Definition
A method of area navigation (RNAV) position determination in which the aircraft computes its position by measuring distances from two or more DME ground stations, supplemented by an Inertial Reference Unit (IRU) that maintains position information using onboard motion sensors. The IRU bridges gaps in DME coverage and improves position accuracy when fewer than two suitable DME stations are available.
Plain English
A way for the aircraft to know where it is by measuring how far it is from two ground-based distance stations, with an onboard motion-sensing system filling in whenever those distance signals are weak or unavailable.
Context Anchor
Seen in RNAV departure and instrument procedure discussions when describing what navigation sources an aircraft may use to meet the required path accuracy.
Derivation
DME stands for Distance Measuring Equipment, a ground-and-aircraft system that times a radio signal's round trip to calculate distance. IRU stands for Inertial Reference Unit, from the Latin 'inertia' (sluggishness, resistance to change) -- it senses the aircraft's own motion without needing outside signals. The combined name simply lists the position sources used together: two DME inputs plus an IRU as backup and refinement.
Why Pilots Care
Provides a reliable backup navigation capability for RNAV operations when satellite signals are lost or unavailable.
Analogy
It is like finding your location by measuring how far you are from two known landmarks, then using a motion sensor to keep track of your movement between updates.
Intuition Check
DME/DME/IRU does not mean the pilot simply has two DME displays in the cockpit. It means the navigation system is using two DME distance sources plus inertial tracking to compute aircraft position.
Example Sentence 1
The aircraft was certified for DME/DME/IRU navigation, allowing the crew to fly the RNAV departure even in an area with limited GPS coverage.
Example Sentence 2
During preflight, the crew confirmed the DME/DME/IRU system was aligned and ready for the RNAV procedure.