Oktas8:
As an aside....
The description above does not apply to all countries. In some, the RNP specification excludes satellite-only based position fixes. Satellite fixing as part of a full FMS installation is fine, of course
ICAO has, thus far, adopted the U.S. criteria (FAA Order 8260.52) for RNP AR IAPs. The qualification process is quite rigorous and contained in a separate advisory circular (AC 90-101A). (Both the criteria Order 8260.52 and AC 90-101A are posted on the FAA's website.)
In addition to GPS position a minimum of one IRU is required for extraction in the event of a failure of GPS. This is not required to operate to RNP 0.30 minimums so long as the missed approach does not require RNP of less than 1.0 (telescoping missed approach).
As a practical matter thus far all aircraft approved for minimums of less than RNP 0.30 (to as low as RNP 0.10) and/or missed approaches of RNP less than 1.0 all have triple IRUs because the airplanes were designed for oceanic.
Also, RNP AR requires a lot more than GPS and IRUs. The FMS and flight computer redundancy is far greater than with "ordinary" RNAV. TAWS is mandatory as well. Thus, the integrity, alerting, and monitoring is quite rigorous and necessary to achieve a target safety factor of E10-7 that 2XRNP will not be breached.