Simviation Forum - A VOR positioning question

Feb 17^th, 2008 at 10:19pm

Dr.bob7 Offline
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well VOR stations are always pointing magnetic north so there tilted why do they all appear perfectly north no matter where on FSX? do i need to see a actulal pictur e of it or does FSX put them all at a default north facing?

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Reply #1 - Feb 17^th, 2008 at 11:47pm

Brett_Henderson Offline
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I'm not sure what you're asking...

Magnetic variance is represented pretty well in FSX..

Here's a screenshot of my position, exactly on the 360 radial (according to the VOR)...

That's the Appleton VOR.. the variance is 6~7 degrees west there.. as you can see.. I'm about 6 degrees west of true north.

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Reply #2 - Feb 18^th, 2008 at 4:06pm

OTTOL Offline
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Brett_Henderson wrote on Feb 17^th, 2008 at 11:47pm:

I'm not sure what you're asking...

Magnetic variance is represented pretty well in FSX..

Here's a screenshot of my position, exactly on the 360 radial (according to the VOR)...

That's the Appleton VOR.. the variance is 6~7 degrees west there.. as you can see.. I'm about 6 degrees west of true north.

Never mind the original question. It looks like you started to head outbound on, about, the 280 degree radial, then you did a J-turn and headed north. How the heck did you back your plane up?!? Shocked

.....so I loaded up the plane and moved to Middle-EEEE..........OIL..that is......

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Reply #3 - Feb 18^th, 2008 at 5:05pm

Brett_Henderson Offline
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LOLOL

I slewed into position. I wanted to be on the 360 radial exactly...

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Reply #4 - Feb 18^th, 2008 at 9:44pm

beaky Offline
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You may just need to check more VORs in the sim- many areas have very little magnetic variation, so magnetic North would be very close to true North.

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Reply #5 - Apr 6^th, 2008 at 3:13pm

Nexus Offline
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A word of caution: Not all VORs are oriented Mag. north.
I should know since I failed an exam just because of that question. Embarrassed

JAA ATPL BOOK 12 (Operational Procedures) says:
"In areas where the rate of change of magnetic variation becomes excess (close to the north magnetic pole), VOR beacons are oriented true north to assist grid navigation. VOR's in the Canadian Northern Control Area are oriented true north.
Where the the primary heading information is derived from an IN system, care must be taken to monitor the system (by reference to any other aid or method) for degradation or failure."

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Reply #6 - Apr 6^th, 2008 at 6:20pm

beaky Offline
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Nexus wrote on Apr 6^th, 2008 at 3:13pm:

A word of caution: Not all VORs are oriented Mag. north.
I should know since I failed an exam just because of that question. Embarrassed

JAA ATPL BOOK 12 (Operational Procedures) says:
"In areas where the rate of change of magnetic variation becomes excess (close to the north magnetic pole), VOR beacons are oriented true north to assist grid navigation. VOR's in the Canadian Northern Control Area are oriented true north.
Where the the primary heading information is derived from an IN system, care must be taken to monitor the system (by reference to any other aid or method) for degradation or failure."

A very good point; I'd forgotten about that one.

Back to the OP: maybe drbob is asking why the VOR icons in Map view are aligned with true north...?
At any rate, the functional radials of the VORs in MSFS are aligned as they should be... although I'm not sure about the exceptions Nexus notes.

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Reply #7 - Apr 11^th, 2008 at 9:59pm

Dr.bob7 Offline
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I was gone for a while but i read up on the magnetic positioning, ive printed a few isogonic charts so i should be fairly close with navigating

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Reply #8 - Apr 11^th, 2008 at 10:15pm

beaky Offline
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Dr.bob7 wrote on Apr 11^th, 2008 at 9:59pm:

I was gone for a while but i read up on the magnetic positioning, ive printed a few isogonic charts so i should be fairly close with navigating

You're better off with aeronautical charts... they show the isogonic variation for a given area, in addition to the usual stuff. Wink

But again, as far as the VORs go (in the sim as in real life), they are already skewed for local variation... in other words, the 360 radial should point to local magnetic north, not true north.

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Reply #9 - Apr 12^th, 2008 at 8:29pm

Splinter562 Offline
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I'll throw another curve ball in here.

If you are flying along a Low or High Altitude Federal Airway using GPS, you will find that many times, the magnetic course the GPS tells you to fly and the radial indicated on the chart differ by several degrees. Why is this?

Bonus points for anyone who can answer the question without goggling it.

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Reply #10 - Apr 12^th, 2008 at 9:37pm

Brett_Henderson Offline
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I'm going to honestly answer quickly, off the top of my head... I'll come back to see how stupid I am.. Huh

Radials are based off their VOR's position.,, but you can be quite far away from a VOR, while tracking one of its radials... hence at a place where the difference between true and magnetic north, is different than what they are AT the VOR..

I tried "goggling"... there's no place to enter a search argument at www.goggle.com Tongue

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Reply #11 - Apr 12^th, 2008 at 10:29pm

Splinter562 Offline
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Humm, spell check didn't catch me. It doesn't think that "Googling" is a real word.... but for some reason "Goggling" is. Google may not be much help on this one anyway. I didn't come up with much in my attempt to find a good solid reference to a regulation to cite the answer. I think the answer may be in ICAO's "International Standards and Recommended Practices, Annex 10", but I can't find a free copy of it on the net. I did find another reference that at least confirms the answer, though doesn't explain it.

As for your answer Brett, that is what I though for the longest time too. It does account for some of the difference, especially in places where the isogonic lines are closely spaced (like Alaska). But in many places that would only account for fractions of a degree. There is another, much more obscure reason that the numbers can be off by several degrees.

I'll let a few more people take a whack at it before I post the answer.

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Reply #12 - Apr 13^th, 2008 at 12:05am

Brett_Henderson Offline
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I'm truly not researching this... I'll keep poking at it..lol

Ok.. I'm picturing a plane flying along.. The GPS knows where it is; it knows where it wants to be tracking, and knows where it actually is tracking.. hmmmmmm Huh

I don't use GPS at all in the sim, and use it mainly as a DME and for airspace monitoring, when I fly for real.. so this is still a guess..

If I remeber correctly, there was a GPS function for calculating wind direction and velocity (just thinking out loud). Since, (in the planes I fly anyway) the GPS gets no compas or DG input, BUT, it's trying to get you to make your track match the course... hmmm, this is tough Undecided

(is this error replicated in MSFS ?)

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Reply #13 - Apr 13^th, 2008 at 12:12am

Brett_Henderson Offline
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OK.. I cheated and Googled it. I'll take credit for having thought that might happen.. but would have NEVER guessed that that potential problem would ever go unchecked long enough to amount to several degrees (sorry if that's a hint)..

Good one

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Reply #14 - Apr 13^th, 2008 at 6:39pm

Splinter562 Offline
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No other takers? I guess all the points go to Brett on this one.

VOR radials are aligned to magnetic north... when the station is built. The earth's magnetic field continues to change, but the alignment of the 360 radial does not. Given that most VOR stations were built in the 60s, the difference between the 360 radial and magnetic north can be several degrees off.

Here is an example:
The St. Petersburg (PIE) and Sarasota (SRQ) VORs are 31 NM apart and connected by the Victor Airway V579. The charted outbound course from PIE on this airway is 166, but the inbound course to SRQ is 169. Both locations presently have the same magnetic variation, so why the difference. Well, the PIE VOR was built in 1965, when the magnetic variation was 1E, the SRQ VOR was built 20 years later in 1985, when the variation was 2W, 3 degrees of difference. On top of that, if you were to plug in PIE to SRQ into a GPS unit, it would probably tell you that the desired magnetic course is 171, because the current magnetic variation is closer to 4W today.

http://www.airnav.com/cgi-bin/navaid-info?id=PIE&type=VORTAC&name=ST+PETERSBURG
http://www.airnav.com/cgi-bin/navaid-info?id=SRQ&type=VORTAC&name=SARASOTA

Another interesting fact on this topic. The isogonic lines on your US VFR Sectional use the values from 2005, when the last World Magnetic Model was published. The shift is slow enough that the difference between the 2005 values and today's values is small. If you look closely, you will probably see the lines jump by several miles in 2010 when the next World Magnetic Model is released.

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Reply #15 - Apr 13^th, 2008 at 6:51pm

Brett_Henderson Offline
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And it just goes to show ya... Leave those GPSs off Cool

.. or at the very least, use a VOR when racking radials..use the GPS as a DME and to keep your butt out of controlled or restricted airspace... Cheesy

Seriously though... here's the kicker. The VOR reciever and gauge can be off, too. I think +/- 4 degrees is considered, useable. (or is that 4 degrees difference between to VOR gauges, tuned to the same VOR constitutes a VOR check ?)

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Reply #16 - Apr 13^th, 2008 at 7:42pm

Splinter562 Offline
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[Those squemish about FARs should turn away now]

VOR checks are based on indicated bearing to the station. Which for a CDI means centering the needle and reading the bearing.

For a "dual systems VOR (units independent of each other except for the antenna)", the maximum permissible variation is ±4°. For a single VOR you need to use an airborne or ground VOR check point (listed in your handy A/FD). For ground checkpoints the max error is ±4°. For airborne checkpoints the max error is ±6°.

To legally use the receiver for IFR it has to be checked using one of the methods above (or a VOR Test signal) within the preceeding 30 days.

References:
AIM 1-1-4
FAR §91.171

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Reply #17 - Apr 13^th, 2008 at 8:22pm

Brett_Henderson Offline
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LOL

Don't bother feeling squeemish about FARs.. it's a waste of energy.

I remember one session at the club... Me, a CFI, a DE, a 20,000hr pilot, and an aviation attorney. We found more than one FAR section where a consensus on what it said could not be reached, or if it was even logical.. Shocked

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