Performance-based navigation

Historically, aircraft navigation specifications have been specified directly in terms of sensors (navigation beacons and/or waypoints). A navigation specification that includes an additional requirement for on-board navigation performance monitoring and alerting is referred to as a required navigation performance (RNP) specification. One not having such requirements is referred to as an area navigation (RNAV) specification.

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. The navigation specifications provide specific implementation guidance in order to facilitate global harmonisation.

Under PBN, generic navigation requirements are first defined based on the operational requirements. Civil aviation authorities then evaluate options in respect of available technology and navigation services. A chosen solution would be the most cost-effective for the civil aviation authority, 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 or RNP system.

PBN offers a number of advantages over the sensor-specific method of developing airspace and obstacle clearance criteria:

1. reduces the need to maintain sensor-specific routes and procedures, and their costs. For example, moving a single
   approaches, missed approaches
   , etc. Adding new sensor-specific procedures would compound this cost, and the rapid growth in available navigation systems would soon make sensor-specific routes and procedures unaffordable;
2. avoids the need for developing sensor-specific operations with each new evolution of navigation systems, which would be cost-prohibitive. The expansion of satellite navigation services is expected to contribute to the continued diversity of RNP and RNAV systems in different aircraft. The original basic
   Global Navigation Satellite System (GLONASS) will further improve GNSS performance. The use of GNSS/inertial
   integration is also expanding;
3. allows for more efficient use of
   noise mitigation
   );
4. clarifies how RNAV systems are used; and
5. facilitates the operational approval process for civil aviation authorities by providing a limited set of navigation specifications intended for global use.

Within an airspace, PBN requirements will be affected by the communication, surveillance and air traffic control (ATC) environments, the navaid infrastructure and functional and operational capability needed to meet the ATM application. PBN performance requirements also depend on what reversion, non-RNAV means of navigation are available and what degree of redundancy is required to ensure adequate continuity of operations.

To achieve the efficiency and capacity gains partially enabled by RNAV and RNP, the FAA will pursue use of

Performance-based navigation (PBN) is ICAO's initiative to standardise terminology, specifications and meanings. One example is to standardise the terminology used around APVs (Approaches with vertical guidance). All APVs have until recently been designated as RNAV-approaches, while these in fact are RNP-approaches with respect to the PBN implementation. All APVs require on-board performance monitoring and alerting, so the system cannot only be capable of navigation down to the required degree of accuracy, but also needs to continuously monitor the performance and be capable of alerting the pilot if its performance falls below that which is required.

These approaches had some confusing names and designations on charts, and the changeover is currently being conducted across all member states. The two types of RNAV-approaches have traditionally been named RNAV (GNSS) and RNAV (RNP) respectively, where the former is the traditional straight-in approach from the final approach fix, and the latter is a more complex approach that curves in the horizontal plane after the final approach fix which requires authorization for it to be commenced (AR = Authorization Required). The correct naming and designation for these approaches under the PBN implementation are RNP and

In procedurally controlled airspace, separation minima and route spacing on RNP specifications are expected to provide a greater benefit than those based on RNAV specifications. This is because the on-board performance monitoring and alerting function could alleviate the absence of ATS surveillance service by providing an alternative means of risk mitigation.

Transition to PBN

It is expected that all future RNAV and RNP applications will identify the navigation requirements through the use of performance specifications rather than defining specific navigation sensors.

The Valley of Mexico will be the first in Mexico where the performance-based navigation system is used, which will allow the new

On-board performance monitoring and alerting capabilities fulfill two needs, one on board the aircraft and one within the airspace design. The assurance of airborne system performance is implicit for RNAV operations. Based upon existing airworthiness criteria, RNAV systems are only required to demonstrate intended function and performance using explicit requirements that are broadly interpreted. The result is that while the nominal RNAV system performance can be very good, it is characterised by the variability of the system functionality and related flight performance. RNP systems provide a means to minimise variability and assure reliable, repeatable and predictable flight operations.

On-board performance monitoring and alerting allow the air crew to detect whether or not the RNP system satisfies the navigation performance required in the navigation specification. On-board performance monitoring and alerting relate to both lateral and longitudinal navigation performance.

On-board performance monitoring and alerting is concerned with the performance of the area navigation system.

• "on-board" explicitly means that the performance monitoring and alerting is affected on board the aircraft and not elsewhere, e.g. using a ground-based route adherence monitor or ATC surveillance. The monitoring element of on-board performance monitoring and alerting relates to flight technical error (FTE) and navigation system error (NSE). Path definition error (PDE) is constrained through database integrity and functional requirements on the defined path, and is considered negligible.
• "monitoring" refers to the monitoring of the aircraft's performance as regards its ability to determine positioning error and/or to follow the desired path.
• "alerting" relates to the monitoring: if the aircraft's navigation system does not perform well enough, this will be alerted to the air crew.

RNAV and RNP specific functions

Performance-based flight operations are based on the ability to assure reliable, repeatable and predictable flight paths for improved capacity and efficiency in planned operations. The implementation of performance-based flight operations requires not only the functions traditionally provided by the RNAV system, but also may require specific functions to improve procedures, and airspace and air traffic operations. The system capabilities for established fixed radius paths, RNAV or RNP holding, and lateral offsets fall into this category.

Fixed radius paths

Fixed radius paths (FRP) take two forms:

1. the radius to fix (RF) leg type is one of the leg types that should be used when there is a requirement for a specific curved path radius in a terminal or approach procedure. The RF leg is defined by radius, arc length and fix. RNP systems supporting this leg type provide the same ability to conform to the track-keeping accuracy during the turn as in straight line segments. Bank angle limits for different aircraft types and winds aloft are taken into account in procedure design.
2. the fixed radius transition (FRT) is intended to be used in en-route procedures. These turns have two possible radii, 22.5 NM for
   high altitude routes (above FL
   195) and 15 NM for low altitude routes. Using such path elements in an RNAV route enables improvement in airspace usage through closely spaced parallel routes.

Aircraft operating in the North Atlantic airspace are required to meet a minimum navigation performance specification (MNPS). The MNPS specification has intentionally been excluded from PBN because of its mandatory nature and because future MNPS implementations are not envisaged.^[5]

Future developments

It is likely that navigation applications will progress from 2-dimensional to 3-dimensional/4-dimensional applications, although time-scales and operational requirements are currently difficult to determine. Consequently, on-board performance monitoring and alerting is still to be developed in the vertical plane (vertical RNP) and ongoing work is aimed at harmonising longitudinal and linear performance requirements. Angular performance requirements associated with approach and landing will be included in the scope of PBN in the future. Similarly, specifications to support helicopter-specific navigation and holding functional requirements may also be included.

See also

• Index of aviation articles

References