwave energy to bounce off both the surface and side of the feature and direct the majority of the microwave signal back to the sensor.
Flat, smooth surface generally appear black or dark because all of the energy is being reflected away from the sensor. A calm body of water will appear dark in radar imagery. A forest canopy is an example of a rough or diffuse surface and will appear grey in color with varying texture. Rougher surfaces scatter more energy in all directions, including back to the sensor. Therefore rough surfaces appear brighter. Surface roughness is described as a function of wavelength and the angle of incidence of the incoming radiation. Depending on the wavelength and angle of incidence a surface may produce different backscatter.
Dielectric Constant
The dielectric constant is a measure of the reflectivity and conductivity of a material. In the microwave range most dry natural materials on Earth have a dielectric constant in the range of 1 to 8. Water on the other hand has a dielectric constant of approximately 80. The presence of moisture greatly increases the dielectric constant of a material therefore greatly increases the amount of radiation reflected back. Dry soils have low dielectric constant and low radar reflectivity. Wet soils are strong reflectors due to the high dielectric constant. Moist and partially frozen soils will have intermediate values and backscatter. Once a surface becomes entirely flooded it acts as specular reflector, resulting in low backscatter. Flooded areas appear dark in SAR images.
Wavelength of Energy
Longer wavelength bands (P and L bands, 100-15cm) can penetrate forest canopy and reflect off of standing tree trunks. These wavelengths are used to detect the amount of wood in a forest and estimate forest biomass. The shorter wavelengths (C and X bands, 3-5.8cm) are used to detect smaller features like twigs and leaves. The longer the microwave wavelength, the greater the penetration of vegetation canopy.
Examples
Vegetation Response
Generally if a plant canopy is dense there will be strong backscatter. Vegetation that has a greater moisture content will return more energy than drier vegetation because of the increased dielectric constant. The wavelength of the microwave energy also changes the relative amount of backscatter depending on the vegetation types. Radar can be used to identify different types of vegetation by comparing the amount of backscatter from different microwave bands (wavelengths). The below images are of a forested area in Wisconsin, both radar images were acquired the same day but using different wavelength microwave energy. In both images the lakes appear dark due to the specular reflection from the surface and the forest appears grey. A tornado passed through the area approximately ten years prior to these images. A significant amount of re-growth occurred in the following ten years. In the left image, the tornado damage isn't visible because the shorter wavelength C band radar mainly reflects off the leaves and branches in the canopy. We are unable to distinguish the younger trees from the older more established trees because the canopies have similar backscatter. The longer wavelength L band radar reflects primarily off of the trunks of trees, making the younger trees visible and the tornado scar apparent.
Figure adapted from Lillesand, Kiefer and Chipman.
Moisture Content and Flooding
The images below were captured by RADARSAT-1 over Cambodia during the monsoon season. The first image is during the dry season, the MeKong river appears darks and the surrounding areas in shades of grey. The cities appear very bright. After severe rains significant flooding occurred. In the second image the flooded areas appear black due to the specular reflection of the surface. The outer areas actually appear brighter because they aren't flooded, but do have increased soil moisture compared to the dry season.
Color Composites
Similar to multispectral imagery, color composites can be created from radar images when radar data is collected from multiple wavelengths or bands. The color composites can highlight features that interact with different microwave wavelengths. The SAR Spaceborne Imaging Radar aboard the space shuttle Endeavour collected radar data in the L bands (horizontally transmitted and received) and the L-band (horizontally transmitted and vertically received) and the C-band. The below images of Mount Pinatubo in the Philippines were acquired by the space shuttle in 1994. The main volcanic crater on Mount Pinatubo erupted in June 1991. The color composites were made by displaying the L-band (horizontally transmitted and received) in red; the L-band (horizontally transmitted and vertically received) in green; and the C-band (horizontally transmitted and vertically received) in blue. The red on the high slopes of the volcano shows the distribution of the ash deposited during the 1991 eruption, which appears red because of the low cross-polarized (horizontally transmitted and vertically) radar return more energy in the C and L bands. The dark areas coming down from the slopes are smooth mud flows that continue to flood.
