• Andreas

A magnetic disorder

Aktualisiert: Apr 27


When a 200-million-dollar passenger jet approaches a major airport somewhere on this planet and receives some directions from air traffic control (so-called „RADAR vectors”), they are expressed as magnetic heading [1]. This almost embarrassing fact tells us two things:

  • In aviation, we still mostly use a magnetic north reference, despite the fact that technology would enable us to always use true north

  • Sometimes, instructions are given as a heading, even if the goal is to achieve a certain track. (For RADAR vectors, the air traffic controller aims to establish the aircraft on a certain track, but the procedure dictates that he converts it to an estimated heading, based on the current wind and aircraft speed, before communicating to the pilot [1]).

This article will focus on bullet number one: The use of magnetic north.

While this practice is obviously based on historical equipment capability, it is very hard to understand why it has been able to survive until today. Interestingly, the maritime counterpart of ICAO, the International Maritime Organisation (IMO) has changed to true north as the primary reference decades ago [7]. So how exactly does the use of magnetic north affect aeronautical navigation? It simply appears everywhere: We paint numbers on the runways according magnetic north, we adjust VOR’s, instrument procedures and even the controller's RADAR screens [5]. Needless to say that it costs a fortune to keep all of this up-to-date.

If you are a perfectionist, looking at an IFR enroute chart can drive you crazy! You will find, that sometimes the same track is depicted with 090°, going from west to east, but labelled 271° going the other way (even without any meridian convergence effects)! The cherry on the cake is usually, that your on-board FMS displays an even different number…

This confusion is caused by different application of magnetic variation (the difference between true and magnetic north). National authorities, procedure designers and avionics manufacturers use different methods to cope with magnetic variation [5]. Generally, procedures are designed with reference to true north and then converted to magnetic north at a later stage for publication. Likewise, modern GNSS avionics perform all the calculations in true and convert it to magnetic for display using a database of variation values [5]. And here it gets tricky: Depending on the leg-type and fix involved, the variation value can be determined by the state, the "station declination" in case of VOR's or by an internal model of the avionics manufacturer [8]. There are two generally accepted models of magnetic variation:, The World Magnetic Model (WMM) and the International Geomagnetic Reference Field (IGRF) [2] [3]. Both are used in aviation amongst air navigation service providers and avionics manufacturers and are typically valid for five years only [5]. Further, the update interval of an airport’s variation and the variation of the associated instrument procedure and navaid may be different [5]. Two VOR’s connected by an airway (old-school) can have different variation update cycles! The fact that variation is called "station declination" in VOR siting standards only adds to the confusion [5].


Figure 1: Ancient magnetic compass

Why bother you say?

The topic is probably as pressing, as never before! This is much more than just a “cosmetic issue”. Not only have we seen anomalies in aircraft guidance systems performance during ILS approaches in high-latitude areas that were caused by changes in variation [5], even the most sophisticated Inertial Reference System (IRS) can have severe limitations, if you force it to display magnetic information [6]. Additionally, there are many cases, where a precise heading information is crucial, such as automatic landing or the use of Synthetic Vision Systems (SVS), Enhanced Vision Systems (EVS) or Combined Vision Systems (CVS) [6]. It is not very nice if the magic SVS picture and the EVS picture do not line up...

So would it be a big move to change everything to true north? Certainly not, as we shall see shortly. By now, we understand that modern avionics already use true north for all the calculations and only convert it to magnetic numbers for display purposes [4] [5]. But wait, it gets even better: In fact there are regions, where all the runways, navaids and procedures are aligned with true north as of today: Canada’s Northern Domestic Airspace (NDA) is one example [6]. Driven by extreme values of variation and magnetic compass unreliability, Canada has long-term experience in using true north. In the early days of polar air navigation, an ingenious device, called "astro compass" was used to determine the aircraft's true heading [9] (see Figure 2). Together with an almanac and a little bit of practice, it is possible to obtain fairly accurate true headings.


Figure 2: My personal astro compass (very old, but it still works)

Actually, Canada is at the forefront to get the ICAO convinced to implement true north referencing globally, as the issue was presented in “WP/147” at the 12th ICAO Air Navigation Conference [6]. Not only would this save an incredible amount of money, it would also simplify everybody’s life, from the avionics manufacturers, down to the ATPL students trying to memorize the mnemonic of how to apply variation;-) For transport category aircraft, which typically have inertial reference systems on-board and often even a switch to change the north-reference from magnetic to true, the change is a no-brainer from an equipage point-of-view. Smaller aircraft often use some magnetic sensors to align their heading systems and therefore a true-heading is not readily available without additional measures [6]. Some avionics even perform the “variation-database-trick” backwards and provide true heading based on magnetic heading and the application of variation from a database…

Open questions regarding equipment capabilities for smaller aircraft should not stop us from moving forward towards a world where everything is referenced with respect to true north. Especially in a time, where inertial sensors are becoming more and more available at a low cost and we might see high-grade gyros appearing in more general aviation applications.

In fact, let me get back to bullet number two at the beginning: Why do we need heading information on a small aircraft in the first place? Apart from SVS-applications and some wind calculations, I would claim that it is possible to simply use true track, which can be readily provided by GNSS.

Next time when you see ground workers pulling up at the runway threshold getting ready to change the runway designator from 24 to 25 (or whatever), think about it…

Rev/20200327

References:

[1] ICAO, PANS-OPS Doc 8168 Vol 1, Fifth edition, 2006

[2] NOAA National Geophysical Data Center & British Geological Survey Geomagnetism Team, World Magnetic Model, https://www.ngdc.noaa.gov/geomag/WMM/DoDWMM.shtml, 09/2018

[3] International Association of Geomagnetism and Aeronomy, IGRF model, https://www.ngdc.noaa.gov/IAGA/vmod/index.html, 09/2018

[4] FAA, AC20-138D, change 1&2, 2016

[5] Performance based operations Aviation Rulemaking Committee (PARC), Magnetic Variation Review and Recommendations report, 2013

[6] NAV CANADA, Magnetic vs. True North (presentation), 2017

[7] International Maritime Organisation, STANDARD MARINE COMMUNICATION PHRASES, NAV46/WP.3,2000

[8] RTCA, DO-229D, Minimum Operational Performance Standards for Global Positioning/Wide Area Augmentation System Airborne Equipment, 2006

[9] Royal Canadian Air Force, Polar Navigation, vol. 2, 1941

#avionics #navigation #variation #north #compass #inertial #GNSS #RADAR #vectors

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