From Glossary of Meteorology
A scanning technique in airborne Doppler radar that makes use of both the aircraft motion and scanning to the side of the aircraft to survey the volume of atmosphere surrounding the flight track.
Typically, the radar antenna is located in the tail of the airplane and rotates continuously about an axis aligned with the airplane. The pointing direction may be normal to the flight track or, as common for dual-beam airborne radars, may include pointing angles both forward and rearward of the direction normal to the flight track. The fore and aft directions provide measurements for dual-Doppler analysis that can be used to determine the three-dimensional velocity vector of the echoes.