back scatter

noun

1. the scattering of particles or radiation, such as sound waves, X-rays, or alpha-particles, by the atoms of the medium through which they pass, in the backward direction
2. the radiation or particles so scattered
3. a technique whereby very long-range radars locate targets hidden by the curvature of the earth. Radar beams are reflected off the underside of the troposphere onto the target and the return beams, similarly reflected, are measured