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Area Navigation (RNAV)

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Definition

Area Navigation (RNAV) is a key enabler of Performance Based Navigation (PBN). It is a family of navigation specifications which permit the operation of aircraft on any desired flight path; RNAV allows aircraft positions to be continuously determined wherever the aircraft are within the coverage of navigation aids or within the limits of a self-contained system capability (or a combination of these) rather than only along the tracks between individual ground navigation aids.

How it works

RNAV is enabled through the use of a navigation computer. Waypoints are input into the computer either manually (but this has limited capabilities) or automatically with an integrated database. The flight crew then creates a route as a series of waypoints in accordance with the flight plan. The computer estimates the aircraft position using the fitted navigation sensors and compares the estimation to the defined route. Deviation between the estimated position and the defined path creates guidance information. In order to perform RNAV, aircraft must be equipped with an RNAV system.

Historical development

Various types of ground-based RNAV systems have been available from terrestrial sources over the years; these were originally dependent on low frequency (LF)/very low frequency (VLF) radio signals. Examples included Sonne/Consol, Omega and LORAN-C. RNAV moved to positions derived from very high frequency (VHR) omnidirectional range (VOR) radials (up to 62nm slant distance) and/or distance measuring equipment (DME) distances. Inertial navigation systems (INS) can be used to maintain prior tracking for up to 2 hours. As RNAV accuracy improved, it began to play a vital role in increasing air traffic management (ATM) efficiency whilst also sustaining safety performance.

The advent of Global Navigation Satellite Systems (GNSS), mainly in the specific form of the United States (US) Global Positioning System (GPS), brought a new opportunity to derive an accurate three-dimensional (VNAV) position as well as a highly accurate two-dimensional (LNAV) position over an area not restricted by the disposition of ground transmitters. RNAV of sufficient accuracy is seen as ultimately providing a replacement for all ground-based navigation aids. Although the most extensive GNSS presently available is the GPS coverage of the US, there is also the Russian Global Navigation Satellite System (GLONASS) and the partially operative European GNSS, GALILEO. New satellites are being launched to enlarge the GALILEO constellation, which are gradually improving GALILEO’s availability worldwide. The constellation is expected to be completed by 2020 when GALILEO will reach full operational capacity.

Although the use of GNSS input for RNAV has made this method of navigation truly global, it has led to the availability of a very wide range of accuracy in RNAV - and therefore the uses to which it can be put - depending on how GNSS data is used. RNAV use of GNSS ranges from hand held GPS, as an aid to day VFR navigation, to the use of approach procedures which meet the highest accuracy and integrity criteria.

RNAV navigation specifications

The International Civil Aviation Organization’s (ICAO) PBN Manual identifies four navigation specifications under the RNAV family: RNAV 10, RNAV 5, RNAV 2 and RNAV 1.

RNAV 10, which is designated as RNP 10 in the ICAO’s PBN Manual, is an RNAV specification for oceanic and remote continental navigation applications.

RNAV 5, also referred to as Basic Area Navigation (B-RNAV), has been in use In Europe since 1998 and is mandated for aircraft using higher level airspace. It requires a minimum navigational accuracy of +/- 5nm for 95% of the time and is not approved for use below MSA.

RNAV 2 supports navigation in en-route continental airspace in the United States.

RNAV 1 is the RNAV specification for Precision Area Navigation (P-RNAV). It requires a minimum navigational accuracy of +/- 1nm for 95% of the time. Qualifying systems must have the ability to fly accurate tactical offsets; P-RNAV routes must be extracted directly from the FMS data base and must be flown by linking the R-NAV system to the FMS/autopilot. In addition, flight crews are restricted from manually adding waypoints to the route. This level of navigation accuracy can be achieved using DME/DME, VOR/DME or GNSS. It can also be maintained for short periods using inertial reference systems (IRS) and the length of time that a particular IRS can be used to maintain P-RNAV accuracy without external update is determined at the time of equipment certification. It should be noted that if GNSS is not used as a source then two independent ground-based sources are required to meet P-RNAV minimum requirements apart from specified short periods of INS ‘backup’, which is a more stringent requirement than for some older flight management system (FMS). P-RNAV is used to provide more routes and terminal area procedures and may be used down to the final approach fix (FAF) on designated approach procedures.

RNAV vs RNP

Under the PBN concept, in addition to RNAV navigation specifications there exists the required navigation performance (RNP) family of navigation specifications. RNAV and RNP navigation specifications are substantially very similar; they only differ in relation to the performance monitoring and alerting requirement which applies to RNP navigation specifications. This means that if the RNP system does not perform the way it should then an alert should be provided to the flight crew. In practical terms what this means is that air traffic control (ATC) can have greater confidence in the track keeping performance of the aircraft and this greater confidence translates into being able to place routes closer together. It should be noted, however, that some P-RNAV/B-RNAV aircraft have RNP capability, i.e. on-board performance monitoring and alerting may be available (even though it is not required for either P-RNAV or B-RNAV).

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