What are the differences between radar and optical sensors?
SAR sensors, like any radar sensor, can work without solar illumination and under many weather conditions. They can be used at night as well as during the day because they provide their own illumination. Optical images are influenced by atmospheric conditions (for example the presence of clouds, smoke, aerosol or fog can turn an image useless); visible light has short enough wavelengths to respond to all the individual boundaries between air and water droplets. At each boundary the light is reflected to a new direction, and by the time it escapes the cloud, information on the light’s original direction is hopelessly lost. Radar signals can instead penetrate clouds because the microwaves are not subject to multiple scattering as visible light is. The radar signals in fact are only affected while entering and exiting the cloud. Because they don’t suffer multiple bounces, the radar waves are relatively undistorted by clouds (from http://www.asf.alaska.edu/3_1.html).
