Area Navigation - Background

Background

The continuing growth of aviation increases demands on airspace capacity, thus emphasizing the need for optimum utilisation of available airspace. Improved operational efficiency derived from the application of area navigation techniques has resulted in the development of navigation applications in various regions worldwide and for all phases of flight. These applications could potentially be expanded to provide guidance for ground movement operations.

Requirements for navigation applications on specific routes or within a specific airspace must be defined in a clear and concise manner. This is to ensure that the flight crew and the air traffic controllers are aware of the on-board RNAV system capabilities in order to determine if the performance of the RNAV system is appropriate for the specific airspace requirements.

RNAV systems evolved in a manner similar to conventional ground-based routes and procedures. A specific RNAV system was identified and its performance was evaluated through a combination of analysis and flight testing. For land-based operations, the initial systems used very high frequency omnidirectional radio range (VOR) and distance measuring equipment (DME) for estimating position; for oceanic operations, inertial navigation systems (INS) were employed. Airspace and obstacle clearance criteria were developed based on the performance of available equipment, and specifications for requirements were based on available capabilities. Such prescriptive requirements resulted in delays to the introduction of new RNAV system capabilities and higher costs for maintaining appropriate certification. To avoid such prescriptive specifications of requirements, an alternative method for defining equipment requirements has been introduced. This enables the specification of performance requirements, independent of available equipment capabilities, and is termed performance-based navigation (PBN). Thus, RNAV is now one of the navigation techniques of PBN; currently the only other is required navigation performance (RNP).

RNAV and RNP systems are fundamentally similar. The key difference between them is the requirement for on-board performance monitoring and alerting. A navigation specification that includes a requirement for on-board navigation performance monitoring and alerting is referred to as an RNP specification. One not having such requirements is referred to as an RNAV specification. An area navigation system capable of achieving the performance requirement of an RNP specification is referred to as an RNP system.

As a result of decisions made in the industry in the 1990s, most modern RNAV systems provide on-board performance monitoring and alerting, therefore the navigation specifications developed for use by these systems can be designated as RNP.

Many RNAV systems, while offering very high accuracy and possessing many of the functions provided by RNP systems, are not able to provide assurance of their performance. Recognising this, and to avoid operators incurring unnecessary expense, where the airspace requirement does not necessitate the use of an RNP system, many new as well as existing navigation requirements will continue to specify RNAV rather than RNP systems. It is therefore expected that RNAV and RNP operations will co-exist for many years.

However, RNP systems provide improvements in the integrity of operation, permitting possibly closer route spacing, and can provide sufficient integrity to allow only the RNP systems to be use for navigation in a specific airspace. The use of RNP systems may therefore offer significant safety, operational and efficiency benefits. While RNAV and RNP applications will co-exist for a number of years, it is expected that there will be a gradual transition to RNP applications as the proportion of aircraft equipped with RNP systems increases and the cost of transition reduces.