VOR Radial confusion
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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
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
Airbus38.
Whoops !! I stand corrected. .....Lamberts Conformal Conic.
There's always something..............
Whoops !! I stand corrected. .....Lamberts Conformal Conic.
There's always something..............
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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.
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.
VRP (per Pooleys): N54 55.70 W006 55.50
DUNGV (per Aerad): N54 53.73 W006 47.40
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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.
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.
Last edited by roadrabbit; 8th Dec 2017 at 21:42. Reason: clarity
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.
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.
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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?
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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.
If the annual error resulting from variation change is affecting the accuracy of your navigation, you're doing really well.
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.
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.
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.
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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.
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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?
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?
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.
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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.
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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.
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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:
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
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 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
- Other Pprune conversations 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
Last edited by Charley; 12th Dec 2017 at 11:30.
