VOR Radial confusion

[mccourtm]

How come the published radials for way points differ to what you would read off the map. For example in the Pooleys guide EGAE is given as "BEL 117.2 314 39" which I interpret as saying that EGAE is 314 degrees magnetic from the Belfast VOR with a distance of 39miles. Yet if you were to plot that line on the map you would get 311 magnetic.
This isn't just in Pooleys, if you look at waypoints on the CAA approach charts you'll find that there is a difference of a few degrees as well.
Any ideas as to what I'm missing here:confused:

mmcc

[High Wing Drifter]

I get near as 314 degress as I can differentiate with the Jepp Low Alt Enroue chart and an IFR plotter. Dumb question because you already mentioned magnetic, but are you using the VOR's magnetic hook/pointer?

[mccourtm]

Yes, I've taken magnetic into account, i.e., I'm drawing a straight line, measuring the true heading and then adding the variation (local to the vor). I am, however, using the CAA VFR sectionals and I've tried it on both quarter and half mil.

[A V 8]

OK, maybe I have got the wrong end of the stick here

I'm drawing a straight line, measuring the true heading and then adding the variation (local to the vor).

but why would you want to do that? The compass rose surrounding a VOR on a VFR Map is already in degrees magnetic, not true. Is the BEL VOR located on an aerodrome? (Not got a chart handy at the mo) If so, it's probably not going to be right in the centre of the aerodrome. Draw a line from the centre of the VOR to the centre or EGAE. Where does the line intercept the VOR compass rose?

Also, how old is the chart/pooleys that you're using. Remeber the magnetic variation changes from time to time.

Edited again to say I dont mean to sound patronising;)

[FlyingForFun]

I don't have my instrument plates to hand, so I can't quote an exact example, but from one plate to another, the radial of a specific point from a VOR can vary by a couple of degrees, even on plates published by the same publisher, for airports in the same country, with the same date.

For example, if you look at a selection of Aerad plates for airports like Southampton, Alderney and Guernsey, as well as comparing these to the Aerad airways chart covering the same area, and find the radial from SAM to ORTAC, I think you will find that it can be either 206, 207 or 208 degrees.

Unless you have a glass cockpit, I wouldn't worry because you can't set the OBS or CDI that accurately! If you do have glass, though, then you're bu**ered!

FFF
--------------

[mccourtm]

AV8, the VOR compass rose on the chart increments in 10 deg units, so I'm simply using the true heading + var for accuracy. The charts that I'm using are current, as is Pooleys.
I've no doubt that I can replicate this on any of the CAA sectionals so I must be missing something:ugh:

[drauk]

311 deg (magnetic) and 39 nm are the correct values. See:

http://fly.dsc.net/u/Plan?pid1=11251551&pid2=11251931

Your flight guide and/or chart would have to be VERY old to account for a 3 degree change.

[mccourtm]

FFF, I see what you are saying about the descrepancy. I first noticed this when I was first attempting to use a GPS and entering my own waypoints by using radial/distance (hence the accurate take off the map). Then after I had taken note of the radial/dis for a VRP I came across a ref to it in a CAA visual approach plate and noticed that mine was different. Since then I've used L/L as waypoints references but it still confuses me.

[mccourtm]

Drauk, that's a nice tool. My charts are up to date. Look at the current approach chart for EGAE http://www.ais.org.uk/aes/pubs/aip/pdf/aerodromes/32AE0807.PDF
It shows Dungiven as being R312 from BEL. Again, if you measure it you'll find that it's 308M

[GOLF_BRAVO_ZULU]

This may sound like a daft suggestion but are you allowing for convergence relative to the projection of the VFR chart you are using?

[mccourtm]

G B Z, I'm not really sure what you're saying but I take it that it has something to do with a flat map of curved surface??? If this is the case I'm not sure that a distance of 38 miles would show up an error of 3 to 4 Deg.
I'm phyically drawing a line from the centre of the VOR and extending it to the waypoint. Then with the protractor I measure the true heading and add the variation to it.

[S-Works]

VORs are shown as magnetic radials not true radials.

[mccourtm]

I know that VOR radials are shown as magnetic radials, that's why I'm adding the variance (that is the var' where the VOR is located) to the measured true track from the radial, and the reason that I'm not just putting a ruler over the vor and directly reading the magnetic heading is because the VOR compass rose has only 10 deg increments!

[S-Works]

you put a plotter over the VOR and read the mag radial direct from it. you do not need to correct it for variance........

[Sleeve Wing]

"This may sound like a daft suggestion but are you allowing for convergence relative to the projection of the VFR chart you are using?"

Heavy stuff, GBZ !
Isn't convergence only applicable to Great Circle tracks on a Lamberts.
Usual half and quarter mil CAA charts are Mercator...........:ok:

[rustle]

Isn't this all a bit academic anyway?

The tolerance (or allowable error) from a VOR is +/- 4 degrees
The tolerance (or allowable error) on a OBS is +/- 1 degree

If you were 60nm from the VOR and 1 radial (degree) off track and both the VOR and your kit were exactly correct that equates to 1nm lateral displacement which, whilst IFR would be iffy*, VFR would let you see what you were supposedly "over"

* Which is why it would never be used for an IFR approach

Are you measuring your angle to the ARP or a runway?

What is the problem you are trying to solve?

[mccourtm]

Rustle, as you say, my issue is a bit academic but I was just concerned that my meaurment of a radial differed by up to 4 deg from that as is published in the "current" Pooley and CAA maps. Needless to say I haven't gone through every one but those that I looked at differed to what was published, so I was wondering if I was missing something, i.e., could I trust myself to measure a VOR radial in the air if I was trying to pinpoint my position.
Look at the link that I posted earlier. The link is to the current CAA chart for Londonderry airport (EGAD). Dungiven is a VRP to the south east and it is also used for an instrument app. The radial is shown as R312 from BEL but if you measure it you'll find that it's actually 308. This isn't a "one off" as I've come across it in Pooleys where it gives you the position of an airfield referenced to a nearby VOR and again there's a difference of up to 4 deg for all the references, no matter what the airfield is.:confused:

[S-Works]

What/why are you adding for your conversion from mag to true?

The correct way to read a VOR from a chart is with a chart plotter lined up to the magnetic flute on the top of the VOR rose. When I measure the examples you are giving I get with a degree (subject to my eyesight) of the published radials.

So I suspect you are not measuring correctly or have a misunderstanding of how to plot a VOR radial on a chart. You do not measure true and then add variation you measure mag and you plot mag. Thats what the flute on the VOR rose is for.

[S-Works]

OK, I have just replicated what you are doing. You have a fundamental misunderstanding of how to plot a VOR radial!

If I draw a line from BEL to EGAE and measure true course, I get 305d add 6d from the variation on the chart and I get your 311d radial.

If I use the radio nav plotter from my Jepp IFR kit on the flute on the VOR radial I get 314d.

The VOR station is corrected for the variation at that point. The chart gives an average across the variation lines.

You should not plot VOR radials in true and convert you should take them straight from the flute as this is the information displayed on your instrument.

[mccourtm]

Thanks for all your posts. They have all been helpful.
I agree, put the plotter in line with the compass rose and off you go.
BTW the errror is more noticeable on the 1/4 mil chart. I did, however, assume that by lining the plotter that way, or adding the variation, or reading the radial of the chart or whatever, that they would all produce the "exact" same result (in theory).
I'm obviously too pedantic:O

[LH2]

Without looking at the particular chart you are using, I would say GBZ is right on the money. Your discrepancy will be accountable to a number of sources, but the projection's angular distortion at the points you are measuring is likely to be the biggest contributor (other significant factors could be, depending on your particular example, your measuring a rhumb line track and comparing against a great circle track, measurement artifacts such as chart or ruler deformation, etc.)

In any case, as you correctly surmise, it is a mostly academic issue. At least for me, I couldn't hold a track to 5 degrees to save my life :}

[Sleeve Wing]

Airbus38.
Whoops !! I stand corrected. .....Lamberts Conformal Conic.
There's always something..............:O

[Islander2]

How come the published radials for way points differ to what you would read off the map. For example in the Pooleys guide EGAE is given as "BEL 117.2 314 39" which I interpret as saying that EGAE is 314 degrees magnetic from the Belfast VOR with a distance of 39miles. Yet if you were to plot that line on the map you would get 311 magnetic.How odd, my 2007 Pooleys says, on p376: "BEL 117.20 311 39.5"

Also, I might be able to help with your confusion here:
Dungiven is a VRP to the south east and it is also used for an instrument app. The radial is shown as R312 from BEL but if you measure it you'll find that it's actually 308.Dungiven VRP and DUNGV holding fix are not the same place:

VRP (per Pooleys): N54 55.70 W006 55.50
DUNGV (per Aerad): N54 53.73 W006 47.40

[S-Works]

Variation shift? My 2006 gives 314/39 which matches the 2006 chart I plotted it on. You are so posh having new charts and guides Chris!!!! :p

[Islander2]

You are so posh having new charts and guides Chris!!!! :pAs ever, no expense spared! ;)

[roadrabbit]

It is a long (very long!) time since I studied VOR radials, but I seem to recall they work differently to, say, an NDB, in one very important way. They radiate radials and not a single radio signal. Thus when built, they are aligned as accurately as possible to magnetic north and that becomes their permanent reference. As time passes, magnetic north changes, as in annual change in magnetic variation.

Thus an error creeps in and the VOR no longer aligns accurately with current magnetic north.

Thsi will explain why you cannot measure the true direction from a VOR to another point, apply variation, and obtain the correct radial. Even when using the VOR rose on a chart it will not necessarily give you the correct radial (apart from the lack of accuracy in setting and measuring) even if aligning with the 'flute'. The only way to obtain the exact radial is to accurately position a VOR receiver over the geographical position required, tune in the VOR and measure the radial.

Anyone who has seen a VOR station will recognise it immediately - it is a low circular building with lots of vertical 'posts' around the perimeter of its flat roof - not easily moved. However, I suspect modern VORs might be electronically 'rotatable' to align with current magnetic north.

[Broadlands]

Just for the hell of it
SkyDemon gives 305T, 309M and a distance of 40nm.
Thought I would see what it came up with. I have not tried the Jepp on my iPad yet.

[eckhard]

As roadrabbit says, the VORs are not necessarily aligned with mag north. The difference between true north and the VOR reference is called declination in the USA. As far as I know, the declination can be adjusted from time to time.

You will often find that two VORs at the opposite ends of an airway don’t line up; e.g. the airway starts off on the 060 radial from VOR ‘A’ but then arrives at VOR ‘B’ on the 242 radial (062 inbound), even though the airway is plotted as a straight line.

The reason can be due to different variation at the two VORs but over a relatively short distance is more likely to be due to a difference in the two declination values.

[MarcK]

Aligning to the VOR rose should give you the correct radial, as the rose is itself aligned with the VOR.

[BackPacker]

As roadrabbit says, the VORs are not necessarily aligned with mag north. The difference between true north and the VOR reference is called declination in the USA. As far as I know, the declination can be adjusted from time to time.

I wonder how often the VORs are actually aligned with the current magnetic north.

My gut feeling is that the alignment itself is rather easy - just a software parameter in a program somewhere. But as you've noticed, there's lots of places where the VOR is referenced. Not only do you need to update all the approach and departure plates for any nearby airfields, but even airfield plates as far as 50 nm away may have their positions referenced to your VOR (magnetic) radial and distance, so their plates need updating as well.

That will make realigning a VOR a major undertaking, and I can imagine that that's only going to happen when *really* needed - for instance when it's outside the published tolerances for a VOR. Maybe after something like 10-20 years?

[Pilot DAR]

Occasionally, runways have to be renumbered too, though this would be based upon the variation becoming greater/less than the next 5 degrees. I remember when Toronto's runways were renumbered for this reason.

If the annual error resulting from variation change is affecting the accuracy of your navigation, you're doing really well.

[Charley]

Magnetic Declination can be the cause of these errors, as rightly suggested.

When a VOR is installed or receives a regular calibration, it will have its datum realigned with magnetic north. Afterwards the VOR will be flight tested by one of the navaid calibration companies; Flight Precision used to do it with King Airs but I gather there's a few companies now, e.g. Cobham.

The information you 'need' (if you want to be geeky about it) is all in the AIP.

For instance, this is the entry for Belfast VOR:

BELFAST
VOR/DME
(2.9°W (2019))
(decl.: 4.0°W)

The magnetic variation is suggested as 2.9W for 2019 but the declination is still set to 4W. Therefore you can expect to see a 1° error between mag bearings and radials for the time being, until it is datum is next realigned.

So, strictly speaking, radials are not referenced to either true north or magnetic north; radials are referenced only to the datum at the VOR. Kudos to eckhard for bringing it up.

[scifi]

I think RoadRabbit has it 100% correct, the VOR radials are set in stone, or more precisely their aerials are set in concrete on the roof of the building.


So from a practical point of view, if you were flying from Donegal to Belfast, via Londondery, the first part of your flight you would only look at your Compass / DI, and fly approx 95 degrees magnetic. Then for the Londondery to Belfast leg, you would only look at your VOR meter, which would be set on the 314 VOR radial. Your DI and VOR will read differently.
.

[custardpsc]

I am pretty sure it is trivial to change the orientation of a VOR. Although the antennae are fixed, it is the relationship between the phase of the 'rotating' signal versus the omni reference signal that gives the radial. Changing the phase of the signal is undoubtedly trivial.

At the risk of thread drift, anyone ever used or seen used a transponder ILS approach ? This works on exactly the same principle, to generate a pseudo ILS, valid only for one aircraft. Are there any in actual daily use?

[Charley]

I think RoadRabbit has it 100% correct, the VOR radials are set in stone, or more precisely their aerials are set in concrete on the roof of the building.

You would be mistaken.

VOR's actually emit two radio signals, one is an omnidirectional master signal and the second is a directional, rotating azimuth signal. The phase of the azimuth signal changes as it rotates at 30Hz, such that the phase of the azimuth signal only matches the phase of the master signal when the two are aligned, i.e. at Station North. Hence the difference in phase between the signals is used by the receiving avionics to determine the radial from the station.

The master signal can and is changed periodically to bring Station North to broadly match Magnetic North over a coming period. That exact period changes according to national requirements; some VOR's in the US go for a long time without being adjusted, those in some European states are adjusted much more frequently.

It is important though to note that during that period, Station North will only exactly match Magnetic North for just part of that period. The difference between Station North and Magnetic North is the Station Declination and it is listed in the AIP. Please note that declination and variation are not the same thing.

Since a VOR receiver is only measuring the difference in phase betweem two signals, there is nothing about the system that is set in concrete and unchangeable. Engineers periodically 'adjust' station north by changing the phase outputs.

[patowalker]

Please note that declination and variation are not the same thing.

Are you sure? I thought it was deviation that sometimes gets confused with declination or variation.

[scifi]

Hi Charley, you say... 'The master signal can and is changed periodically to bring Station North to broadly match Magnetic North over a coming period.'


Looking at my old 1992 Pooleys, it gives the 314 39 radial for this example, so not much maintenance has occurred for 25 years. In this time the Magnetic Variation will have changed by 3.33 degrees, which could be the difference the OP has observed.
.

[Charley]

Hi Pato

In everyday aviation, declination and variation are used interchangeably (and notably with one term being more common than the other depending on which side of the Atlantic you find yourself on). Within the specific context of VOR's, deviation is the difference between system north and true north, variation is the difference between magnetic north at that geographic location and true north. The latter will change over time, the former will not unless manually adjusted at the station. AIUI.

Hi Scifly

Presumably then the VOR you refer to has never had an error so great that adjustment need be necessary. Bear in mind that an adjustment will have implications, in that IFR charts will need amending, any IAP's referencing that navaid will need amending, etc etc. It's relatively simple to adjust the station, it's all the stuff that need be done after (flight checking, amending AIP, docs, charts etc) which tends to result it in being done only when beyond a certain tolerance.

I have some source documentation for this but it's too large to upload so I did a Google search for some references instead. As it happens, the largest number of hits are from Pprune on occasions this has been asked before. More reading below for those who might wish follows:


An example of the effect (http://www.avionicswest.com/Tips/Tip17.html) and why the declination/error doesn't actually matter as long as you fly the radial selected (but will look wrong if you draw your leg on a map and measure mag track that way)
A similar discussion on Skyvector (https://skyvector.com/content/magnetic-course-numbers-few-degrees)
Other Pprune conversations (https://www.google.co.uk/search?q=vor+declination+site:www.pprune.org) where this has been explained before, so I will save the keyboard


As I said before, there are a few companies that do navaid calibration, but a few years back there was only really one in the UK, Flight Precision, callsign 'Calibrator'. I'm sure there are some ex-Calibrators kicking about these forums, perhaps they might chime in. ;)

Cheers all
Charley

[patowalker]

Thanks Charley.