EARTH SCIENCE > SPECTRAL/ENGINEERING > RADAR > SIGMA NAUGHT
The conventional measure of the strength of radar signals reflected by a distributed scatterer, usually expressed in dB. It is a normalized dimensionless number, comparing the strength observed to that expected from an area of one square meter. Sigma naught is defined with respect to the nominally horizontal plane, and in general has a significant variation with incidence angle, wavelength, and polarization, as well as with properties of the scattering surface itself (see speckle, and statistics).
ESA MetOp Glossary, http://www.esa.int/esaME/ESAYRHT7YYC_index_0.html