Area Navigation - RNAV - Systems[PDF]
- 1 Introduction
- 2 RNAV System architecture
- 3 Sensors
- 4 Database
- 5 Procedure coding
- 6 Conclusion
- 7 See also
- 8 Reference
- 9 Author
The position of the aircraft is known using various sensors that can compute its position. RNAV can then be summed up as the ability of an aircraft to navigate, computing change of tracks from one point to another, using only coordinates.
The RNAV system may also be connected with other systems, such as auto-throttle and autopilot/flight director, allowing more automated flight operation and performance management. Despite the differences in architecture and equipment, the basic types of functions contained in the RNAV equipment are common.
RNAV System architecture
RNAV systems are designed to provide a given level of accuracy, with repeatable and predictable path definition, appropriate to the application. The RNAV system typically integrates information from sensors, such as air data, inertial reference, radio navigation and satellite navigation, together with inputs from internal databases and data entered by the crew to perform the following functions:
- Flight plan management
- Guidance and control
- Display and system control
The navigation function computes data that can include aircraft position, velocity, track angle, vertical flight path angle, drift angle, magnetic variation, barometric-corrected altitude, and wind direction and magnitude.
The flight planning function creates and assembles the lateral and vertical flight plan used by the guidance function:
- More advanced RNAV systems include a capability for performance management where aerodynamic and propulsion models are used to compute vertical flight profiles matched to the aircraft and able to satisfy the constraints imposed by air traffic control.
- A performance management function can be complex, utilizing fuel flow, total fuel, flap position, engine data and limits, altitude, airspeed, Mach, temperature, vertical speed, progress along the flight plan and pilot inputs.
An RNAV system provides lateral guidance, and in many cases, vertical guidance as well.
- The lateral guidance function compares the aircraft’s position generated by the navigation function with the desired lateral flight path and then generates steering commands used to fly the aircraft along the desired path.
- The vertical guidance function, where included, is used to control the aircraft along the vertical profile within constraints imposed by the flight plan
Display and system controls provide the means for system initialization, flight planning, path deviations, progress monitoring, active guidance control and presentation of navigation data for flight crew situational awareness.
- Simple navigation can be based upon a single type of navigation sensor such as GNSS:
- Advanced navigation can be based upon a variety of navigation sensors such as GNSS; Inertial system (IRS) and VOR/DME, but the management of navigation is only based on GNSS:
- Complex multi-sensor systems can use a variety of navigation sensors including GNSS, DME, VOR and IRS to compute the position and velocity of the aircraft:
- Satellite-based: Global Navigation Satellite System (GNSS)
- Ground-based: VOR / DME
- Ground-based: DME / DME
- Ground-based: LORAN (obsolete and not used on IVAO)
- Self-Contained Systems: Initial Reference Units (INS & IRS)
As the aircraft progresses along its flight path, if the RNAV system is using ground navaids, it uses its current estimate of the aircraft's position and its internal database to automatically tune the ground stations in order to obtain the most accurate radio position.
See below the table indicating all existing PBN designators:
|Area||RNAV category||All sensors||GNSS||DME/DME||VOR/DME||DME/DME/IRU||LORAN|
|RNP APCH with BARO VNAV||S2||-||-||-||-||-|
|RNP AR APCH with RF||T1||-||-||-||-||-|
|RNP AR APCH without RF||T2||-||-||-||-||-|
For example, a typical airliner pilot using complex FMC should include in his flight plan: PBN/B1C1D1O1S2 For example, a typical IFR general aviation pilot equipped with a simple GPS should include in his flight plan: PBN/B2D2O2S1
Global Navigation Satellite System (GNSS)
- Positioning system: it allows positioning an object everywhere on earth in relation to its coordinates and its altitude.
- Augmentation system: it allows making sure the positioning system’s integrity is reliable, thus there is no gap in positioning signal.
Examples of a few GNSS positioning systems:
- GPS (USA)
- GLONASS (Russia)
- Galileo (Europe)
- Compass (China)
- IRNSS (India)
- QZSS (Japan)
- Height and integrating dimension:
The position is computed from the distances to the satellites. Aircraft can use up to 6 satellite signals:
- 4 signals is basic positioning
- 5 signals will allow detecting a faulty signal: RAIM function
- 6 signals will allow determining which satellite is faulty: FDE function
Accuracy and Integrity
GNSS must meet essential criteria to ensure flight safety:
- Accuracy: amount of errors between computed and true position
- Integrity: ability to alert the user when accuracy decreases
- Continuity: amount of time the system will operate without interruption
- Availability: amount of time the system is actually able to function
Receiver Autonomous Integrity Monitoring (RAIM) enables to achieve integrity when using GNSS. It enables detecting a discrepancy in satellite signal, which leads to a decrease in position accuracy. Since the monitoring is continuous, the pilot can be immediately alerted when inaccuracy hit a critical threshold, generally the required specification.
For ABAS-based approach (LNAV and LNAV/VNAV), RAIM must be operative to ensure Required Navigation Performance (RNP).Some systems have RAIM built-in predictions, enabling to know whether RAIM function will be available or not in a specific location at a specific time.
The Fault Detection and Exclusion (FDE) function allows the user deselecting a faulty satellite to ensure continuity and availability of GNSS.
That’s why all approach operations are RNP specifications and not RNAV specifications. In order to achieve this degree of precision, GNSS signals are correlated with augmentation systems.
There are three types of augmentation systems:
- Satellite-Based Augmentation System (SBAS)
- Ground-Based Augmentation System (GBAS)
- Autonomous/Aircraft-Based Augmentation System (ABAS)
Examples of a few SBAS augmentation systems used for LPV & LNAV/VNAV approaches:
- WAAS (USA)
- EGNOS (Europe)
- MSAS (Japan)
Example of GBAS augmentation system used for GLS approaches:
- LAAS (USA)
Examples of ABAS augmentation systems used for LNAV & LNAV/VNAV approaches:
- Redundant position cross feeding comparison (GNSS & DME / DME for instance)
- RAIM / FDE
VOR / DME
- VOR/DME coordinates
- aircraft actual radial from the station
- aircraft actual distance to this DME
DME / DME
- Both DMEs coordinates
- Aircraft actual distance to these DMEs
Long Range Navigation (LORAN)
LORAN is a radio transmission system developed during World War II. The goal was to enhance long range navigation such as Atlantic crossing for ships and airplanes. Basically, it is a super VOR which can be tracked up to 1,500 miles.
INS & IRS
After manual insert of the initial position, and a timed warm-up and computation (up to 20 minutes), the system was self-contained and autonomous to perform various calculations: wind aloft, track to coordinates, present position, ground speed, etc…
Remaining in the cockpit of most airliners, it serves as backup in case of GNSS failure.
This database is defined with a cycle database update:
- Each cycle is defined by the year on two digits followed by the number of the cycle in the year. At the time of the writing, cycle 1609 is effective (9th cycle of 2016).
- An AIRAC cycle is valid only for 28 days.
- After 28 days, a new AIRAC cycle is published
AIRAC cycle contains pretty much the same elements of a country Aeronautical Information Publication (AIP):
Example: Jersey EGJJ – RNAV (GNSS) RUNWAY 08 via LAPLI IAF
In relation to RNAV, database study is particularly important as it follows strict conventions. There are two main coding particularities: waypoint naming and leg types.
- VOR/NDB or an airport: named using their identifier (i.e. LND VOR, EGLL airport)
- Waypoint when they are non-physical waypoint: defined by their coordinates and named using 5 or 6 letters (i.e. MERIT, ROMAM…)
- RNAV waypoint located in an RNAV approach route: waypoints are named such as the two last letters of the ICAO identifier of the airport plus 3 figures XXnnn (n=figure, X=letter) (i.e. RS604)
- Constructed waypoint for FMC: named using a defined radial and a distance DnnnX (n=figure, X=letter). The number ‘nnn’ represents the radial in degrees and the X the order of the letter inside the alphabet is the distance located at n NM. (i.e. D206J = Radial 206° 10NM)
Example: SLO2A arrival Dakar GOOY
Example: Around Toulouse LFBO Airport, RNAV waypoints are named BO508, BO509, BO510…
- CIxx/CSxx as a waypoint where the final course should be established, generally the IF
- FDxx/FIxx/FNxx/FSxx as where the final descent should be initiated, generally the FAF/FAP
- Maxx/MDxx as the missed approach point of a procedure
- RWxx as the runway threshold (often used for descent altitude-distance check).
- Any step down fix will have a proper waypoint
- Left: Tallinn EETN – VOR (Overlay) RUNWAY 08
- Right: Cape Town FACT – VOR Z RUNWAY 01
A leg is the segment joining two points. Depending on the intended flightpath, it is defined by a path type and a terminator. It results in 14 different leg types.
|Constant DME arc||A||A||Altitude|
|Direct track||D||D||DME Distance|
|Course from a fix to||F||F||Fix|
|Holding Pattern||H||I||Next Leg|
|Constant radius||R||R||Radial termination|
Example of Leg types
|CA||Course to an Altitude|
|CF||Course to a Fix|
|DF||Direct to a Fix|
|FA||Fix to an Altitude|
|FM||Fix to a Manual Termination|
|HA||Racetrack Course Reversal (Altitude Termination)|
|HF||Racetrack (Single Circuit – Fix Termination)|
|HM||Racetrack (Manual Termination)|
|TF||Track to a Fix|
|RF||Constant Radius Arc|
|VA||Heading to an Altitude|
|VI||Heading to an Intercept|
|VM||Heading to a Manual Termination|
If you notice a discontinuity or a difference with published flightpath, aircraft should be manned as to fly the right flightpath.
- Track to intercept a fix radial
- Timed base turns
- Procedure 45/180 and 80/260 turns
Description of the procedure: Study case: Bastia LFKB – NDB RUNWAY 34
Given the MSA, the pilot should enter the racetrack at BP. Then he should perform a procedure 45/180 turn before getting back onto the final axis course. As shown on the ND, the Airbus MCDU is not able to transcript the approach and the pilot should make most of it manually.
Note that a waypoint is created to materialize the end of the procedure turn.
RNAV is a brilliant navigation method to optimize traffic flow using the power of GNSS even though it implies tons of new rules, standards and recommendations to implement for every actors of the aviation industry.
However the future is already marching on, as the evolution of RNAV is already being developed and enhanced: the Required Navigation Performance (RNP)
- VID 200696 - Creation
DATE OF SUBMISSION
- 00:29, 14 May 2021
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- The content of this documentation is intended for aviation simulation only and must not be used for real aviation operations.