RNP-1 certification is required, either in the basic Airplane Flight Manual or via Supplemental Type Certificate. Can you get your airplane certified without GNSS? Some GNSS installations can't even be certified below RNP-5, due to [lack of] other installed equipment...
In the US, FAA Advisory Circular 20-138D (Sec 6-4, 6-8, 7-3, 9) discusses requirements for RNAV-1/2 and RNP-1/2 using INS/IRU with DME-DME updating equipment
Last edited by Intruder; 27th Aug 2014 at 00:18.