Magnetisk nord (MN) - Sann Nord (TN) bør ta over i 2030 - AW&ST

Ref. innlegget mitt den 31.1. (Red.) The Moving Target Of Magnetic North Bill Carey A surprise shift in the Earth’s magnetic field has caused scientists to revise the standard mathematical model used to orient navigation and heading reference systems before its next planned update. While the effect on aviation is pronounced only at far northern latitudes, the need for a revision brings attention to the consequences of an ever-changing magnetic field. Citing “unplanned variations in the Arctic region,” the U.S. National Oceanic and Atmospheric Administration (NOAA) on Feb. 4 published an “out-of-cycle” edition of the World Magnetic Model (WMM), a joint effort with the British Geological Survey that was not due for an update until 2020. The update was released and available before the partial U.S. government shutdown that lasted from Dec. 22-Jan. 25, although a NOAA website that hosts the model was temporarily taken down. Recent ‘magnetic jerk’ surprises scientists Magnetic North Pole drifts toward Siberia Canada floats true north reference system The WMM was last released in 2015, providing a predictive model of the movement of the northern magnetic pole that was supposed to be reliable for five years. But more rapid movement of the magnetic pole from the Canadian Arctic toward Siberia—attributed to shifting of the Earth’s liquid iron core—necessitated the out-of-cycle update. “Right after the 2015 mag-model came out, there was what scientists call a ‘magnetic jerk’ in the field that caused it to accelerate in a way that was unanticipated,” explains Michael Paniccia, a geodetic earth scientist with the U.S. National Geospatial-Intelligence Agency (NGA). “These jerks are fairly common; they happen every so often,” adds Paniccia, whose Defense Department agency supplies data for the WMM. “What made this one so bad was that it happened right after the [2015] model was released. That is what spawned the need for this out-of-cycle update.” This is the first such out-of-cycle update of the model, which dates back to the 1950s, according to Paniccia. Until the 1990s, the U.S. Navy gathered data on the magnetic field using aircraft; since then, data primarily is collected by satellite-borne magnetometers. The European Space Agency claimed credit for helping to measure the accelerated drift of magnetic north—from around 15 km (9 mi.) annually 50 years ago to 55 km currently—using its Swarm constellation of three satellites launched in November 2013.

The green line indicates 0 declination, or the angle between magnetic north and true north. Red lines are positive (east of north) declination; blue lines are negative (west of north) declination. Background is a map of magnetic total field intensity. Credit: NOAA

Movement of magnetic north is shown over time. Yellow squares are from expeditions to the Arctic that used compasses and magnetometers to physically find pole locations. Modeled pole locations from 1590 to 2020 are circles progressing from blue lines to yellow. Credit: NOAA

The recent magnetic jerk caused magnetic variation to exceed the 1-deg.-error limit allowed by military specification at the north polar cap, with the affect pronounced at 55 deg. N. Lat. and above, NGA officials say. “Typically, in the past, we’ll come up right on that 1-deg. level at the five-year update,” notes Paniccia. But the magnetic field more recently has shifted at greater speed, “which is causing more error than in years past.” For aviation, magnetic variation affects the design and operation of instrument procedures and ground-based navigation aids and the numbering of runway ends according to magnetic compass direction. Aircraft attitude and heading reference systems using magnetic north for directional guidance must be aligned with inertial reference units and flight management systems (FMS) operating in true north that calculate declination—the difference between true and magnetic north—before presenting information to the pilot. “The whole reason for the magnetic model is pilots fly everything in magnetic [north] but GPSs and flight-management systems do everything in true [north],” says David Atkinson, NGA Digital Aeronautical Flight Information File (DAFIF) program officer. “The only reason for a magnetic presentation at all is for the pilot’s display to line up.” With a 1-deg. error in direction, an aircraft flying at 600 mph for 4 hr. would be off course by around 40 mi., estimates Atkinson, who works part time as a line pilot with commuter airline Air Choice One, based in Concord, Missouri. The updated WMM will be incorporated with the DAFIF database produced by the NGA and made available for FMS updates through the 28-day AIRAC (for Aeronautical Information Regulation and Control) cycle, says Atkinson. Canada has proposed that International Civil Aviation Organization (ICAO) member states adopt a true north reference system by 2030 for bearings to nondirectional beacons, VOR (VHF omnidirectional range) radials and tracks.

Wichita Dwight. D. Eisenhower National Airport will renumber its three runways to account for magnetic variation as part of a major runway improvement project. Credit: Wichita Airport Authority

In a working paper presented last October during ICAO’s 13th Air Navigation Conference, Canada argues that with the transition from analog to digital aircraft systems, magnetic variation discrepancies are causing operational errors in performance-based navigation procedures, Category 2/3 autoland coupled approaches and landings, and AIRAC coding for course and heading legs. With incorrect heading information, synthetic runways generated by head-up display and synthetic vision systems “do not overlay the real runway.” Managing magnetic variation costs airlines, air navigation service providers and avionics manufacturers millions of dollars a year, the paper says. In Canada, expressing magnetic variation in various publications costs C$500,000 ($378,000) per year, with another C$300,000 spent on “rotating VHF omnidirectional radio ranges and flight-checking modified instrument procedures for magnetic variation changes.” In 2012, the FAA updated the magnetic variation of approaches to Ted Stevens Anchorage International Airport in Alaska to reflect current values, the working paper reports. “This caused a mismatch between the magnetic variation used in various aircraft systems and the navigation database in the flight management system,” it says. “As a result, Boeing aircraft (with the exception of the 737) experienced unacceptable lateral guidance when conducting Cat. 2 and Cat. 3 approaches. . . . Essentially the headings provided by the inertial reference units were arguing at a computer-systems level with the aircraft autoflight system, resulting in the aircraft rolling back and forth when flying the Cat. 2 or Cat. 3 instrument landing system [ILS].” The FAA resolved the problem by returning “magnetic variation to the incorrect, but aircraft-usable value” until operators updated the magnetic variation tables used by aircraft inertial reference systems. Similarly, operators were experiencing unstable Cat. 2/3 approaches at Canadian airports. Upon investigation, it was found that aircraft magnetic variation tables were 10-15 years out of date, according to the working paper. Magnetic variation is an issue for other airports. Wichita Dwight D. Eisenhower National Airport (ICT), the largest commercial airport in Kansas, will renumber its three runways as part of a major runway improvement project beginning this year. Runway compass headings at ICT have shifted 6 deg., or about 1 deg. per decade, since the airport was built in 1954. The FAA allows an inbound compass course variation of up to 5 deg. on an ILS localizer signal matched to the runway—up to 195 deg. for a runway numbered as 19, explains Brad Christopher, Wichita Airport Authority assistant director of airports. At 10, 301 ft., Runway 1L/19R, ICT’s longest, is numbered for compass headings of about 10 deg. when flying to the airport from the south and 190 deg. when flying from the north. With the 6-deg. drift of magnetic north, the headings now exceed the allowed variation so that runway ends should be renumbered 2L/20R. “Now we’re at 196 [magnetic heading]; we are clearly in the range that it is appropriate to change that from Runway 19 to 20,” says Christopher. “We’ve known about this phenomenon for quite some time; it didn’t just slip up on us.”

Aircraft taxi toward takeoff from Runway 1L/19R at Wichita Dwight. D. Eisenhower National Airport. Credit: Wichita Airport Authority

The estimated $10-12 million runway improvement project, which is funded mostly by federal Airport Improvement Program grants, calls for replacing the keel, or middle, section of runway pavement and paved runway shoulders, as well as the airfield electrical system. The airport expects to sandblast the current markings and renumber runway ends, but that is the easy part, says Christopher. “The hard part is the ancillary changes to airfield guidance signs with runway designations. These sign panels all have to be replaced,” he says. A software-based airfield lighting-control system will have to be updated. Christopher expects the work associated with redesignating runways will cost about $500,000. The rapid movement of magnetic north that caused NOAA to issue an out-of-cycle edition of the WMM could lead mapping provider Jeppesen to update its NavData electronic database “whenever the new mag variation models are released by NOAA,” says Andrew Riedel, manager of Jeppesen corporate technical standards. “This mainly affects a field we output to our NavData customers called Dynamic Mag Var. This is important and critical data to some of our FMS equipment manufacturers,” he adds. While there is no immediate impact for its digital or paper chart products, the Boeing subsidiary based in Englewood, Colorado, could see an increase in source information changes by civil aviation authorities (CAA), with some countries processing navigation bearing changes more often to keep up with magnetic variation in certain regions, Riedel says. The company has proactively approached several CAAs to inquire about their plans. If “a large country releases mass changes,” Jeppesen would publish a customer briefing bulletin to inform its customers, Riedel says. Jeppesen emphasizes that it is speaking of magnetic variation shifts in the north polar region. A complete reversal of the Earth’s magnetic field to the south “may have different and more drastic impacts.” That is something that happens over several thousand years, NOAA says.