A Squared, I think you are correct.
Now one question. Let say you have an integrated navigation system (like on the airbus) that computes your position and accuracy (and also your Required Navigation Performance), by using your gps in priority but also Dme, vor, ndb and dead reck with the intertials if you are in big ****.
Now you loose your 2 gps. However you still receive dme and/or vor coverage and your navigation performance is still computed and (theoretically) within limits for approach. Would it mean you can still start your RNAV approach?
The next day, you loose not only your 2 gps but also all your antenas that include vor/dme/adf just approach, bad day...luckily your fms is still serviceable to calculate your actual and required navigation performance for an RNAV approach...now your 2-3 irs become your primary means of navigation, but because of short time they didn't drifted much and your nav perf shows "good to go"...so do you do your RNAV approach?
Thanks for your feedback, that's an interesting topic.