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3. RNAV System Description

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RNAV System Description

1. FMS or RNAV
Use of the terms FMS and RNAV in literature is sometimes confusing. Sometimes they are described as being separate systems, sometimes as being one and the same.
To end this confusion ICAO has decided to use ‘RNAV’ when referring to the navigation part of a system.
E.g.: RNAV being the navigation part of a Flight Management System.
Over the years, RNAV has evolved from a simple point to point navigation system to a complex system, which, by integration into an FMS, is capable of managing the flight, reducing workload and taking over monitoring and controlling tasks. Systems now have the ability to navigate accurately in three dimensions and even the fourth dimension is being implemented rudimentarily by means of modes such as Required Time of Arrival (737).

Most FMSs are based on and certified as multi sensor systems. For new aircraft, GPS has been added as an additional sensor as part of the multi sensor system.
Apart from selecting GNSS for position calculation (as primary sensor when available) alternative sensors such as e.g. DME, VOR and IRS are available and can be used in various combinations.

The FMS always selects a sensor according to a pre-programmed order of preference.
When GNSS is available on board and in space, it is used as the primary sensor. Next in the order of preference is DME/DME, provided DMEs, stored in a separate update file, are available and meet the accuracy criteria as checked by the system. The outcome of this check (FMS calculated position) must be within a certain margin from the FMS predicted position before the update is accepted. The so-called ‘validation and reasonableness check’. This check is done constantly, by using a filtering logic, which continuously recalculates the predicted position and matches this with the outcome of a measured position (DME/DME or VOR/DME) extrapolated to the same point in time.
If none of these radio positions fall within the tolerances predicted by the system, the FMS will revert to IRS only navigation. Normal strap down figure is GPS, DME/DME, VOR/DME, IRS, although the order may depend on the actually measured and predicted accuracy. VOR/DME updating may be disabled by the operator as a preference.
Multiple IRS can therefore be seen as the basic reference position.
In all cases where the aircraft is out of range of sufficient (accurate) radio stations, IRS is the primary sensor, taking into account the IRS drift rate as measured with the last radio update. This drift vector (track and distance) is applied as a correction to the ‘IRS only’ position. Continuing IRS drift will still cause the accuracy of the position to degrade slowly until an area is entered where radio updating is possible again.
Newer systems include RNP capability, which uses the above mentioned position prediction method to give, as a monitoring tool, an estimate of the accuracy of the aircraft’s position through an ANP (Boeing) or EPE (Airbus) value to the crew.
In case of a fall back to IRS only navigation, this estimated position error will increase as the only navigation system available is the time related IRS.

4. GNSS INPUT (For GPS click here)
For updating, multi sensor systems like FMS give the highest priority to GNSS, when available and acceptable. At least 4 satellites must be in view to calculate a 3 dimensional position. To increase the integrity of the position, needed when GPS would be the sole means of navigation, 6 satellites must be in view.
Position accuracy is extremely high. Even when downgrading (for military purposes) is applied it is still 100m on a 95% probability basis. Normally however, civil accuracy is ± 10m; in fact more accurate than a DME/DME position.
Global availability is of course another interesting part of GNSS. It allows for accurate position determination over oceanic and other remote areas where limited or no VOR or DME coverage is available.

    NOTE: There are legal implications involving the use of GPS due to the lack of controllability by the State where the signal is used.
    Answers vary from applying mitigating techniques, reducing the influence of false signals (e.g. RAIM) to checking the reasonableness of the received signals by other sensors.
    International experts are still discussing whether the current legal system, as laid down in the ICAO Convention, sufficiently covers a global positioning system which is usable cross-border.
    Operators may elect to accept liability when sufficient mitigating techniques are assumed to exist. States may deny or accept responsibility for or even restrict use of GPS signals within their area of jurisdiction.

FMSs use separate radio beacon update files in which all VORs and DMEs within 200 NM of the aircraft’s position are stored. Beacons for updating are selected from this file whereby preference is given to stations with an intersect angle of 90° relative to the aircraft position. In any case, the intersect angle must fall between 30° and 150°. Any other intersection angle will result in a too large position error probability. Additionally, filters are applied to verify usefulness of the station, such as signal strength etc.
Station identification however, is not part of the selection process. Idents are ‘unknown’ to this part of the system and stations radiating for example ‘on test’ will continue to be used indiscriminately.

If DME/DME updating is not possible, the next best option is VOR/DME, provided the station is suitably located. Accuracy depends on the radial and DME error. Radial errors are always distance related; DME errors only for older stations (i.e. those commissioned before 1989) and therefore stations used for updating must be within a maximum distance from the aircraft position. Limitations vary, depending on the equipment manufacturer. Accuracy may be reduced further because the VOR’s declination is normally derived from a geomagnetic program and may differ somewhat from the actual declination.

If suitable radio updates are not available, the FMC will revert to ‘IRS only’ navigation. System accuracy then becomes time related, where accuracy decreases with time. Eventhough newer, laser gyro, (triple mixed) IRSs have become increasingly accurate, they still suffer from this time related accuracy degradation. Developments in this field are continuing.

ICAO Doc 8168 (PANS-OPS) specifies criteria for the design of RNAV routes.
The backbone of RNAV routes is the location of the waypoints and their designation as ‘fly-by’ or ‘fly-over’. To ascertain the required protection area along the nominal track, it is essential to know whether the turn to intercept the next track should be started before or after passing the active waypoint. Waypoints are flagged (marked) as fly-by or fly-over in the database. Minimum distance between waypoints is a function of ground speed, course change and type of waypoint and speed limitations may be added in order to control the required airspace during turns. Additionally, vertical navigation can be supplemented by storing minimum, maximum and mandatory crossing altitudes with waypoints in the database.

The limitation to store conventional routes by means of point to point coding, as in the first generation databases, became obsolete with the introduction of the so-called ‘Path and Terminator’ (PT) concept in FMS/EFIS equipped aircraft.
In this PT concept, conventionally designed routes are cut into small parts and translated into segments defined by a route segment and a termination point.


Climb straight ahead to 8000ft, then left via track 045 to ABC is translated into the following paths and terminators:
Fix to Altitude 8000ft: FA to 8000
Course 045 to Fix ABC: CF to ABC
FA and CF are the path and terminators
Climb straight ahead to 8000ft, then turn right, direct to ABC becomes:
Fix to Altitude 8000ft: FA to 8000
Direct to ABC: DF to ABC
As no track is specified after reaching 8000ft, the aircraft is free to fly ‘direct’ to ABC
Other common Path and Terminators for RNAV use are:
TF : Track between Fixes.
The ‘point to point’ method, preferred for RNAV and RNP leg coding due to its simplicity and high grade of turn anticipation freedom fr the FMS.
Radius to fix. New leg type to accurately control the lateral path during a turn. (Required for RNP but difficult to code and one of the reasons why PRNAV will be used as a transition to RNP1 operations.)
From a fix via a track (course). Stored magnetic track may cause incorrect connection to the next track, especially with long segments or when the next segment is a CF leg (Course to Fix).
Racetrack Course Reversal or HA, HF and HM Leg types. Define racetrack pattern course reversals at a specified database fix. Leg Time or Distance is included as a data field. The three codes indicate different path termination types:
HA = Altitude Termination
HF = Single circuit terminating at the fix.
HM = Manual Termination.

Conventional (TMA) routes are based on VOR radials, DME distances, fixes, NDB bearings and barometric altitudes. Complex manoeuvres are possible by using a series of navigation beacons combined with turns, initiated at an altitude, a fix or even both. Complexity increases because of airspace optimisation and environmental demands. Keeping pace with the ever-increasing complexity of route design, coding rules and FMC software became more and more complex as well. Currently the ARINC 424 coding standard allows 23 path and terminators.

An indispensable part of the system is the navigation database. All officially published routes are stored into a database using the ARINC 424 coding convention standard.
Data is stored in a master database in separate files, such as airway files, navigation facility files, waypoint files, airport files etc.
En-route data is separated from TMA data because the complex TMA procedures are based on the PT concept while for en-route, simple, straight segments are loaded (Remember the TF leg philosophy of the DC 10 system).
Navigation databases are maintained by commercial suppliers, of which only a few exist world-wide. With increased use of RNAV without an underlying conventional route structure, the integrity of navigation data has become increasingly important.
Prior to application of RNAV within the TMA and below the Minimum Sector Altitude, the database supplier must meet certain quality assurance standards.

The database supplier maintains a master database, which is continually updated with information from AIPs, AIP Supplements and NOTAMs.
At least two weeks before the AIRAC cycle date the database supplier sends an updated selection of the master database to the FMC manufacturer for conversion from ARINC 424 code into FMS readable code. This means that changes, becoming available after that closure date can not be stored in the database.
The selection of data is based on the customer’s demands; for each aircraft/FMC combination a selection is made and the data is tailored to the software capabilities of the FMC.
The FMC manufacturer performs (limited) consistency checks with the previous cycle, may modify data and converts the data into the FMC machine language. FMC readable data is then stored in a customer database and copied onto a disk for loading on board the aircraft.
The FMC contains two data cycles, the previous, and the current cycle. Upon receipt of the new data, it is used to replace the old cycle, normally several days before it becomes effective. This gives the department responsible for loading some lead-time for their process. On reaching the new effective date, the pilot can select the new, now effective data and the old data is retained as a back up.

Source: FCRG

See also:
History of RNAV
Area Navigation Definitions
RNAV System Description
Required Navigation Performance (RNP)
RNAV Development
RNAV System Limitations
Global Positioning System (GPS)

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