CNS/ATM resource kit – Chapter 6: Performance-based navigation

The navigational component of CNS/ATM is performance-based navigation or PBN, which is becoming more common worldwide. For more than 15 years, Australian aviation has been adopting GNSS-based area navigation in place of ground-based navigation aids such as non-directional beacons (NDB) and VHF omni-range (VOR) for the en route, terminal and approach phases of flight.

This chapter will introduce PBN, discuss PBN-based approach designs and look at deeming provisions for GNSS-equipped aircraft.

What is PBN?

Performance-based navigation (PBN) is the internationally recognised regulatory framework for implementing area navigation, with an emphasis on GNSS as the enabling technology.

PBN includes the definition of navigation specifications in terms of the accuracy, integrity, continuity and functionality required for various types of operations. It uses on-board equipment such as global navigation satellite systems (GNSS) receivers, stand-alone navigators, and integrated navigation systems.

PBN is absolute navigation—the aircraft determines its current latitude and longitude, and where it is in relation to the intended flight path. As long as the aircraft has a means of determining its current position, it can operate anywhere within coverage of the relevant GNSS system.

This contrasts with traditional relative navigation, based largely on fixed ground-based navigation aids which guide aircraft along published routes via waypoints defined by the aids.

What does performance mean?

Under PBN, airspace and route design take into account the aircraft operations in the region, and the capability of aircraft flying in it.

Both aircraft and flight crew must meet performance standards for the route, which may change with the flight phase (en route, approach etc.) and the class of airspace in which the aircraft is flying.

Specifications

PBN encompasses two types of navigation specifications:

  • RNAV (area navigation), and
  • RNP (required navigation performance).

The difference between the two specifications is that on-board performance monitoring and alerting is required for RNP but not for RNAV. RNAV requires independent performance monitoring of an aircraft’s position.

RNP has parallel lateral performance requirements and can be supported by a variety of technologies. In Australia, RNP operations require GNSS but can be supplemented by inertial systems.

Transition from conventional navigation to RNP
Transition from conventional navigation to RNP

RNAV

The RNAV family of navigation specifications were created by ICAO to consolidate the disparate approvals developed by countries around the world, including:

  • US RNAV Type A and B
  • European B-RNAV and P-RNAV
  • Australian AUSEP and GPS OCEANIC.

While it remains possible to operate using RNAV based on DME/DME, DME/VOR or inertial navigation systems, Australia lacks the substantial infrastructure required to do so. For this reason, GNSS will be the basis of navigation for most aircraft.

RNAV defines fixes by name, latitude and longitude. These area navigation fixes allow planning of routes which are less dependent on the location of navaids.

RNP

In an aircraft using a stand-alone GNSS, the functionality requirements of RNP are achieved through the use of receiver autonomous integrity monitoring (RAIM).

Integrated area navigation systems employ several sources of information, such as inertial and GNSS, to provide highly accurate navigation. They use aircraft autonomous integrity monitoring that are equivalent to RAIM.

Further information on augmentation systems is available in Chapter 3.

RNAV specifications, except oceanic and remote RNAV 10 (RNP 10), are not implemented or used in Australian airspace.

Australia is implementing the following navigation specifications
Australia is implementing the following navigation specifications

Although RNP 10 is a commonly used specification, it actually belongs in the RNAV family. This is a product of history.

Benefits of PBN

Performance-based navigation allows pilots, operators and air traffic control to make the best use of advances in navigation technology and brings increased safety, efficiency and environmental benefits.

The International Civil Aviation Organization (ICAO) says that PBN helps the aviation community by reducing congestion, helping to maintain reliable all-weather operations at even the most challenging airports, conserving fuel, protecting the environment, and reducing the impact of aircraft noise. The benefits can be seen in the table below.

This table contains benefits of PBN
Benefit Development
Reduced separation standards for all phases of flight As the skies become busier, PBN allows the most efficient use of available airspace, through appropriately managed reductions in separation standards during the en route, approach and landing phases. Australia’s airways system can handle more aircraft and do this more safely within time and airspace constraints.
PBN and GNSS allow straight-in approaches ICAO data shows that straight-in approaches are 25 times safer than circling approaches. Adding vertical guidance to the approach brings a further safety gain.
Reduced reliance on radio-navigation aids through widespread use of GNSS-enabled PBN Most airports are served by satellite-based approaches. In many cases, these have replaced approaches using NDB and VOR radio-navigation aids. These ground-based aids are 70-year-old technology, which is becoming increasingly expensive to install and maintain. About 180 ground-based navaids were switched off from May 2016 as part of Airservices Australia’s navigation rationalisation project. The remaining aids are retained for contingency navigation only.
Reduced track miles/fuel burn/carbon dioxide emissions during landing approaches PBN reduces unproductive flight time, unnecessary delays and fuel burn, providing obvious economic benefits to operators and the environment. Advanced PBN applications now under development will deliver further efficiencies through time-of-arrival control and continuous descent arrivals (CDFA). Four dimensional air traffic management will bring even more efficiency, with aircraft operating on direct routes at optimum altitudes, thus avoiding the congested arrival holding pattern.
Global harmonisation ICAO’s PBN navigation standards are being applied worldwide for use by any authorised operator from any ICAO state. This means that certifying both operators and aircraft will be much easier, and aircraft will be operating to global standards.

The importance of accurate position estimates

Technologies such as ADS-B help overcome some of the limitations of ground-based navigation aids in en route position finding. But they are not a substitute for good communication practices.

A number of safety incident reports have involved pilots either not arriving at a reporting point within two minutes of their estimate, or not updating their estimate when it was outside the two minutes, as required in the Aeronautical Information Package (AIP).

The majority of these reports involved aircraft in a climb, descent, regaining track after a diversion, or around the Australian flight information region boundary.

In areas outside radar or ADS-B coverage, air traffic controllers use the aircraft track, altitude and position estimates advised by the pilot to provide separation from other aircraft or airspace. This means that if your tracking, altitude or estimates provided to ATC are not accurate, it is possible that your separation with other aircraft or airspace may also be compromised.

Further information is available from the Airservices Australia website.

Potential sources of error with area navigation

The RNAV and RNP specifications define the accuracy required in both the cross-track (lateral) and along-track (longitudinal) dimensions.

Lateral navigation. Aircraft tracking and positioning errors may lead to navigation being less accurate than required. Three errors in on-board performance monitoring and alerting contribute to the total system error (TSE), and are shown in the illustration below:

  • path definition error (PDE). This occurs when the path defined in the RNAV system does not correspond to the desired path i.e. the path expected to be flown over the ground
  • flight technical error (FTE). This relates to the autopilot’s ability to follow the defined path or track, including any display error
  • navigation system error (NSE). This refers to the difference between the aircraft’s estimated position and its actual position.
Potential sources of error with area navigation
Potential sources of error with area navigation

Longitudinal navigation specifications define requirements for along-track accuracy, which includes navigation system error (NSE) and path definition error (PDE).

There is no flight technical error (FTE) in the longitudinal dimension, and PDE is considered negligible.

The along-track accuracy affects position reporting (e.g. ‘10 nm to ABC’) and procedure design (e.g. minimum segment altitudes, where the aircraft can begin descent once crossing a fix).

The on-board performance monitoring and alerting requirements in the RNP specifications are defined for the lateral dimension for the purpose of assessing an aircraft’s compliance. However, NSE is considered as a radial error so that on-board performance monitoring and alerting is provided in all directions.

How PBN provides more flexibility than conventional navigation

Standard instrument departure (SID) and standard terminal arrival route (STAR)

SIDs are designated IFR departure routes linking an airport or runway with a significant point, normally on a designated air route, at which the en route phase of flight commences.

STARs are designated IFR arrival routes linking a significant point, normally on an air route, with a point from which a published instrument approach procedure can be commenced. Major airports typically have a ‘family’ of STARs which link major air routes to instrument approach procedures.

Leg types

A leg type describes the desired path proceeding, following, or between waypoints on a procedure. Tracks are intercepted to and from stations and waypoints with reference to navigation aids/systems using ground-based and satellite-based navigational systems.

Leg types are identified by a two-letter code that describes the path (e.g. heading, course, track, etc.) and the termination point (e.g. the path terminates at an altitude, distance, fix, etc.). Leg types used for procedure design are included in the aircraft navigation database, but not normally provided on the procedure chart. The path and terminator concept defines that every leg of a procedure has a termination point and some kind of path into that termination point.

Instrument approaches

ICAO has introduced a method of classifying instrument approaches—Type A and Type B. Details are contained in ICAO Annex 6 Part 1 Chapter 4.

Approaches are then flown using either a two dimensional (2D) or a three dimensional (3D) methodology.

Use of GNSS for instrument approaches

ICAO recognises GNSS and augmented GNSS signal in space (SIS), and traditional ground-based aids, as suitable technologies to support a range of 2D and 3D approaches.

The official ICAO term for RNAV(GNSS) approaches (previously called GPS APPROACH in Australia) is now RNP APCH, although RNAV(GNSS) will be around for some time until charts and databases are updated to the new ICAO charting standards.

2D approaches

Two dimensional approaches use lateral guidance only. Examples are NDB, VOR, localiser (LLZ) or GNSS (required navigation performance—RNP).

With 2D approaches it is the pilot’s responsibility to adhere to all step-down altitudes and use the minimum descent altitude (MDA) procedure.

With the advent of GNSS, including its various augmentations, a range of different 2D approaches are possible. These include:

This table contains various augmentations, a range of different 2D approaches
Approach Description
RNP APCH–LNAV

The superseded RNAV/GNSS approach (APCH) is replaced by the new RNP APCH lateral navigation (LNAV) approach definition.

The aircraft must be equipped with an appropriately authorised TSO-C129 sensor or navigator, a TSO-C145 GNSS sensor, or a TSO-C146 stand-alone GNSS system. This equipment must been installed correctly as described in CASA Advisory Circular 21-36.

LP Localiser performance (LP) uses satellite-based augmentation (SBAS)-provided lateral guidance to tolerance similar to ILS LOC.
LNAV+V Modern GNSS receivers may have the capability to present an ‘advisory’ vertical profile for the final segment. These are often called LNAV+V. This profile is generated by the receiver and is not based on an underlying approach design. While using this form of guidance, pilots are responsible for any step-down altitudes and must use the MDA procedure.

3D approaches

Three dimensional approaches use both lateral and vertical guidance, with the vertical profile provided by the guidance system. A decision altitude (DA) minimum procedure is used.

Instrument landing systems (ILS), microwave landing systems (MLS) and ground-based GNSS augmentation landing systems (GLS) can provide Cat I, II or III level of minimums.

There are several types of RNP APCH with 3D vertical guidance, and they differ in the way in which they source their vertical guidance information.

This table contains several types of RNP APCH with 3D vertical guidance
Approach Description
RNP APCH–LNAV/VNAV (baro-VNAV)

Barometric vertical navigation (baro-VNAV) uses a combination of lateral guidance and a vertical profile generated by on-board equipment from the instrument approach design database. A suitable aircraft pitot static and barometric system is required. The flight management system (FMS) calculates the descent path using this barometric information.

An important limitation of baro-VNAV is that the actual path flown by the aircraft is dependent on the ambient air density. A temperature higher than ISA will result in a steeper approach path and temperatures lower than ISA will result in a lower descent profile.

Baro-VNAV approaches may have published temperature limits and will not be available should the ambient temperature lie outside the permitted range.

LPV

Localiser performance with vertical guidance (LPV) requires a satellite-based augmentation system (SBAS) service and currently provides approaches equivalent to an instrument landing system (ILS) Cat 1.

LPV can only be conducted in a defined SBAS service area, such as the US wide area augmentation system (WAAS), or the European geostationary navigation overlay service (EGNOS).

RNP AR APCH Required navigation performance—authorisation required (RNP-AR) is a type of RNP operation that allows for defined curves in the flight path and uses baro-VNAV for vertical guidance. Special authorisation is required—see ICAO Doc 9905.
Overlay approaches Some aircraft equipment is authorised to fly approaches for which the aircraft is not technically fitted. For example, while in an SBAS service area, the receiver may be able to be used to conduct (or overlay) a baro-VNAV approach. However, without SBAS, the aircraft may not be fitted to fly a stand-alone baro-VNAV procedure.

Approach plates

A single instrument approach plate may contain a mix of 2D and 3D approaches. Care must be taken to ensure that correct piloting procedure is used including recognition of the type of minimums presented.

Enabling legislation

Civil Aviation Order (CAO) 20.18 (Aircraft equipment—basic operational requirements), deals with the equipment required for PBN and ADS-B and affects all IFR operators in Australia.

Civil Aviation Order (CAO) 20.91 (Instructions and Directions for Performance-based Navigation) allows for this equipment to be used for PBN, both in Australia and overseas.

CAO 20.91 contains deeming provisions which mean that:

  • aircraft equipped with stand-alone GNSS systems with aircraft flight manual entries for RNP 1, RNP 2, or RNP APCH-LNAV, or installed in accordance with CASA advisory circular 21-36, and flown by suitably qualified pilots, meet the equivalent PBN requirements
  • aircraft equipped with integrated avionics systems using GNSS only for area navigation are also covered by the deeming provisions.

Aircraft with flight management systems (FMS), such as some newer commuter/regional aircraft, will need to obtain navigation authorisations from CASA. The PBN standards also provide for IFR helicopter-specific operations, such as in metropolitan areas and for offshore support.

CAO 20.91 and its associated advisory circular provide operating instructions and airworthiness requirements for IFR pilots flying aircraft using PBN.

Pilot and operator obligations

Pilots in command of IFR flights must only use RNAV or RNP if they are qualified to do so.

The aircraft operator must also:

  • hold, or be deemed to hold, a navigation authorisation for the relevant PBN specification
  • ensure that each member of the flight crew satisfies the requirements in the relevant appendix (1–13)
  • ensure that each member of the flight crew conducts the flight according to the authorisation.

Navigation databases

Since navigation under PBN relies on area navigation, the aircraft navigation system must carry a navigation database. Under the requirements of the CAO:

  • the database must be valid for the current AIRAC cycle (refer to AIP GEN 3.1 for further information)
  • all terminal routes (SIDs, STARs and approaches) must be loaded from the database and may not be modified by the pilot except as provided for in CAO 20.91.

Aircraft equipment

Information on aircraft equipment is available in Chapter 5.

Key points

  • PBN has two requirements—the pilot must be suitably trained and qualified and the aircraft must be appropriately equipped.
  • The introduction of PBN allows pilots, operators and air traffic control to make the best use of recent advances in navigation technology, and brings increased safety, efficiency and environmental benefits.
  • The difference between the RNAV and RNP navigation specifications is that on-board performance monitoring and alerting is required for RNP but not for RNAV operations.
  • Area navigation operates by first determining the aircraft’s present position in terms of latitude and longitude, and then where this position is in relation to the intended flight path.
  • In Australia, if you have a GNSS-equipped aircraft approved for IFR operations, you do not need to make any changes.

Resources

Further reading

  • CASA (2010). Harmonising navigation: Performance-based navigation—global harmonisation. Flight Safety Australia, January–February. Retrieved April 2017.
  • CASA (2011). Mixed Blessings: The dangers of GPS reliance. Flight Safety Australia, January–February. Retrieved April 2017.
  • CASA (2011). Global harmony: Performance-based navigation standards are being aligned across the world. Flight Safety Australia, September–October. Retrieved April 2017.
  • ICAO (2013). Performance-based navigation manual.
  • ICAO (2016). PBN ikit. Retrieved April 2017.

References

Published date: 23 June 2021
Online version available at: https://www.casa.gov.au//search-centre/safety-kits/cnsatm-resource-kit/cnsatm-resource-kit-chapter-6-performance-based-navigation
Back to top of page