Radar Backscatter as a Function of Incidence Angle
The brightness on a radar image depends on how "reflective" the surface is. Bright parts of a radar image are strong radar reflectors on the ground, while dark parts of the image represent surfaces that reflect very little energy back to the radar. This reflectivity is called "radar backscatter", and varies as a function of incidence angle and the type of surface that is being imaged.
Here we illustrate the way that backscatter varies for three surfaces as a function of incidence angle. We can see that a smooth surface acts like a mirror when the angle is small, but that the radar backscatter falls off quickly with angles greater than 20 degrees because the signal bounces off the surface in a direction that is away from the radar (much like a stone bounces off a lake when thrown at a shallow angle). The opposite is true for a rough surface. At steep angles (incidence angle less than 20 degrees), a lot of energy is scattered in many directions so that the total backscatter is lower than from a smooth surface at the same angle. Change this incidence angle to a value of 40 or 50 degrees, and a rough surface still produces a lot of random scatter; indeed this is now more than the backscatter from a smooth surface at the same angle.
Using this technique of changing the angle of incidence (which means that we collect data on more than one orbit) it is possible to map the distribution of surfaces (such as lava flows) that have different roughnesses. We will show you examples of this on page 3 of this tutorial.
Go on to Page 3 of this Remote Sensing Tutorial to see how radar backscatter characteristics are used to help map geologic surface units.
Author: Pete Mouginis-Mark
Copyright by P. Mouginis-Mark
Curator: Lori Glaze