If a meter-rule is used to measure surface roughness, then any surface fluctuation of the order of 1 cm or less will be considered smooth. On the other hand, if a surface is examined under a microscope, then a fluctuation of the order of a fraction of a millimiter is considered very rough. In SAR imaging, the reference length scale for surface roughness is the wavelength of the microwave. If the surface fluctuation is less than the microwave wavelength, then the surface is considered smooth. The land surface appears smooth to a long wavelength radar.
Little radiation is backscattered from the surface.
The same land surface appears rough to a short wavelength radar. The surface appears bright in the radar image due to increased backscattering from the surface. The C band is useful for imaging ocean and ice features. However, it also finds numerous land applications. The L band has a longer wavelength and is more penetrating than the C band.