Performance-based navigation was identified in ICAO’s Future Air Navigation System concept of the early 1990s, which defined required navigation performance  capability as a parameter “describing lateral deviations from assigned or selected track as well as along-track position-fixing accuracy on the basis of an appropriate containment level.”

The concept has evolved over the years, explained ICAO’s Erwin Lassooij, and the organization’s recognition that performance and capability do not describe the same thing is reflected in its current definition of RNP as “a statement of the navigation performance necessary for operation within a defined airspace.”

RNP types are normally specified in terms of a number that corresponds to “a containment value expressed as a distance in nautical miles from the intended position within which flights would be at least 95 percent of the time.” For example, RNP4 represents a navigation accuracy of plus or minus four nautical miles 95 percent of the time.

Both RNP4 and RNP10 have been implemented in oceanic and en route airspace, but the need for RNP in terminal airspace has highlighted the lack of containment criteria to support realistic separation standards with the required 99.9-percent probability. As a result, RTCA developed the RNP Rnav concept (DO 236), which focuses on containment integrity and continuity rather than functional integrity.

ICAO defines integrity as “the ability of a system to provide timely warnings to users when the system should not be used for navigation”; RTCA DO 236 defines containment integrity as “a measure of confidence in the estimated position, expressed as the probability that the system will detect and annunciate the condition where the total system error is greater than the cross-track containment limit.”

Harmonizing RNP Implementation

The result is that there are two different standards using similar terminology, and the fact that the ICAO RNP manual does not provide adequate guidance for implementation complicates matters further. Moreover, individual states and regions have implemented differing interpretations of Rnav and RNP.

So far three levels of RNP have been implemented: RNP10 and RNP4 in en route and oceanic airspace, and RNP 0.3 in terminal airspace. However, the conservative obstacle separation criteria adopted for terminal airspace, which govern the minimum clearances allowed between terrain or obstacles and the flight path detailed in a procedure, mean even RNP 0.3 has provided little benefit so far.

ICAO established the RNP special operational requirements study group to develop a common understanding of the RNP concept and the relationship between RNP and Rnav functionality. The group agreed that there is a need for navigation specifications with and without containment integrity, deciding that operations with containment integrity should be designated RNP and those without should be designated Rnav.

Existing Rnav requirements without containment–the European basic Rnav (B-Rnav) and an equivalent used in the Middle East, RNP5–will be designated B-Rnav. A new navigation standard, known as continental Rnav (C-Rnav) and probably based on the U.S. Rnav Type A, will be developed for applications requiring two-nautical mile accuracy in continental en route as well as terminal airspace. And the existing U.S. Rnav Type B and European precision Rnav (P-Rnav) will become terminal Rnav (T-Rnav).

The U.S. and Eurocontrol have agreed that aircraft and operators approved for T-Rnav operations by their state of registry will also meet the requirements for operation in U.S Rnav Type B and P-Rnav airspace. ICAO plans to publish the
terminal Rnav standard in the revised RNP manual.

Terminal approach and departure operations have normally been sensor-specific, so separate designs have been required for each of several approach aids, such as VOR/DME, DME/DME, the basic global navigation satellite system and space- and ground-based augmentation systems.

To avoid the demand on procedure development and publication resources this entails and the resulting operational inflexibility, the RNP concept needs to be applied to the approach phase of flight. Containment will be required if operational benefits are to be achieved, and the RNP special operational requirements study group is developing relevant operational requirements.

The “spider in the web of all ICAO navigation documentation” will be the performance-based navigation manual, Lassooij said. Due to be published in July along with the RNP approach procedure design manual, it will consist of three parts covering the concept, implementation guidance and a compendium of navigation specifications.

To discourage the proliferation of national and regional standards, ICAO Annexes 2, 4, 6, 10, 11 and 15 will be revised to reference the new standards, with state consultation scheduled for August and application to follow three months later.