People not involved in map-related sports or professions probably don't think about "north" much. I think most people have learned that "true north" and "magnetic north" are not the same, but I worded the title of this post the way I did to cover the fact that magnetic north is also not the same as itself over time. The earth's magnetic field varies somewhat randomly on a daily basis and also steadily over time. It still amuses me when I come across concrete manifestations of the unconstant nature of magnetic north, and I saw one in a NOTAM recently.
110345 CYVR VANCOUVER INTL
YVR- VANCOUVER VOR 115.9 ROTATION, ADD 4 DEG TO ALL PUB APPROACH RDL ASSOCIATED WITH YVR. SPECIFIC RDL ISSUED BY ATC SHALL BE ADHERED TO AS PER THE RECEIVED AND ACKNOWLEDGED CLEARANCE
TIL 1105050901
YVR is a VOR -- a navigational radio station -- near the CYVR airport and it "tells" a pilot that her airplane is on a particular radial, i.e. that it is located a particular magnetic direction from the VOR ground station. She uses that information to navigate in the vicinity of the airport, perhaps to do an instrument approach that follows a specified radial straight to a runway, or to stay out of a restricted area that she has determined from the chart to be west of a particular radial. A number of radials are published on charts as named routes, for example on the expired chart I have here, the 037 radial from the YVR VOR was designated as part of the V304 airway to Calgary, sort of like Portage Avenue in Winnipeg forms part of the Trans-Canada Highway. A pilot tracking a radial makes heading corrections as needed to compensate for any crosswind, but she expects that in zero wind she will be flying on the same magnetic heading as the radial while tracking away from a station, and on the reciprocal heading while tracking towards a station. (To get to the station on the 000 radial, you fly 180, or south). A pilot who filed a flight plan from Vancouver to Calgary along V304 would expect to be flying on a heading of 037, and because the published minimum obstacle clearance along that radial is 9000', she would also expect that by keeping the 037 radial selected and centred on her VOR at 9000' that she wouldn't fly through any clouds with crunchy centres.
A VOR does not use the orientation of the earth's magnetic field to function, but it is set up and calibrated so that its zero radial runs due magnetic north of the station. That means that when the earth's magnetic field shifts, the YVR 037 radial still runs along a safe path between YVR and the next VOR on the airway. The pilot might just have to fly a different heading to stay on the airway. But after a number of years the accumulated magnetic shift is enough that Nav Canada wants to recalibrate the VOR such that the 037R really does run in the 037 direction. So they rotate the actual VOR signal. I'm pretty sure they just go in there and electronically adjust the direction in which the station sends its signals, but it's more fun to imagine that they jacked the whole thing up (they're usually about the size of a garage, except round) and cranked it around four degrees counterclockwise. But once they've done that, the 037 radial no longer runs through that safe route towards Calgary. It might pass a lot closer to some pointy rocks. So they have to amend all the publications so that what once specified the 037 radial now specifies the 041 radial, and so on all the way around. There are about fourteen airways defined off the YVR VOR. And they do amend all the publications that show those airways, and pilots or their companies are required to buy new ones every fifty-six days, but they didn't rotate the VOR on the same day as the new publications come out. This NOTAM informed me that until the new chart became effective, on May 5th, that I should add four degrees to any published radial from YVR. The new chart labels the 041 radial as V304, and the 039 radial subsides into unpublished obscurity.
There was a Nav Canada Challenger at YVR while I was there, probably in town to check the alignment of the airways and approaches.
16 comments:
I always hated it when my students asked me to explain why magnetic headings were used for just about everything -- runways, NavAids, and so on. I hated that question because I could never provide a satisfying answer. I'd usually respond with something along the line of, "Runway headings are magnetic because some (ancient) planes don't have gyroscopic heading indicators, and the poor pilots flying them have to navigate by the magnetic compass."
Thirty years ago, most students bought that explanation. Maybe they bought it because GPS units were not available to civilians at any price back in those days. Or, perhaps they were just being polite. Heck, I don't know.
I never could figure out how to respond to astute students that pointed out that you can't really fly according to a magnetic compass anyway, unless the air happens to be perfectly still. Turning and turbulence will make the compass swing all over the place, and besides, you have to correct for the crosswind. Why not forget the plight of pilots flying antique aircraft, set all gyro compasses and runway headings to true north and be done with it? And while we're at it, why don't we standardize all VORs and ILSs and other NavAids to true north as well?
Does anyone know why we don't use true north for all navigation? Other than because that's the way it's always been done in the past?
As someone who flies antique aircraft (a 1943 J3 cub and 1946 Champ) I say hrrrmph!
You can navigate by compass, it's a little wiggly and all, and maybe not too accurate. But I'm a student and I've used it just fine for my cross country trips. I'm also only travelling at a meager groundspeed so it takes quite a lot longer to accumulate any serious errors.
On the flip side, it's not like I'm using a VOR anytime soon unless I buy a handheld that does that.
Of course the whole ATIS is magnetic but METAR's are true would go away too.
Oddly, I never asked that question. I believe we navigate by coordinates based on magnetic north because we are following in the unbroken tradition established when someone (some ancient Phonecian? Who discovered the principle of the magnetic compass?) discovered a way to maintain a course on a cloudy night.
It would kind of make sense to switch. It wouldn't actually put more work on the people navigating with compasses, because the charts have gridlines on true north, so we plan in true and convert to magnetic anyway.
It would definitely be one for old captains to annoy their young FOs about in reminisce.
Data pilot,
Unless you're flying an airplane which navigates from an inertial reference, your heading indicator, gyroscopic or not, is oriented to a magnetic reference, is by a device that senses the earth's magnetic field.
Does anyone know why we don't use true north for all navigation?
Yes. Because the vast majority of planes flying have no means of determining the aircraft's orientation relative to true north
Because the vast majority of planes flying have no means of determining the aircraft's orientation relative to true north.
Actually, yes they do. All you have to do is add the variation (difference between true and magnetic) to the reading from the compass. You're supposed to correct for deviation (compass errors) anyway so why not one more step.
It always seemed to me that the best way would be to have a little knob (non-magnetic!) which you turn to set the position of the lubber line on the compass to match the variation.
I think IRS equipped aircraft do determine true north by sensing the Earth's rotation and the local gravitational field while they're aligning.
For GNSS (GPS) procedures we have the ridiculous situation where variation is deliberately added in to headings to be subtracted out again.
Use true.
(And why do we CC e-mails? When did you last use any carbon paper?)
Perhaps part of the explanation for the enduraance of Magnetic in a (developed) world of satellite aids is that in that other world many many people are obliged for a variety of reasons to navigate - in the generic sense - in a much less sophisticated surface environment .
Reading Aviatrix's post about the Vancouver VOR's compass rose being rotated 4 degrees (a significant amount) and the change being announced via NOTAM stuck in my craw. Pilots need the best possible tools to avoid flying into high-altitude, crunchy clouds -- which are plentiful near Vancouver. Rotating VOR radials is at best a temporary solution.
I mean, we're talking about Canada, the Home of the Wandering Magnetic North Pole and the Land of Squirrelly Compasses. If any nation has an impetus to lead the way to standardization on true north navigation, Canada does. It's not 1935 any more.
If if those of us living south of the border are really lucky, our nation might take notice and adopt true north navigation as well. Although, given that we're still measuring temperatures in Fahrenheit and buying food by the pound, I'm not going to hold my breath.
It should be noted for the purpose of this discussion that Canada does use true north navigation north of a line I may find for you later. The runway numbers have a T in them to remind pilots that they are true. I'll see if I can find out if anyone else does this, or it's Canada alone.
I'll see if I can find out if anyone else does this, or it's Canada alone.
The runway direction at Thule, Greenland is true.
Actually, yes they do. All you have to do is add the variation.....
Sigh.... OK yeeees, it *is* possible to *compute* true north by adding variation to magnetic heading. I assumed that was obvious to all.
Allow me to rephrase:
Because the vast majority of planes flying have no means of *directly* determining the aircraft's orientation relative to true north, by sensing true north.
Better?
For GNSS (GPS) procedures we have the ridiculous situation where variation is deliberately added in to headings to be subtracted out again.
No, at the code level, the navigation is being done in Earth centered Earth Fixed Cartesian coordinates, the inherent reference system of GPS. Magnetic heading, and latitude/longitude for that matter are all computed for display.
I wonder if the big white letters painted on runways have ever been changed somewhere because magnetic North has shifted.
One thing that amazes me about air-navigation (to make a word out of words) is how everything, be it ever so high-tech, is built to be in perfect harmony with what was used in the first half of the 20th century.
Yay for interoperability.
However, making true North the norm would probably not even mess with the idea of having DC-3s and A321s find the same runway, using some different navaids.
Yes, zb - our local international used to have 29L/R parallels. They're now 30L/R.
zb: Yes, as Sarah attests, it is not an everyday occurrence, but eventually magnetic north shifts far enough that the runway heading is no longer within 5 degrees of its number and it gets changed. Sometimes at night you can see the painted out numbers underneath.
I've been flying for only a few years and here in France I know a few airstrips which numbers have changed because of magnetic North changed.
It's not uncommon.
I'm very happy to fly in a place where the magnetic declination is 0° :-) (well it's between 0° and 5° so in practice for my little VRF flights I can usually neglect it).
A Squared: No, at the code level, the navigation is being done in Earth centered Earth Fixed Cartesian coordinates, the inherent reference system of GPS. Magnetic heading, and latitude/longitude for that matter are all computed for display.
Of course the GPSs internally work in ECEF coordinates but I wasn't talking about that: I was talking about the procedures which are published as magnetic radials. However, as I recall a single variation is used for the whole procedure so you can wind up using a nav aid with a different variation for a GNSS procedure as for a normal one.
Probably a weird question to ask here, but does anyone know this site? Did anyone order a shirt here? The shirts look really good but I don't know if I can trust the site www.pilotbooth.com
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