Monday, October 23, 2017
Transport Canada - Aviation Safety Letter Area Navigation in Canada Issue 2/2010

Area Navigation in Canada Issue 2/2010

by Ian Johnson, Civil Aviation Safety Inspector, Aerodromes and Airspace Standards, Standards, Civil Aviation, Transport Canada

Airborne navigation has progressed from maps, watches and sextants, to ground-based navigation aids (NAVAID) (non-directional beacons [NDB] and VHF omnidirectional ranges [VOR]), to self-contained navigation systems such as inertial navigation systems (INS) and space-based systems (e.g. GPS). A minimum navigation performance specification (MNPS) for the North Atlantic was published in 1979 by the International Civil Aviation Organization (ICAO), marking the beginning of navigation harmonization. The intent was to standardize the navigation performance of aircraft crossing the Atlantic from North America to Europe in order to manage air traffic in a safe and efficient manner and increase safety. By using managed Mach cruise speeds and specifying a level of navigation-system accuracy (initially, the required position accuracy allowed a 60-NM across-track by 60-NM along-track spacing between aircraft), aircraft could be spaced more effectively, thereby saving air operators time and fuel. As the skies became more crowded over the years and the distances travelled increased, greater accuracy in navigation became necessary not only for oceanic airspace but also for domestic airspace. The earlier tolerance for navigation error gave way to the “be exactly at this position, at this time” necessity of today’s busy airspace. This has led to the development of additional navigation specifications for specific types of airspace.

Initially, civil aviation authorities regulated aircraft navigation capability by requiring the carriage of specific navigation units (e.g. VOR or distance-measuring equipment [DME]). Then area navigation (RNAV) system use became commonplace in the 1970s. These early units used input from long-range systems (OMEGA, LORAN) and ground-based NAVAIDs to fix positions. As costs decreased, stand-alone inertial navigation systems (INS) began to be widely utilized and positional accuracy increased significantly. With this greater level of accuracy and reliability, highly sensitive systems utilizing multiple sensor inputs were developed and put into service. Satellite navigation constellations, inertial reference platforms, and ground-based NAVAIDs are all integrated by flight management systems (FMS) today to determine the position of an aircraft. An example of a stand-alone sensor with integrated capabilities available would be a combination GPS-inertial reference unit (IRU).

Early navigation practices meant an aircraft’s position could be in error literally by miles. Today’s systems can establish a position to significantly less than a mile. These technological advances have created many different levels of possible system accuracy, redundancy, and performance monitoring. RNAV progressed to required navigation performance (RNP), which has now evolved into the ICAO performance-based navigation (PBN) concept. RNP and RNAV are sub-specifications of PBN; RNP has additional technical requirements above and beyond RNAV. In order to have a consistent global approach to navigation, standards are being harmonized through PBN. Rather than specifying the exact navigation equipment aircraft need to carry, ICAO has created PBN specifications. This means that a navigation specification will state the accuracy, integrity, continuity, performance monitoring and alerting, and signal in space required. The system accuracy required is stated after the type of specification, for example, RNP 4, RNAV 5. The 4 and 5 represent the +/- NM along-/across-track accuracy performance the aircraft’s navigation system must meet. An RNP-type navigation system will continuously monitor its position and alert crew members if the aircraft has the potential to stray outside of allowable airspace boundaries. The airspace boundary is an area equivalent to twice the RNP value. For example, the RNP-4 lateral boundary is a corridor 8 NM in width.

The basic navigation categories are as follows:

Area navigation (RNAV)—A method of navigation that permits aircraft operation on any desired flight path within the coverage of station-referenced NAVAIDs, within the limits of the capability of self-contained aids, or a combination of both.

Required navigation performance (RNP) system—An RNAV system that supports on-board performance monitoring and alerting.

Performance-based navigation (PBN)—RNAV based on performance requirements for aircraft operating along an air traffic system route, on an instrument approach procedure, or in a designated airspace.

Certain levels of navigation performance are infrastructure-based, meaning the number of DME or VOR/DME facilities available affects the aircraft system’s ability to resolve its location. A navigation system may be capable of an accuracy level of only 2 NM, due to the number and proximity of facilities. Yet given enough facilities, the same system may provide an accuracy level of 1 NM. For example, because the RNAV-1 and RNAV-2 specifications can be dependent on infrastructure, the two specifications are combined into one by ICAO and the Federal Aviation Administration (FAA): RNAV 1/2. The use of satellite systems provides a unique capability independent of ground-based infrastructure. RNAV or RNP arrivals or departures can be implemented at airports that have either minimal or non-existent ground-based NAVAIDs—potentially a much more cost-effective way to provide approach services.

With the advent of reliable and accurate navigation systems for commercial and private aircraft, operators can now take advantage of these capabilities in certain en-route and terminal airspaces. Specifications currently in place or being developed are:

Area of applicationNavigation accuracy (NM)Designation of navigation standard (current)Designation of navigation standard (new)Requirement for performance monitoring and alertingGNSS required
Oceanic/Remote* 10 RNP 10 RNAV 10
(RNP 10 label)
No No
Oceanic/Remote 4 RNP 4 RNP 4 Yes Yes
En route-Continental 5 B-RNAV RNAV 5 No No
En route-Continental and Terminal** 2 US RNAV “A” RNAV 2 No No
Terminal** 1 US RNAV “B”P-RNAV RNAV 1 No No
Terminal 1 Basic RNP 1   Yes Yes
Terminal 1 Advanced RNP 1   Yes Yes
Terminal/
Approach
1/0.3 RNP APCH   Yes Yes
Terminal/
Approach
1/0.3 or less RNP AR APCH   Yes Yes


* Time limits apply to certain DME/DME/IRU systems.

** RNAV 1/2 can be infrastructure-based.

RNAV and GPS procedures have been in effect in Canada for some time now, and operators are aware of their benefits. Operators are currently using PBN arrivals, approaches, and departures at various airports to reduce flight time, fuel burn, carbon emissions, and noise footprints. RNP procedures into mountainous airports have the potential to enable lower weather minima than those possible with traditional NAVAIDs.

In the future, PBN will enable continuous descent arrivals (CDA) and required time of arrival (RTA) approaches (i.e. the flight will be cleared to arrive at the runway threshold within a specific window of time). It has the potential to increase efficiencies at high-volume airports and provide better access to smaller airfields. Combined with automatic dependent surveillance-broadcast or -contract (ADS-B and ADS-C, respectively) and controller-pilot data link communications (CPDLC), PBN specifications could allow higher traffic densities on oceanic or remote routes. PBN’s inherent potential to optimize flight routes, improve flight safety, and also reduce emissions makes it an attractive tool for aviators in Canada.

References:

1. ICAO Performance-Based Navigation Manual, ICAO Doc 9613

2. TC Advisory Circular (AC) 123R “Use of Global Positioning System for Instrument Approaches”

3. FAA AC 90-105 “Approval Guidance for RNP Operations and Barometric Vertical Navigation in the U.S. National Airspace System”

4. FAA AC 90-101 “Approval Guidance for RNP Procedures with SAAAR”

Other information:

1. Transport Canada Aeronautical Information Manual (TC AIM)

2. AIP Canada (ICAO) COM section.

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