VOR cone of confusion

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John777

Pre-takeoff checklist
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I am preparing for my instrument checkride and I am still not sure why cone of confusion forms over the VOR.

Can anyone give me a brief explanation?

Thanks.
John.
 
If you were directly over the VOR, are you flying toward it or away from it?

What people often label as zone of confusion when crossing a VOR is that the pointer goes full scale. If you're 10 feet from a VOR and pass it abeam, you will be more than 6 deg offset. Draw a picture if that isn't obvious.
 
Let us say that I am Flying aWay from it
 
all I know about it is that, the plane is picking up multiple radials and becoming unreliable.
 
More technically, VOR are implemented with a ring of directional antennas (used to be a rotating antenna) paired with an omnidirectional antenna. The phase difference between the signals received from the two broadcasts tells you what radial you're on.

The problem is that directional isn't perfectly directional. Directly above the VOR in a cone, the signals from the directional antennas blend together because this is where they overlap. In this area, the measured phase difference is unpredictable and weak. So, you get poor registration on the CDI.
 
If you are directly above a VOR, a moment ago, you were approaching it, and a moment from now, you'll be flying away from it. Right now, it's not determined.

At the same time, you have moved the needle full scale one way or another, since it's only measuring a +/- 6 deg cone in front and behind. If you're a microscopic amount left, it will read full scale right. A microscopic amount right, and it's full scale left. The zone of confusion is because all this changes very, very rapidly over a small area.

The technical name for this is a "gimbal lock," and there are several analogies. Like, defining bearing to an object at zenith.
 
arnoha said:
More technically, VOR are implemented with a ring of directional antennas (used to be a rotating antenna) paired with an omnidirectional antenna. The phase difference between the signals received from the two broadcasts tells you what radial you're on.

The problem is that directional isn't perfectly directional. Directly above the VOR in a cone, the signals from the directional antennas blend together because this is where they overlap. In this area, the measured phase difference is unpredictable and weak. So, you get poor registration on the CDI.
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Thank you sir. I asked on this discussion board because I do not see any explanation on IFH or other books. If you could post the good reference source, that would be great.
 
John777 said:
Thank you sir. I asked on this discussion board because I do not see any explanation on IFH or other books. If you could post the good reference source, that would be great.
Click to expand...

I don't have one. The Wikipedia article on VORs is pretty good at describing the operation, but it doesn't really do justice to the cone of confusion issue other than to say it exists. What's IFH?

Basically, the directional antenna cannot be perfect. For one, it's physically impossible to produce such a thing. Antennas always leak to some degree in all directions. For another, a truly perfect directional antenna would be useless. You need continuous coverage for all 360 degrees. You can't have an infinite number of antennas, so each antenna pattern must have some overlap with the one beside it. In fact, modern ones reduce the number of needed antennas by having very well designed overlap. If enough overlap happens (cone of confusion), the CDI is unable to report anything intelligent.
 
Do you ask your CFII any of these questions? In any event, you are overthinking your check ride prep. Relax and focus on the fundamentals. You'll have plenty of time after you get your ticket to learn stump the dummy stuff to amaze your friends and confound your enemies.
 
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John777 said:
I am preparing for my instrument checkride and I am still not sure why cone of confusion forms over the VOR.

Can anyone give me a brief explanation?

Thanks.
John.
Click to expand...

I guess the other question is...has anyone ever been asked on an instrument check ride; "why?"

I wasn't. Rather I was only asked to explain what it was, to which I responded; "the area right above a VOR where your CDI goes apesh** until you come out the other side."

That satisfied my check ride guy! ;)
 
"Since electromagnetic radiation is dipole radiation, it is not possible to build an antenna that radiates coherently equally in all directions, although such a hypothetical isotropic antenna is used as a reference to calculate antenna gain.

The simplest antennas, monopole and dipole antennas, consist of one or two straight metal rods along a common axis. These axially symmetric antennas have radiation patterns with a similar symmetry, calledomnidirectional patterns; they radiate equal power in all directions perpendicular to the antenna, with the power varying only with the angle to the axis, dropping off to zero on the antenna's axis. This illustrates the general principle that if the shape of an antenna is symmetrical, its radiation pattern will have the same symmetry.

In most antennas, the radiation from the different parts of the antenna interferes at some angles. This results in zero radiation at certain angles where the radio waves from the different parts arrive out of phase, and local maxima of radiation at other angles where the radio waves arrive in phase. Therefore the radiation plot of most antennas shows a pattern of maxima called "lobes" at various angles, separated by "nulls" at which the radiation goes to zero.


A rectangular radiation plot, an alternative presentation method to a polar plot.
The larger the antenna is compared to a wavelength, the more lobes there will be. In a directive antenna in which the objective is to direct the radio waves in one particular direction, the lobe in that direction is larger than the others; this is called the "main lobe". The axis of maximum radiation, passing through the center of the main lobe, is called the "beam axis" or boresight axis". In some antennas, such as split-beam antennas, there may exist more than one major lobe. A minor lobe is any lobe except a major lobe.

The other lobes, representing unwanted radiation in other directions, are called "side lobes". The side lobe in the opposite direction (180°) from the main lobe is called the "back lobe". Usually it refers to a minor lobe that occupies the hemisphere in a direction opposite to that of the major (main) lobe.

Minor lobes usually represent radiation in undesired directions, and they should be minimized. Side lobes are normally the largest of the minor lobes. The level of minor lobes is usually expressed as a ratio of the power density in the lobe in question to that of the major lobe. This ratio is often termed the side lobe ratio or side lobe level. Side lobe levels of −20 dB or smaller are usually not desirable in many applications. Attainment of a side lobe level smaller than −30 dB usually requires very careful design and construction. In most radar systems, for example, low side lobe ratios are very important to minimize false target indications through the side lobes."
 
I just had a really bad flashback from my Electromagnetic Fields and Waves class in college
 
The farther you are from the VOR, the more stable your CDI will be. The closer you approach to it the less stable the CDI will be making it more sensitive, since the VOR transmits laterally and not vertically. When you're diectly overhead the CDI receives mixed signals and is therefore erroneous.
 
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MAKG1 said:
The technical name for this is a "gimbal lock," and there are several analogies. Like, defining bearing to an object at zenith.
Click to expand...
That's certainly NOT the technical term for the VOR behavior (in fact, cone of confusion is correct). There are no gimbals to lock in a VOR system. Gimbal lock is a degenerate case with multidimensional rotation where you end up with two of what normally would have been orthogonal rotations in the same plane.
 
flyingron said:
That's certainly NOT the technical term for the VOR behavior (in fact, cone of confusion is correct). There are no gimbals to lock in a VOR system. Gimbal lock is a degenerate case with multidimensional rotation where you end up with two of what normally would have been orthogonal rotations in the same plane.
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There are no gimbals to lock in an AHRS either, but a PFD will exhibit gimbal lock behavior as well.

To/from is ill defined at crossing, as is the bearing to the station (and hence the course deviation). It's a conventional gimbal lock. Even with a perfect antenna, there will still be a zone of confusion because of that. The station has a bearing, elevation, and rotation, like any target on the ground. The bearing and rotation are degenerate at nadir.

Virtually all navigation, guidance and tracking problems exhibit some form of gimbal lock somewhere in parameter space, when expressed as angles.
 
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It's not any sort of gimbal lock. Gimbal lock requires multiplane rotation (even if not physical gimbals) to occur. You can colloquially call any navigational failure a gimbal lock, but that's incorrect (and certainly not as you asserted the "TECHNICAL TERM" for it).

The cone of confusion is not unlike flying your whiskey compass very close to the magnetic pole. It just exceeds the system's error tolerances to respond appropriately.
 
flyingron said:
It's not any sort of gimbal lock. Gimbal lock requires multiplane rotation (even if not physical gimbals) to occur. You can colloquially call any navigational failure a gimbal lock, but that's incorrect (and certainly not as you asserted the "TECHNICAL TERM" for it).

The cone of confusion is not unlike flying your whiskey compass very close to the magnetic pole. It just exceeds the system's error tolerances to respond appropriately.
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Yes, and the "multiple rotations" are bearing, elevation, and rotation.

The magnetic pole has a gimbal lock as well. It is a generic condition of spherical polar coordinates with a position angle, and there are dozens of examples. It's very limiting -- and wrong -- to limit it to yaw, pitch and roll.

Some of us do this sort of stuff for a living.
 
Some of us do, and we understand the terms. Yes, it applies to any three rotations about a point, I never claimed it was limited to yaw, pitch, and roll. But you're still wrong. There's only ONE rotation in VOR. A VOR doesn't have rotations other than parallel to the earth. Getting direcdtly over the axis of that rotation is not "gimbal lock", Neitehr is getting right over the pole with a compass.

Next you'll be calling precession gimbal lock.
 
John777 said:
I am preparing for my instrument checkride and I am still not sure why cone of confusion forms over the VOR.

Can anyone give me a brief explanation?

Thanks.
John.
Click to expand...


If you're already confused, wouldn't that mean you understand the confusion?
 
bobmrg said:
"Since electromagnetic radiation is dipole radiation, it is not possible to build an antenna that radiates coherently equally in all directions, although such a hypothetical isotropic antenna is used as a reference to calculate antenna gain.

The simplest antennas, monopole and dipole antennas, consist of one or two straight metal rods along a common axis. These axially symmetric antennas have radiation patterns with a similar symmetry, calledomnidirectional patterns; they radiate equal power in all directions perpendicular to the antenna, with the power varying only with the angle to the axis, dropping off to zero on the antenna's axis. This illustrates the general principle that if the shape of an antenna is symmetrical, its radiation pattern will have the same symmetry.

In most antennas, the radiation from the different parts of the antenna interferes at some angles. This results in zero radiation at certain angles where the radio waves from the different parts arrive out of phase, and local maxima of radiation at other angles where the radio waves arrive in phase. Therefore the radiation plot of most antennas shows a pattern of maxima called "lobes" at various angles, separated by "nulls" at which the radiation goes to zero.


A rectangular radiation plot, an alternative presentation method to a polar plot.
The larger the antenna is compared to a wavelength, the more lobes there will be. In a directive antenna in which the objective is to direct the radio waves in one particular direction, the lobe in that direction is larger than the others; this is called the "main lobe". The axis of maximum radiation, passing through the center of the main lobe, is called the "beam axis" or boresight axis". In some antennas, such as split-beam antennas, there may exist more than one major lobe. A minor lobe is any lobe except a major lobe.

The other lobes, representing unwanted radiation in other directions, are called "side lobes". The side lobe in the opposite direction (180°) from the main lobe is called the "back lobe". Usually it refers to a minor lobe that occupies the hemisphere in a direction opposite to that of the major (main) lobe.

Minor lobes usually represent radiation in undesired directions, and they should be minimized. Side lobes are normally the largest of the minor lobes. The level of minor lobes is usually expressed as a ratio of the power density in the lobe in question to that of the major lobe. This ratio is often termed the side lobe ratio or side lobe level. Side lobe levels of −20 dB or smaller are usually not desirable in many applications. Attainment of a side lobe level smaller than −30 dB usually requires very careful design and construction. In most radar systems, for example, low side lobe ratios are very important to minimize false target indications through the side lobes."
Click to expand...

Yeah, that. That is basically what I was trying to give a super-simplified explanation for. The actual cone of confusion is a result of the combination of several overlapping antenna patterns.
 
flyingron said:
Some of us do, and we understand the terms. Yes, it applies to any three rotations about a point, I never claimed it was limited to yaw, pitch, and roll. But you're still wrong. There's only ONE rotation in VOR. A VOR doesn't have rotations other than parallel to the earth. Getting direcdtly over the axis of that rotation is not "gimbal lock", Neitehr is getting right over the pole with a compass.

Next you'll be calling precession gimbal lock.
Click to expand...
You don't understand how three dimensional navigation works. Elevation angle is important to the instrumentation even if it doesn't display it. Yes, it it a gimbal lock, and precession isn't.

It's people who don't understand these things that have kept me employed for years, as I fix the messes they make. Keep firing away...

A PERFECT antenna will still generate an ambiguous reading at zenith. Or do you dispute that as well?
 
John777 said:
I am preparing for my instrument checkride and I am still not sure why cone of confusion forms over the VOR.

Can anyone give me a brief explanation?

Thanks.
John.
Click to expand...
Better question: What are you supposed to do when you enter the cone of confusion?
 
It would be really cool if there was something on the indicator that told you if you were going to or from a VOR. At what point do you enter a new VOR destination?
 
GlennAB1 said:
It would be really cool if there was something on the indicator that told you if you were going to or from a VOR. At what point do you enter a new VOR destination?
Click to expand...

You mean like that To/From flag? Also the point at which you change to a new VOR is marked right on an IFR chart, and if it's not it's the midpoint between the 2 VORs.
 
Why it happens is a matter of physics, and some folks threw some pretty detailed explanations out.

The VOR's are broadcasting a signal out horizontally away from the transmitter site. Just like the airport's lighted beacon emits visible light away from the rotating beacon site. When you are directly overhead of the VOR, the indications received by your instrument are unreliable enough for it to resolve a radial or To/From indication. Just like when you are DIRECTLY over the beacon site you might have trouble discerning what color the light is, or which way its pointing.

From a practical, test and checkride standpoint, what is more important is what you do when enter (and identify being in) the zone/cone of confusion, which is usually "nothing" if your plan is to maintain course, or turn to your new heading (using the gyrocompass) if your plan changes course with station passage. Once you get out of the zone of confusion you can center up on the desired radial.
 
StinkBug said:
You mean like that To/From flag? Also the point at which you change to a new VOR is marked right on an IFR chart, and if it's not it's the midpoint between the 2 VORs.
Click to expand...

I'm really hoping he/she was being sarcastic...
 
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