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I am trying to understand how to determine the Navigation Accuracy for a given flight phase of an Instrument Flight Procedure, from the US Standard for PBN Instrument Procedure Design (Order 8260.58A). However, I am unable to understand how to determine the aforementioned accuracy under certain circumstances, given that, for example in case of A-RNP navigational specification and STAR flight phase, it states that:

1-2-4.c, Table 1-2-1 A-RNP permits a range of scalable RNP lateral navigation accuracies. Apply the largest RNP for the flight phase unless a smaller value is required to achieve a desired ground track or is operationally required. RNP APCH applies to final flight phase.

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My question is, in other words, why a desired ground track would require a smaller navigation accuracy value? Under what circumstances such a ground track would be desirable and, since the goal is to provide an adequate Obstacle Clearance Area, doesn't this mean that smaller accuracy value would lead to a smaller obstacle clearance area?

It is a really technical question and I do not expect to be a popular one, but I would really appreciate if someone could point me in the right direction.

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Im more familiar with EASA then FAA but im sure there are similarities:

why a desired ground track would require a smaller navigation accuracy value?

RNP 2: the aircraft must be within ± 2 nm of the indicated position 95% of the flying time (Required for Basic RNAV).

RNP 1: the aircraft must be within ± 1 nm of the indicated position 95% of the flying time (Required for Precision RNAV).

So flying in a congested airspace (for example the NAT-OTS routes) ATC can accommodate more aircraft with smaller separations.

the goal is to provide an adequate Obstacle Clearance Area

Yes, as well as separation and spacing of aircraft.

doesn't this mean that smaller accuracy value would lead to a smaller obstacle clearance area?

In theory yes. But Obstacle clearance requirements around Airports and high terrain take into account all aircrafts including general aviation and older aircraft without PBN, RNP equipment so while you are right with more accuracy the approach and departure corridors lead to smaller clearance areas due to the regulations the actual clearance areas could stay the same.

More likely there will be changes at the approach and departure procedures (points, fixes and routes).

Hopefully you found something helpful.

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