RNAV (Area Navigation) allows an aircraft to navigate on any paths that are defined on arbitrary points within the coverage of sufficient navigation aids.
Before RNAV, navigation points were either at a naviad (VOR, NDB etc) or at an intersection (e.g. radial 250 of VOR ABC, with radial 170 of VOR XYZ).
RNAV allows airspace designers more freedom in designing routes. The waypoints and routes are then being coded into a database in the FMS and the pilot only follows the magenta line
.
For RNAV to work, the aircraft needs to have means to calculate where it is with a certain accuracy. This can be done based on GNSS or multiple DME receivers sometimes coupled with the IRU.
RNP (Required Navigation Performance) is basically an extension to RNAV. In addition to knowing where the aircraft is and is going to be, the RNP system is required to have failure monitoring and alerting on it. IF something goes wrong with the navigation system, (either on board or on the ground/ in space) the system will be able to detect the failure and alert for it.
B-RNAV requires track keeping accuracy of 5 NM 95%,
P-RNAV requires track keeping accuracy of 1 NM 95%
RNP 5 requires track keeping accuracy of 5NM 95%, with a probability of exceeding 10NM without detection and alerting less than 0.00001 (10^-5).
see also:
http://www.faa.gov/air_traffic/publi...m/aim0102.html