From Glossary of Meteorology
(Also called deflection-modulated indicator.) One of two general classes of radar indicator in which the sweep of the electron beam is deflected vertically or horizontally from a base line to indicate the existence of an echo from a target.
The amount of deflection is usually a function of the echo signal strength. Common types of amplitude-modulated indicators give only the slant range between radar and target. However, since the characteristics of the target signal can be easily observed with this type of indicator, it is very useful in radar meteorology.