Airport Runway Names Shift with Earth’s Magnetic Field

Earth's magnetic field is constantly changing and while large-scale changes, such as a complete reversal of the magnetic field, happen over several thousand years, smaller changes over shorter periods of time affect navigation, including for aviation. This shifting magnetic field affects airports and airline operations, including alphanumeric airport runway names, which indicate headings—crucial information for pilots.

The Fairbanks International Airport renamed runway 1L-19R to 2L-20R in 2009, for example, when magnetic north shifted enough to mandate a change. And the airport operators know—from NOAA’s World Magnetic Model and other sources—that they'll likely need to update the name again in 2033, said Sam Loud, spokeswoman for the Alaska airport. That model, developed by CIRES, University of Colorado Boulder, NOAA and other researchers, provides guidance for everyone who needs accurate navigation tools.

If you are one of the millions of Americans catching flights for the upcoming Thanksgiving holiday, take a few minutes to give thanks for the complicated science behind the simple runway label.

The World Magnetic Model (WMM) is a data-based, mathematical representation of Earth’s large-scale magnetic field used for navigation, orientation, and heading references. It’s critical to the map apps in virtually every smart phone and is the standard navigation tool for the U.S. Department of Defense, the North Atlantic Treaty Organization, the Federal Aviation Administration (FAA), and more. NOAA’s National Centers for Environmental Information (NCEI) and the British Geological Survey produce an updated model every five years.

“Several agencies use the World Magnetic Model to update their navigation aids, with our software and online calculators,” said Arnaud Chulliat, the CIRES scientist in NCEI who leads the project.

The World Magnetic Model is a large-scale representation of Earth’s magnetic field. Image: NOAA/NCEI

The data that go into the World Magnetic Model come from many sources, primarily satellite observations and a global network of 120 magnetic observatories. Satellites help scientists  separate various contributions to the magnetic field, according to Chulliat. Earth’s core contributes to the magnetic field, but so do magnetic rocks in Earth’s subsurface and electrical currents in the atmosphere and beyond.

“We’re looking for the signal from Earth’s core,” Chulliat said. “To get that, we combine satellite and ground signals into the model and use various mathematical and modeling techniques to separate core from non-core signals.”

The NOAA NCEI team provides its customers with the WMM and associated software that they can download, embed in other systems, or use to write software of their own. There’s also an online magnetic field calculator into which a user can enter a set of coordinates to get a precise magnetic field value for that location.

Those tools need periodic updates, and here’s why: Earth’s inner core is solid, while the outer core is liquid. Both are primarily made of iron and nickel; flows within the outer core generate 95 percent of the geomagnetic field, including the slow, secular variation that is tracked by the WMM.

Fluxes in the outer core trigger unpredictable changes in Earth’s magnetic field. For example, over the past few decades the North magnetic pole has been drifting toward Siberia at an irregular speed, and that affects navigation accuracy. Updates to the WMM ensure accurate magnetic field declination—the difference between true north and magnetic north.

Earth’s inner core is solid, while the outer core is liquid and can trigger changes in the magnetic field. Image: European Space Agency and ATG/Medialab

NCEI scientists and their partners continuously survey the magnetic field to precisely map the present field and its rate of change and then extrapolate changes into the future. The team issues major updates every five years; the 2020 update will come out at the end of 2019.

The aviation industry relies on magnetic compasses as primary instruments for navigation, alongside GPS instruments, so shifts in Earth’s magnetic field affect many aspects of aircraft navigation, including instrument landing systems, air traffic procedures, and runway designations. Airport runways are perhaps the most visible example of a navigation aid updated to match shifts in Earth’s magnetic field.

COMPASS WITH RUNWAY OVERLAY. Image: CIRES

By FAA rules, runways are numbered according to the points on a compass, from 1-36, reflecting the magnetic compass reading to the nearest 10 degrees, and dropping the last digit. In addition, runway numbers are connected to the direction a plane is traveling—so while on a handheld compass, south corresponds to 180 degrees, or 18 in runway terms, if a plane is on runway 18–36, then it’s heading north, according to the designation on a compass with the runway overlay. Large airports have other designators, too, such as L or R for the left or right runway in a parallel pair. For pilots and other navigators, keeping runway names updated to accurately reflect magnetic heading is crucial.

Take Denver International Airport (DIA) runway 17L-35R, for example, which is currently magnetically oriented at 172.5 degrees, according to DIA spokesman Heath Montgomery. When magnetic shifts nudge that bearing by another 4 degrees to 176, that runway will become 18L-36R. “The declination has changed just over 2.5 degrees over the past 22 years since DIA opened,” Montgomery wrote in an email. “Declination is changing by 0.1 degree per year, so about another 30 years or more until the next change at the current rate.”

Magnetic shifts happen faster the closer to the poles you go, so airports at high latitudes must make more frequent adjustments than airports closer to the equator. For Fairbanks International Airport, for example, the interval of change is roughly every 24 years, wrote spokeswoman Sam Loud.

The overall goal for all navigational aids, according to guidelines in FAA Order 8260, is to keep the magnetic variation figure used for guidance as closely aligned to the current computed value as modeled by the World Magnetic Model, within 1 degree, plus or minus.

The WMM team consults regularly with FAA employees who use the model to update runways and other navigation systems. “The FAA uses our model to determine the magnetic field at certain locations and points in time, then sets their instruments and other navigational aids at these points,” Chulliat said.  

The NCEI team is also working to develop new ways to map Earth’s magnetic field. With the current constellation of satellites providing high-accuracy magnetic measurements at low Earth altitude, scientists can map the magnetic field. “But these satellites won’t last forever,” said Chulliat, “so our goal is to come up with innovative and more cost-effective ways to survey the Earth's magnetic field, so the World Magnetic Model stays as accurate as it is now throughout the coming decades.”

As part of the University of Colorado Boulder’s Grand Challenge, the WMM team is working with colleagues in the Aerospace Engineering Sciences department and the Laboratory for Atmospheric and Space Physics at CU Boulder to develop, build, test, and fly the first-ever CubeSat that will collect high-resolution magnetic field data around the globe. The eventual goal is to model the Earth’s magnetic field with a constellation of these miniature satellites.