From ICAO Doc 9613:
18.104.22.168 RNAV 5 operations are based on the use of RNAV equipment which automatically determines the aircraft position in the horizontal plane using input from one or a combination of the following types of position sensors, together with the means to establish and follow a desired path:
c) INS or IRS; and
In the example given, any one sensor or a combination of the following:
Critical DME. A DME facility that, when unavailable, results in a navigation service which is insufficient for DME/DME-based or DME/DME/IRU-based operations along a specific route or procedure.
Which means if you have GNSS on board, and a critical DME (ground station) is out of service, you can still fly that RNAV 5 airway.