Definition
A method by which an RNAV system continuously refines its computed position by simultaneously measuring slant-range distances from two or more ground-based DME (Distance Measuring Equipment) stations and triangulating the aircraft's location from those measurements.
Plain English
The aircraft figures out where it is by measuring how far it is from two ground stations at the same time, and keeps refreshing that fix as it flies along.
Context Anchor
Seen in RNAV departure discussions, especially when the aircraft’s navigation system may use ground-based distance signals instead of GPS to keep its position accurate.
Derivation
DME stands for Distance Measuring Equipment, a system that measures distance by timing radio pulses between the aircraft and a ground station. 'Updating' here means refreshing the navigation computer's position fix — using two DME distances together gives a single point on the map, the way two tape measures from known walls would pin down a spot on the floor.
Why Pilots Care
Provides a reliable backup method to keep the navigation solution accurate when GPS is unavailable or unreliable, preventing position drift during instrument departures.
Analogy
Think of standing in a field with two known landmarks. If you know your exact distance from each, you can mark your position on a map. The aircraft does this many times per second using two DME stations.
Grounding Statement
Two distance circles drawn on a chart cross at one point; that crossing point is the airplane's location.
Intuition Check
Updating does not mean installing new software here. It means the navigation system is correcting the aircraft position it has calculated.
Example Sentence 1
Because the aircraft's FMS was certified for DME/DME navigation system updating, it could fly the RNAV departure even with GPS unavailable, provided adequate DME coverage existed along the route.
Example Sentence 2
During the RNAV SID the pilot monitored the DME/DME navigation system updating to confirm the aircraft remained on the published track.