Definition
A chart or database notation indicating that an RNAV procedure may be flown using DME/DME position updating (two or more ground-based DME stations) to meet the Required Navigation Performance value of 0.3 nautical miles, in lieu of GPS. The aircraft's Flight Management System must be capable of DME/DME position determination and must have access to suitable DME station geometry along the procedure.
Plain English
This note tells you the procedure can be flown without GPS, by using signals from two ground-based distance-measuring stations to keep the aircraft within 0.3 nautical miles of the intended path.
Context Anchor
Seen as a note on some instrument approach procedures, especially RNAV procedures where DME/DME updating is allowed for navigation.
Derivation
RNP 0.3 means the aircraft must stay within 0.3 nautical miles of centerline 95 percent of the time. The 'DME/DME' prefix specifies which navigation source the FMS is allowed to use to achieve that performance. 'Authorized' simply means the procedure designer has verified the DME coverage along the route supports it.
Why Pilots Care
Determines whether an aircraft without GPS can legally and safely fly a Baro-VNAV or RNP approach that specifies 0.3 performance.
Analogy
Think of the airplane fixing its position by measuring from two known points on the ground. If both distance readings are good, the system can figure out where the airplane is, much like finding your spot on a map from distances to two landmarks.
Intuition Check
Do not read “authorized” as a personal clearance from ATC. Here it means the published procedure permits this navigation method, but only for aircraft with the proper approved equipment.
Example Sentence 1
The approach chart was annotated DME/DME RNP 0.3 Authorized, so the crew briefed a backup plan in case GPS was lost during the arrival.
Example Sentence 2
Before departure the dispatcher confirmed the airplane remained DME/DME RNP 0.3 Authorized after the recent avionics update.