z
The Barometric Vertical NAVigation (Baro-VNAV) is a navigation capability of the FMS that computes vertical guidance referenced to a specified vertical path. The vertical guidance is based on barometric altitude.  This means that the Altimetry System Error (ASE) is also a component of the vertical Total System Error (TSE ). 
z
True position
Estimated position

The Total System Error (TSEz) is defined as follows:
TSEz =√ ( FTEz)2+(HCE)2+(ASE)2
Each aircraft operating in airspace where vertical performance is specified shall have a Total System Error  in the vertical direction (TSEz) that is less than the specified performance limit 99.7% of the flying time.
For example, the specified performance at or below 5000ft in ED-75/DO-236 is 160 ft.
There is no integrity and continuity requirement for the vertical navigation.

3.4.  INSTRUMENT APPROACH PROCEDURE DESIGN CRITERIA
The instrument approach procedures are designed using either ICAO PANS OPS or FAA TERPS standards. These standards define the protection area, which must be clear of obstacles. The size and the shape of these protection areas are a function of the type of approach and the accuracy of the associated navigation means.
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3. The RNP Concept
Getting to grips with RNP AR

For RNP RNAV approaches designed with ICAO PANS OPS criteria (DOC 8168 §33 or §35 amdt 12), which are straight in approaches from the IF (Intermediate Fix) with RNP 0.3 NM, the lateral obstacle protection areas include a buffer in addition to the 2RNP half width corridor (see figure below). For example in the final approach leg the buffer is 0.2 NM and the Minimum Obstacle Clearance (MOC) is a fixed value of 250ft.
Note: These paragraphs of PANS OPS are planned to be modified in 2008 or later.

All Airbus aircraft with FMS and GPS, are certified for basic RNP approach in compliance with the ED-75/DO-236 and thus can fly this type of RNP RNAV approaches.
For RNP AR instrument approach procedures designed with the criteria of FAA Order 8260.52 or the ICAO RNP AR procedure design manual, the protected area is limited to 4xRNP (2RNP on both sides of the flight path without buffer) and the value of the RNP can be as low as 0.1 NM. The Required Obstacle clearance is linked to the aircraft Vertical Error Budget (VEB). The VEB has 3 main components, one associated with the aircraft navigation system longitudinal navigation error, the ASE and the FTE.
Note: The definition of the VEB is very similar to the definition of the TSEz given in § 3.3 above.
The density of obstacle is not very often as suggested in the picture below, but it is the situation that the aircraft manufacturer has to take into consideration for the aircraft certification.

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3. The RNP Concept