True RNP has containment area 2 X RNP with no secondaries. This is used only with RNP AR. The RNP AR aircraft has alerting, accuracy, redundancy, and continuity that is far better than ordinary RNAV, The initial approach segment of an RNAV approach is 6 miles wide, 2 mile primary and 1 mile secondary each side of center-line. OTOH, an RNP AR approach can have an initial approach segment as small as RNP 0.10, which is 1,215 feet each side of center-line. An RNP AR initial that small would be unusual, but 0.50 is not unusual. (1 mile each side of center-line with no secondaries.)
Secondaries in approaches other than RNP AR are necessary to provide the required target level of safety with less sophisticated (and less redundant) avionics than RNP AR. The exception is the LPV final approach segment, which is angular and identical to ILS Category 1. But, even LPV and ILS have secondaries of a sort.
Last edited by aterpster; 20th Nov 2016 at 00:23.