From Glossary of Meteorology
(Also called range folding.) In radar meteorology, a sampling problem that arises when echoes located beyond the maximum unambiguous range (rmax) are received as if they were within this range of the radar.
A radar ordinarily computes range to targets by measuring the time interval between the transmission of a pulse and the receipt of the returned signal, assuming that the signal was associated with the pulse just transmitted. However, depending on the pulse repetition frequency, the returned signal may be associated with one of several pulses transmitted prior to the latest one. Therefore, a returned signal, indicated as originating at range r, could have originated at r + rmax (second-trip echo), or r + 2rmax (third-trip echo), etc. A range-aliased echo from a weather target is sometimes recognizable by a distorted shape. It may appear elongated radially or shrunk in azimuth extent because the radial length is unaffected by aliasing and is a correct measure of the target size while the azimuthal width decreases with increasing range from the radar.