MAG2TRUE: An avionics perspective

Eight years ago, I wrote THIS ARTICLE lamenting the use of magnetic North in aviation. The acronym MAG2TRUE has been used to reflect efforts to change the azimuth reference to true North. Things are gaining momentum and it is time to look at this with an emphasis on avionics.

Figure 1: Obtaining true heading is nothing new: historic astro compass (author's collection)

A revolution in the making

On May 1^st 2000, something remarkable happened in the navigation domain: The US DoD terminated the “jittering” of GPS data, also known as Selective Availability (SA) [1]. From one instant to the next, civil GPS receivers became more accurate, opening a plethora of applications.

What if something similar but more profound was about to happen in the not-so-distant future? No decision has been taken yet, but work is underway at ICAO-level to make a move to true North as the primary azimuth reference. There are good reasons for this, and we will look at those from an avionics perspective.

What is the issue?

Lateral navigation has always been of greatest interest to both flight crews and aerospace engineers. Over decades, several terms have been established. The catch: In a series of technological evolutions, aviation ended up using both magnetic and true North references (indicated red and green below):

Figure 2: Air navigation azimuth terminology including magnetic and true North

Historically, the magnetic compass was justified by its simple design and reasonable accuracy. True heading was available even then: celestial instruments such as the astro compass (Figure 1) derived it from the sun or stars, but they required a clear sky, an accurate time reference and considerable crew workload. It was therefore not surprising that magnetic North became the primary azimuth reference. Technology evolved, and so did navigation systems. Transport category aircraft have been equipped with Inertial Reference Systems (IRS) for decades and those systems make the use of a magnetic sensor obsolete. In fact, on a typical modern airliner the enroute navigation problems are solved in true and then converted to magnetic before being displayed to the flight crew. More on that later. Even light GA aircraft equipped with avionics designed in Olathe (KS) can switch to true North using a software setting.

There is more: GNSS receivers work in true. Charts are oriented in true [2]. Procedures are designed in true before being converted to magnetic. Apart from this, there are penalizing consequences of using magnetic North also on the airport side that translate to millions of dollars in wasted money every year [3]. Runway designators have to be changed, procedures revisited. Airlines have limitations for certain airport/aircraft combinations regarding automatic landings, significantly reducing weather resilience. To put this in perspective: One carrier reported costs of around 21 million dollars for updating magvar tables on 200 aircraft [3], changing runway numbers at a single airport (CYYZ) is estimated at 1 million dollars [3].

So, the question must be asked: Should aviation continue to use magnetic North?

Magnetic North is very unreliable

For starters, there are regions of the World where the horizontal magnetic field component is simply too weak to be used by magnetic navigation sensors or changes so quickly that any prospect of using magnetic North is futile [4]. These regions use true North as of today (e.g.: Canada's Northern Domestic Airspace). But even outside these regions, the magnetic field is notoriously unreliable and even unpredictable [4].

Figure 3: Location of magnetic North Pole since 1595 [4] (CC BY 4.0)

Movement of the magnetic North Pole has been erratic and the speed at which it moves has varied over time, in the past exceeding 55 km per year and more recently slowing down again. To make matters worse, there have even been magnetic pole reversals [4]. Historic reversals have taken several hundred or even thousands of years to be completed, but there have also been geomagnetic excursions, i.e. the poles nearly flipped [4].

The angular difference between true North and magnetic North direction, the so-called variation or magnetic declination, is an ever-changing parameter that is captured in the World Magnetic Model (WMM).

Figure 4: World Magnetic Model 2025 showing isogonic lines [5] (public domain)

The WMM is updated every five years and is only an approximation of reality [5]. The lines represent values of constant variation and are referred to as isogonic lines [2]. An alternative to the WMM is the International Geomagnetic Reference Field (IGRF), which is used by some avionics manufacturers [6]. To further complicate things, avionics manufacturers, aircraft operators, procedure designers and airport authorities all use different updating schedules for their variation data!

The primary azimuth reference for aviation is subject to a random variation, captured in a model every five years and updated at different entities at inconsistent intervals. 

Let that sink in.

Technical consequences

Navigation charts are oriented in true North [2]. However, ICAO Annex 4 requires bearings, tracks and radials to be displayed in magnetic North, unless impractical [2]. It therefore becomes obvious that a different application of magnetic variation will lead to inconsistencies. As mentioned earlier, many transport category aircraft do not have a magnetic sensor (apart from the standby compass) and derive magnetic North from true North using a database. But even if an aircraft uses a magnetometer or a fluxgate, problems can arise.

For starters, let us look at where magnetic variation data is used in aeronautical navigation. The vast number of places will make it obvious why there are plenty of opportunities for inconsistencies.

Procedures/Publications

Where	What is stored or used	Notes
AIP AD 2.2	Aerodrome magnetic variation and annual change	ICAO Annex 15 / PANS-AIM defines the resolution (1° for aerodromes)
AIP ENR 4.1	VOR/DME station declination	Station declination is the value used at calibration, not the current model value (see VOR below)
Charts (Annex 4)	Isogonic lines; variation on chart headers and approach charts	Valid as of chart date
Runway designators (Annex 14)	Magnetic azimuth rounded to nearest 10°	Runways get renumbered when drift crosses a rounding boundary
Procedure design (PANS-OPS Doc 8168, national criteria)	Designed in true, published as magnetic tracks, headings and radials (some states add PBN tracks in true)	Many states freeze the variation used for a procedure at a given epoch or date, so it can lag the model. Check the State's AIP policy
ILS / localizer	Published as magnetic track	The physical beam is aligned to the runway; only the published track depends on variation
VOR	360° radial aligned to magnetic north within some tolerance	Station declination changes only on re-alignment, so it can differ from the variation by several degrees (see below)
Holds, SIDs, STARs, airways	Magnetic tracks	Polar and high-latitude routes are typically given in true or grid tracks
ATC and ATIS	Headings and surface winds in magnetic (on the radio)	METAR, TAF and forecast winds are in true, so conversion is needed
Flight planning (OFP)	True and magnetic track and heading	Uses variation values from the planning database

Onboard avionics

Within an aircraft avionics system, several versions of magnetic variation data can be found and they may even be inconsistent!

Source	Content	Notes
Navigation database (FMS)	Airport, navaid and procedure variation from ARINC 424; updated each AIRAC cycle	Used mostly in the terminal area and for procedure legs
Stored model in FMS / IRS / ADIRU / AHRS	WMM, IGRF or proprietary grid, embedded in software or a loadable database	Fixed epoch; changes only with a software or data load. Used everywhere else for MAG heading, track conversion and wind display
GNSS receiver / EFB	Built-in WMM or IGRF, updated with software releases	Some allow AUTO, manual or true selection

NDB

The NDB simply broadcasts an RF signal in an omnidirectional manner. The signal is picked up by the aircraft antenna, and the ADF receiver determines the relative bearing (RB) of the NDB (see Figure 2). To determine a magnetic bearing (QDM/QDR), the system relies on the aircraft heading reference. If the procedure designer uses a different variation epoch than the aircraft, that translates to an observable error [7].

VOR

For the VOR, things are different. Here, the signal itself carries bearing information (i.e. the received signal depends on the position in space). If a pilot uses an RMI or HSI to interpret raw data VOR information, that depends directly on the station declination (i.e. the angular offset of VOR North from true North, as indicated in Figure 2). Additionally, the industry standards of RNAV equipment require station declination to be used if the relevant fix is a VOR [8]. Authorities do their best to keep most VORs aligned with magnetic North, but electronically rotating VORs is expensive. Many countries tolerate 2°, some 3° difference between VOR station declination and magnetic variation. The US FAA has moved to a full 5° maximum difference [8]. This has no effect on the observed track, as long as the publication data is consistent with the VOR station declination [8]. It does however create a human factor issue, as the displayed aircraft track and “magenta line” will no longer fit the published values.

ILS

In 2012, the FAA updated the magnetic variation of record for Ted Stevens Anchorage International Airport (PANC) to reflect current WMM values. The result was a mismatch between the magnetic variation stored in some aircraft inertial/guidance systems and the newly updated value used in navigation databases. Several aircraft began experiencing unacceptable lateral guidance instability on CAT II/III ILS approaches. The FAA's fix was to roll the published value back to the old, technically "incorrect" number until operators had updated their aircraft [6].

The question is not whether variation data is correct, but whether it matches across the systems.

Even in a system like ILS, where the physical beam is aligned with the runway, an inconsistent variation will cause real guidance errors during heading alignment for landing.

SVS/CVS/HUD

Synthetic Vision Systems (SVS) and Combined Vision Systems (CVS) render a synthetic image or cues of terrain and runways based on the aircraft's computed position and heading. If the magnetic variation value used to orient that synthetic scene is off relative to the real world, the synthetic runway image does not overlay correctly onto the real one [9].

Head-Up Displays (HUD) project flight/guidance symbology (including runway alignment cues) onto the pilot's forward view. The same mechanism applies — if the HUD's heading/track symbology is computed from a different magnetic reference than the real values, the projected guidance (e.g. a runway centerline cue) will not line up with the real runway [9].

PBN

As the introduction of PBN marked a shift from sensor-specific navigation to performance-based navigation, there are generally no specific sensors required. It is rather the navigation infrastructure of a state or the aircraft capabilities that can impose sensor requirements on PBN procedures (such as GNSS) [10]. Navigation information is coded using a specific standard, known as “ARINC 424” (this applies not only to PBN). It contains different “leg-types”, such as “heading-to-altitude” (VA), “course-to-fix” (CF) or “track-to-fix” (TF). PBN procedures are developed in true and then converted to magnetic. ICAO Annex 4 contains guidance to add true bearings and tracks in parentheses on the charts [2], which is done by some ICAO member states. The use of “course-to-fix” legs should be avoided where “track-to-fix” is available [11], for reasons indicated below:

Figure 5: TF vs. CF legs with inconsistent variation data

Track-to-fix legs are not affected by variation, since the definition of a TF leg is such that it represents a geodesic path between two fixes [11]. However, the desired track label in the FMS might still show a different value, as this is expressed in degrees magnetic and derived by the FMS [10]. Course-to-fix legs are directly affected by variation and produce “disconnects” on the navigation display (see above). This contributes to the so-called Path Definition Error (PDE) [11].

Weather reports

When planning a flight, pilots use forecast winds expressed in degrees true (TAF). Manual conversion is needed to relate that wind to a runway oriented to magnetic North. More information about surface wind reports in aviation can be found in THIS ARTICLE.

ATC radars, ADS-B and MLAT

Primary and secondary radar antennas are mechanically rotating (or phased-array electronically scanned) and the zero-reference is set during site commissioning by geodetic survey — bearing is derived from where the antenna is pointing at the moment of reply. At the antenna level, this is usually true referenced. Magnetic variation is then applied for display on the controller's screen [3]. ADS-B and MLAT positions are added with geo-referencing. The Mode S EHS track and turn report (BDS 5,0) contains true track, while the heading and speed report (BDS 6,0) contains magnetic heading [12].

True North navigation works: Canada and the maritime domain prove it every day

Canada uses true North reference in the Northern Domestic Airspace [3]. The area is navigated by a wide mix of airframes, ranging from helicopters to airliners. Some have IRS, others manually align their gyro systems to true North. Furthermore, Transport Canada has conducted a flight test to demonstrate a changeover from magnetic to true North [3]. Using an airport in the Southern Domestic Airspace which uses magnetic North, experimental charts were produced and all variation values were set to zero. The test aircraft successfully conducted a mix of conventional (NDB, VOR) and PBN procedures, showing that there is a way to transition to true North. It comes down to coordination across ICAO member states.

Maritime navigation made the same move decades ago!

Over time, gyrocompasses replaced the magnetic compass as the primary heading reference on large vessels. Today, ships of 500 gross tonnage and above are required to carry a gyrocompass or other non-magnetic heading source [13]. The magnetic compass remains as a backup, together with a means of correcting headings and bearings to true [13].

So how to get true heading in a light/legacy aircraft?

This question needs to be addressed in any kind of transition plan. Canada already has experience with all kinds of legacy aircraft in its Northern Domestic Airspace [3]. Some options are the “GPS-NDB method” or “free-gyro operation” with latitude compensation. But clearly there are better options looking into the future:

The database trick “backwards”

Many GA avionics provide a software setting for true North reference. Typically these systems use a magnetometer and a database to derive true heading. That obviously only works in areas where magnetic sensors work.

The future: MEMS-based gyrocompass

Gyrocompassing, the art of finding true North based on the Earth's rotation, is performed routinely in transport category aircraft using IRS. These systems have historically been very expensive, due to the technology involved. Here is the good news: Much cheaper, MEMS-based systems are becoming available, capable of performing unaided gyrocompassing to 1° accuracy or better [14] [15]. This is a very promising enabler for low-cost IRS.

Small GA aircraft certified for VFR only sometimes rely on a direct-reading magnetic compass. In this case, pilots already have to apply compass deviation as of today. Therefore, several sources have proposed a modified deviation table including also the magnetic variation [16]. In any case, visual flight rules rely on pilots observing ground tracks (pilotage). ICAO/European operating rules mandate navigation systems for VFR aircraft that perform flights along routes that cannot be navigated by reference to visual landmarks [17] [18].

ICAO True North Advisory Group (TRUE-AG)

TRUE-AG is ICAO's True North Advisory Group. It advises the Air Navigation Commission (ANC) on a CONOPS and transition plan for replacing magnetic North with true North as aviation's standard azimuth reference. TRUE-AG is not the same as AHRTAG. AHRTAG is a working group of the International Association of Institutes of Navigation, led by NAV CANADA, that did the underlying research [3] [4].

Outlook

After having read this article, it should be obvious that aeronautical navigation in true North is possible, arguably even desirable. The tricky part is how to get there.

In 2027, a report of ICAO's TRUE-AG is due for presentation to the ANC.

One thing is certain: If we do not take action to resolve this issue, the entire industry will continue to waste millions of dollars and face completely unnecessary operating limitations!

After all:

Forcing an aircraft IRS to display magnetic heading and track is comparable to having some form of “selective availability”.

References

[1]	NOAA, «GPS,» [Online]. Available: https://www.gps.gov/selective-availability. [19 09 2026].
[2]	ICAO, «Annex 4, Aeronautical charts,» 11th ed., 2009.
[3]	A. MacKay, «Magnetic to true North,» NAV CANADA, 2022.
[4]	O. T. Pleter und C. E. Constantinescu, «Study on the Transition to True North in Air Navigation,» Aerospace, vol. 10, nr. 912, 2023.
[5]	NOAA, «US/UK World Magnetic Model Epoch 2025.0,» Public domain, 2024.
[6]	FAA Performance based operations Aviation Rulemaking Committee (PARC), «Executive summary: Magnetic variation review and recommendations,» FAA, 2013.
[7]	Boeing, «Correcting the effects of magnetic variation,» Aero, 2004.
[8]	FAA Performance-based Operations Aviation Rulemaking Committee (PARC), «VOR Recommendations,» FAA, 2016.
[9]	ICAO working paper, «13th Air Navigation Conference, True North reference system,» 2018.
[10]	ICAO, «Doc 9613: PBN manual, 5th edition,» 2023.
[11]	RTCA, «DO-236B: Required Navigation Performance for Area Navigation,» RTCA, 2003.
[12]	ICAO, «Annex 10, vol. 4: Surveillance and Collision Avoidance Systems, 5th ed.,» 2014.
[13]	International Maritime Organization (IMO), «SOLAS Convention, chapter 5, Safety of Navigation,» 1974, amended.
[14]	Fraunhofer Institute for electronic nano systems, «Fraunhofer ENAS MEMS Gyroscope,» Fraunhofer, 2026. [Online]. Available: https://www.enas.fraunhofer.de/en/Business_Units/Smart_Systems/Inertial_Components_and_Systems/High-Precision_MEMS_Angular_Rate_Sensor/Gyrocompass_The_use_of_a_fully_integrated_precision_MEMS_gyroscope_in_a_standalone_north-finder.html. [29 09 2026].
[15]	SBG-Systems, «MEMS-based gyrocompass,» SBG, 2026. [Online]. Available: https://www.sbg-systems.com/news/unveiling-worlds-first-mems-based-gyrocompass/. [29 09 2026].
[16]	O. T. Pleter und C. E. Constantinescu, «The Transition to True North in Air Navigation from the Avionics perspective,» Engineering proceedings MDPI, vol. 90, nr. 11, 2025.
[17]	ICAO, «Annex 6, Operation of Aircraft, Part 2 International General Aviation,» 11th ed. 2022.
[18]	EASA, «Commission Regulation (EU) No 965/2012 (Air OPS),» 2026.