Backscatter

Backscatter is the fraction of transmitted radar or acoustic energy that returns toward the source, the basic observable of active remote sensing. In radar it is reported as the normalized radar cross section sigma-naught, in decibels, and it rises and falls with surface roughness: a scatterometer reads sigma-naught to retrieve wind speed because capillary waves grow with wind. A calm slick lowers backscatter and shows dark in a SAR image, while breaking seas and sea ice raise it. In echo sounding, seabed backscatter strength classifies bottom type alongside the bathymetry. The same physics drives altimeter waveform shape and the sigma-naught wind estimate on Jason and Sentinel-6.