Plain English Meaning

A way for the aircraft to know where it is by listening to two ground stations that tell it how far away they are, and using an internal motion-sensing unit as a backup when those ground stations can't be heard.

Full Definition

An area navigation (RNAV) method in which the aircraft determines its position primarily by measuring distances from two or more ground-based DME stations, supplemented by an onboard Inertial Reference Unit (IRU) that provides position information when adequate DME coverage is not available. The IRU bridges gaps in DME signal coverage so that RNAV accuracy is maintained throughout the route.

Context Anchor

Seen on IFR en route charts and in RNAV equipment discussions, especially when a route or operation depends on what navigation sources the aircraft can use.

Derivation

DME comes from ''; IRU from 'Inertial Reference Unit' (inertial = based on motion sensed inside the aircraft itself); RNAV from ',' meaning navigation along any chosen path rather than only -to-station. The combined name simply lists the position sources the uses, in order of priority.