Scattering means the redirection of incident electromagnetic energy by an object. Similar to diffraction, scattering refers to the same physical process, the coherent distortion of an incident wave. However, diffraction as well as reflection can be regarded as essentially forms of scattering. Scattering explicitly describes the “random distortion of waves by elements that are similar in size or less than the wavelength” (Woodhouse, 2005). Thereby, scattering of the incident wave at an object can occur in any directions with varying strength, with the scattering pattern varying with the incident direction. Thus, the term scattering cross section, often denoted by σ, quantifies the effectiveness of a scatterer. In the field of active microwave remote sensing, the backscattering coefficient σ0 is known “as the ratio of the statistically, averaged, scattered power density to the average incident power density” (Fung, 1994).
In passive microwave remote sensing, radiometers measure the intensity of radiation emitted by a body, called brightness temperature TB. Since TB is always less than its physical temperature T, emissivity, defined as e = TB / T, is a measure of how strongly a body radiates at a given wavelength. It varies between 0 (metal) to unity (blackbody).
Emission and scattering are complementary: surfaces that are good scatterers are weak emitters, and vice versa.