Aircraft operators in some parts of the world have already begun to experience the benefits of area navigation (RNAV) and required navigation performance (RNP). These benefits include safer, more efficient operations, increased air traffic management capacity and improved access to certain parts of the airspace and difficult to approach aerodromes. Once aircraft are freed from the limitations of ground based navigation aids and transitioning to satellite based navigation, more flexible and optimized routings become possible, creating more direct routings, saving fuel and reducing CO2 emissions.
Some of the definitions and concepts associated with RNAV and RNP as well as some RNP naming conventions were found to be inconsistent between various parts of the world, resulting in confusion among aircraft operators, manufacturers, regulators and air navigation service providers. This had a negative impact on the implementation of RNAV and RNP applications, slowing buy-in and increasing costs.
Performance Based Navigation (PBN) came into being as a result of collaboration between the International Civil Aviation Organization (ICAO), industry, regulators and air navigation service providers to understand the issues leading to this confusion and to clarify and update the definitions and explanatory material about the RNAV and RNP concepts and applications. To ensure harmonization and consistency, the effort was applied to all areas of flight from oceanic and remote to en-route, terminal area and approach.
From the early days of commercial operations, aircraft have navigated using positions derived in relation to a ground based navigation aid like a Very-High Frequency Omni-Directional Range (VOR), Distance Measuring Equipment (DME) or Non-Directional Beacon (NDB). Because aircraft may fly the most direct routes in relatively few cases, this kind of navigation leads to very inefficient operations and procedures. Additional inefficiencies come from the need to apply large separations buffers to account for the inherent inaccuracies of navigation and to mitigate the effects of eventual operational errors, although this is less of an issue in a surveillance environment.
RNAV was borne as a means of navigating from any point to any other point, whether a point was marked by a navigation aid or not. The points could be defined as latitude and longitude and an aircraft’s position relative to them could be established by a variety of means. For the first time it became practicable to operate on a flight profile that did not have to pass overhead a series of ground based aids.
RNP builds directly on RNAV. When ICAO was faced with the fact that Global Navigation Satellite Systems (GNSS), the navigation infrastructure, aircraft systems and the requirements of the airspace systems were developing faster than what the traditional processes for systems could cope with, the decision was made to define a concept that would allow airspace designers to specify airspace and operational requirements without relying on specific equipment or systems. This concept was named Required Navigation Performance or RNP. The original RNP concept was oriented towards en-route, remote and oceanic airspace and precise navigation and safe separation of routes.
As it had happened so often in the past also with other new ideas, as RNP developed, certain elements were implemented inconsistently while the RNP applications themselves lacked a common basis for interoperability. This led to confusion and slowed down adoption.
When work on guidance also for other phases of flight begun, it was soon realized that it would be impossible to achieve global interoperability with these new concepts unless the assumption on which they are based, like RNP, were consistently applied.
PBN must be seen as the solution that enables future efficiency-enhancing operational concepts.
One of the definitions of PBN is that it is a “framework for defining navigation performance requirements that can be applied to a route, instrument procedure or defined airspace”. PBN, which comprises both RNAV and RNP specifications, provides a basis for the design and implementation of flight profiles that will facilitate airspace design, the flow of traffic and improved access to runways.
In fact one may even argue that the N in the PBN abbreviation should be replaced by an O, for Operations, since the original, navigation focused paradigm is evolving into a paradigm that encompasses much more than navigation in the traditional sense of the word. Performance Based Operations, PBO, would be a more appropriate term to use in the longer term.
The PBN concept specifies that aircraft RNAV system performance requirements be defined in terms of accuracy, integrity, availability, continuity and functionality required for the proposed operations in the context of a particular airspace concept, when supported by the appropriate navigation infrastructure. In that context, the PBN concept represents a shift from sensor-based to performance-based navigation. Performance requirements are identified in navigation specifications, which also identify the choice of navigation sensors and equipment that may be used to meet the performance requirements. These navigation specifications provide specific implementation guidance for States and aircraft operators in order to facilitate global harmonization.
Under PBN, generic navigation requirements are first defined based on the operational requirements. Aircraft operators then evaluate options in respect of available technology and navigation services. A chosen solution would be the most cost-effective for the aircraft operator, as opposed to a solution being established as part of the operational requirements. Technology can evolve over time without requiring the operation itself to be revisited as long as the requisite performance is provided by the RNAV system.
The navigation specification is used by a State as a basis for the development of their material for airworthiness and operational approval. A navigation specification details the performance required of the RNAV system in terms of accuracy, integrity, availability and continuity; which navigation functionalities the RNAV system must have; which navigation sensors must be integrated into the RNAV system; and which requirements are placed on the flight crew.
A navigation specification is either an RNP specification or an RNAV specification. An RNP specification includes a requirement for on-board self-contained performance monitoring and alerting, while an RNAV specification does not.
On-board performance monitoring and alerting is the main element that determines if the navigation system complies with the necessary safety level associated to an RNP application; it relates to both lateral and longitudinal navigation performance; and it allows the flight crew to detect that the navigation system is not achieving, or cannot guarantee with 10–5 integrity as specified in ICAO Doc 9613, the navigation performance required for the operation.
RNP systems provide improvements on the integrity of operations; this may permit closer route spacing and can provide sufficient integrity to allow only RNAV systems to be used for navigation in a specific airspace. The use of RNP systems therefore offer significant safety, operational, efficiency and environmental benefits.
(Source ICAO)