VOR Radials

bqmassey

bqmassey

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Brandon
Hello

I was reading through some ground school lessons online, and a question popped up that I haven't found an answer for. I'd appreciate if someone explain this to me a little bit.

I'm wondering if a VOR has 180 or 360 radials. If I'm flying with the needle centered while magnetic south of the station and flying magnetic north with the flag on "To", am I on the 180 radial or the 180-360 radial?

I think that there are 180 paired radials, but I'm not sure. If this is the case, though, couldn't there be some confusing as to which side of the VOR you're on? With needle centered, OBS on 180, and the flag saying "From", couldn't you either be north of the station flying north or south of the station flying south?

If you are flying on the needle, with the OBS matching your compass heading, and flag saying "To", and you pass over the VOR, what happens? Does the flag just flip? Does the needle fly to a side, since the radial is now on the opposite side of the VOR?

These are just some questions I have. I'd appreciate help with them, but I'll keep trying to find some answers.

Thanks!
Brandon
 
Brandon,

A VOR has 360 radials. It is the VOR head in your aircraft that figures out the to/from flag. Remember the radials are always from the VOR.

A VOR works with an omni directional pulse and a directional beam that rotates through 360[FONT=&quot]°.

Joe
[/FONT]
 
In theory, a VOR has an infinite number of radials -- with the right gear, you can determine your radial to quite a few decimal places. However, in practice, we consider it having 360 radials, all radiating from the center outward. Thus, with a "FROM" flag, you are always on the radial on which the OBS centers the CDI, regardless of the aircraft's actual heading.

Part of your confusion seems to stem from the popular misconception often observed at the beginning of learning VOR that the TO/FROM flag tells you whether you are flying to or from the station -- that isn't so. The VOR system in your plane has no idea which way the plane is pointed -- it only knows which radial it is on, or how far angularly off a given radial it is. Thus, if you are due north of the station, you can center the needle with either 180 or 360 on the OBS. If you set 180, it reads TO; if you set 360, it reads FROM. If we then put the airplane on a turntable and spun it around, the readings would not change.

The TO/FROM flag only tells you whether flying the course dialed into the CDI will take you closer to or farther from the station. However, if the flag says "TO," then you are on the radial which is reciprocal to the course dialed in. Thus, if you are west of the station, and dial in 090, you get a TO flag, which means if you flew a course of 090 from that point, it would take you TO the station -- but you are still on the 270 radial of that station. Note that this will occur even if you're headed north at the time -- again, the VOR doesn't know which way you're actually going, only where you are in relation to the station at that instant.
 
Brandon, Just wanted to thankyou for your post. Glad you asked the question and as you can see you got some good answers. This is what POA is all about.
 
bqmassey said:
I'm wondering if a VOR has 180 or 360 radials. If I'm flying with the needle centered while magnetic south of the station and flying magnetic north with the flag on "To", am I on the 180 radial or the 180-360 radial?
Click to expand...

If you are south of the station, you are on the 180 radial. Regardless of which direction you are flying, if you twist the OBS to 180 the flag will read From, if you twist to 360 it will read To.

There are 360 radials. The VOR receiver does not know which way the airplane is pointing. That is an important thing to understand. When I first started learning I didn't get exactly how reverse sensing worked either.

For example, if you're east of the VOR and you twist the OBS to 090, the needle will be centered and the flag will read From regardless of which direction you are flying. If you twist the OBS to 270, the needle will be centered and the flag will read To.

I think that there are 180 paired radials, but I'm not sure. If this is the case, though, couldn't there be some confusing as to which side of the VOR you're on? With needle centered, OBS on 180, and the flag saying "From", couldn't you either be north of the station flying north or south of the station flying south?
Click to expand...

Again, no part of the VOR system knows which direction you are flying. If the OBS is on 180, the needle is centered, and the flag says From, you are south of the station, period. It is up to YOU to determine whether you want the flag to say to or from. No matter where you are, if you twist the OBS 180 degrees from its original setting the flag will switch from To to From or vice versa.

I guess to answer your base question, there are 360 radials but the VOR receiver allows you to avoid reverse sensing when flying toward the station by reversing the needle's direction of travel when the flag reads "To."

Here's where reverse sensing comes into play. Let's say you're east of the VOR on the 090 radial, and you have the OBS twisted to 090, but you're flying toward the VOR. Because you have 090 on the OBS, the flag will read From (even though you are flying to the station). Say you drift a little bit north of your course (to the right, since you're flying west). The needle will also drift to the right. Why? Again, the VOR receiver does not know which direction you are flying. The flag says From. If you were in the same situation flying away from the station (just north of the 090 radial), the course would be to your right. So, the needle is on the right.

Now, since you're flying to the station, you really want the OBS to be twisted to 270. In that case, the flag will read "To" and the needle will indicate where the course is.

To avoid reverse sensing, you want the OBS to be roughly similar to your heading (The difference between the two is your wind correction angle). In this case, your heading will be roughly 270 (plus or minus the wind correction angle) so you'll want the OBS on 270 if you want to be on the 090 radial and flying toward the station (west).

Another trick you can use is to look at the sides of the nav head. If you have the OBS twisted to 090, North will be on the left side of the instrument and South will be on the right side. Let's say you are in the off-course situation described above, where you're flying to the station, you want to be on the 090 radial but you've drifted a tad to the right (north). Since the flag will read From but you're flying toward the station, the needle will be reverse sensing and will also go to the right. Since South is on the right side, the course is South of your position.

If you are flying on the needle, with the OBS matching your compass heading, and flag saying "To", and you pass over the VOR, what happens? Does the flag just flip? Does the needle fly to a side, since the radial is now on the opposite side of the VOR?
Click to expand...

Effectively, yes, the flag just flips.

In reality there is a "zone of confusion" over the VOR where the flag will indicate loss of signal and the needle may swing side to side, but that is a result of the physics of the signals involved and has nothing to do with understanding VOR navigation.

I hope I've helped and not over-explained. It's kind of tough to do without drawing pictures. I'd suggest you go to Tim's Air Navigation Simulator and play around with it, that should greatly increase your understanding.
 
flyingcheesehead said:
There are 360 radials.
Click to expand...

Good description Kent. Except there are an inifinite amount of radials on the VOR. The VOR facility transmits two signals at the same time. One signal is constant in all directions, while the other is rotated about the station. The airborne VOR receiver equipment receives both signals and looks electronically at the difference between the two signals, and interprets the result as a radial from/to the station. That means that all radials could be possible and there are an inifinite number between 0 and 360 degrees.

Since our CDIs are calibrated in 5 degree incriments we as pilots try to keep to the nearest whole degree. But we can easily get betwen those degrees with out non-detented CDI OBS.


 
smigaldi said:
Good description Kent. Except there are an inifinite amount of radials on the VOR.
Click to expand...

I thought I did plenty well enough confusing him already, thankyouverymuch. :D

Yes, there are an infinite number of radials, but the question was "are there 180 or 360" and conceptually, 360 is the correct answer of those two. (I'm sure that's on an FAA test somewhere :rolleyes:)

Welcome to PoA Brandon. ;) :yes: :goofy:
 
Ron Levy said:
If you set 180, it reads TO; if you set 360, it reads FROM. If we then put the airplane on a turntable and spun it around, the readings would not change...
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Ok, but what if we put the plane on a conveyor belt that would always exactly match the speed of the wheels. Would the VOR take off? hmm?
:D
 
bqmassey said:
Hello

I was reading through some ground school lessons online, and a question popped up that I haven't found an answer for. I'd appreciate if someone explain this to me a little bit.

<snip>

These are just some questions I have. I'd appreciate help with them, but I'll keep trying to find some answers.

Thanks!
Brandon
Click to expand...


Brandon,

You received some good answers here. The single best article that ever helped me "finally get it" was this:

http://www.campbells.org/Airplanes/VOR/vor.html

Its discussion of the history and theory behind the original instruments (that had faces colored into blue and yellow sectors) was also valuable in understanding how today's VOR systems work, and how to easily and quickly interpret what you're seeing. For example, I can tell at a glance now what heading I can turn to if I want to intercept the bearing to or from the station, without any mental math, simply by using the "picture" that the VOR head presents... all thanks to that article.

Hope it helps you too!
 
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Was basically asking the same question. Thanks for the help guys. This kinda stuff helps more than most.
 
bqmassey said:
Hello

I was reading through some ground school lessons online, and a question popped up that I haven't found an answer for. I'd appreciate if someone explain this to me a little bit.

I'm wondering if a VOR has 180 or 360 radials. If I'm flying with the needle centered while magnetic south of the station and flying magnetic north with the flag on "To", am I on the 180 radial or the 180-360 radial?

I think that there are 180 paired radials, but I'm not sure. If this is the case, though, couldn't there be some confusing as to which side of the VOR you're on? With needle centered, OBS on 180, and the flag saying "From", couldn't you either be north of the station flying north or south of the station flying south?

If you are flying on the needle, with the OBS matching your compass heading, and flag saying "To", and you pass over the VOR, what happens? Does the flag just flip? Does the needle fly to a side, since the radial is now on the opposite side of the VOR?

These are just some questions I have. I'd appreciate help with them, but I'll keep trying to find some answers.

Thanks!
Brandon
Click to expand...
A VOR is an entirely analog device. The radial is defined by the relative phase between two signals. There are an infinite number of radials.
 
Right. I got that part. Just how "Part of your confusion seems to stem from the popular misconception often observed at the beginning of learning VOR that the TO/FROM flag tells you whether you are flying to or from the station -- that isn't so." I struggled with that.
 
I think I've got it to where I can learn now though.
 
Jay_G. said:
Right. I got that part. Just how "Part of your confusion seems to stem from the popular misconception often observed at the beginning of learning VOR that the TO/FROM flag tells you whether you are flying to or from the station -- that isn't so." I struggled with that.
Click to expand...

Welcome to PoA where you will get answers to your questions and usually a lot of stuff you didn't ask for. The value of which varies.

You mentioned that you 'struggled with that'. Are you still struggling, or is it cleared up for you?
 
I'm just in need of practice.
 
Greg Bockelman said:
Stirring the pot on an old thread, Mark? LOL
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I'm betting he didn't notice the posting dates. 17+ years, but relevant necro. I'm impressed. I only noticed because Joe Areeda was the first respondent and I haven't seem him here in forever, so I checked the date.
 
I'd suggest that an NDB has radials as well. You just have to do more work to find which one you're on. ;)
 
dbahn said:
I'd suggest that an NDB has radials as well. You just have to do more work to find which one you're on. ;)
Click to expand...
Actually, it has bearings and they move with changes in the local magnetic variation over time, so using the magnetic bearing from 20 years ago, might not fly over the same dirt, whereas with a VOR radial, the radial you set in the OBS and the CDI if it is centered, will go over the same dirt, often even if the VOR was installed 60 years ago unless the VOR was re-aligned.
 
I think we're saying the same thing. Wherever the NDB is you try to fly on a radial to or from that station. The bearing relative to magnetic north may change, but the bearing to the station represents a line from point A to point B, e.g. an initial approach fix and an on-field NDB. Unless either of those change their location, the course over the ground between them remains the same even with a change in magnetic variation.
 
EdFred said:
I'm betting he didn't notice the posting dates. 17+ years, but relevant necro. I'm impressed. I only noticed because Joe Areeda was the first respondent and I haven't seem him here in forever, so I checked the date.
Click to expand...
I typically don’t notice posting dates either. I was just tweaking Mark a little.

I have absolutely no issues with necro-posts. I don’t know why people make such a big deal about it.
 
EdFred said:
I'm betting he didn't notice the posting dates. 17+ years, but relevant necro. I'm impressed. I only noticed because Joe Areeda was the first respondent and I haven't seem him here in forever, so I checked the date.
Click to expand...
I'm not responsible for renewing the thread but I did manage to reply to a very old post :D
 
VOR necro's should be particularly good, as they are answered by people who flew the VOR system, and understand it.

They likely took their check ride in an aircraft that only had a VOR installed.
 
geezer said:
VOR necro's should be particularly good, as they are answered by people who flew the VOR system, and understand it.

They likely took their check ride in an aircraft that only had a VOR installed.
Click to expand...
I think that, in this case, the OP was actually one of the first to fly the VOR system and the responses came from people flying with prototype equipment to R&D the first generation of VOR receivers.
 
My check ride was 1969, with a second generation transistorized VOR COM, which replaced a "Coffee grinder" crank to tune VOR, and 3 crystals in the COM side. That was a long time before 2006, of the original post.
 
geezer said:
My check ride was 1969, with a second generation transistorized VOR COM, which replaced a "Coffee grinder" crank to tune VOR, and 3 crystals in the COM side. That was a long time before 2006, of the original post.
Click to expand...
Did they have jokes in 1969, or was the world too serious for such frivolity? ;)
 
Jay_G. said:
I'm just in need of practice.
Click to expand...
Try using the navigation simulators at https://www.luizmonteiro.com/Learning_VOR_Sim.aspx Very good tools for learning VOR and many other flight and navigation instruments.

dbahn said:
I think we're saying the same thing. Wherever the NDB is you try to fly on a radial to or from that station.
Click to expand...
A VOR radial doesn't move. As the area's magnetic variation shifts, a particular radial is still in the same physical place but it may no longer be the same magnetic bearing from the station. Eventually, the VOR station will have to be realigned.

An NDB does not broadcast any information regarding bearings. You derive that by combining your relative bearing to the station with your magnetic heading. As magnetic variation shifts, the same magnetic bearing to a station will move in relation to the ground.

geezer said:
They likely took their check ride in an aircraft that only had a VOR installed.
Click to expand...
My initial instrument check ride was in 1985 in a 1960 C-310E which was equipped with two VOR receivers, two CDI displays, and one ADF. Didn't even have DME. Didn't even know I was missing out.
 
Regarding reverse sensing, here's a way to think about it differently. If the needle is not centered, then it's swinging left or right. If you look at the radial numbers around the dial, following any of them on the same side as the needle, will take you to radial that is dialed in at the top. "Follow any of them" means turn so that your course matches that number. It doesn't matter whether the flag says TO or FROM.

Put differently: dial in the radial you want to be on, at the top. Then turn to any heading that is on the same side of the dial as the needle. Of course, listen to the station ID first so you don't fly out to Timbuktu following a VOR that is out of service.
 
Larry in TN said:
A VOR radial doesn't move. As the area's magnetic variation shifts, a particular radial is still in the same physical place but it may no longer be the same magnetic bearing from the station. Eventually, the VOR station will have to be realigned.

An NDB does not broadcast any information regarding bearings. You derive that by combining your relative bearing to the station with your magnetic heading. As magnetic variation shifts, the same magnetic bearing to a station will move in relation to the ground.
Click to expand...
What I'm trying to say is that the goal of a NDB approach is to fly a direct line from point A to point B, and that line is a radial from the station (point B) to point A. So in the approach shown below you are flying inbound on the 204 degree radial and outbound on the 024 degree radial to arrive over the airfield. When the magnetic field changes, you still fly that same path over the land but the adjustment comes in your relative bearing, which also has to factor in wind correction.

NDB.jpg
 
dbahn said:
What I'm trying to say is that the goal of a NDB approach is to fly a direct line from point A to point B, and that line is a radial from the station (point B) to point A.
Click to expand...
VORs have radials, NDBs have bearings. It might sound like a meaningless distinction but it is not for the reasons I stated above.

The NDB bearing-to is dependent on your heading and relative bearing. A radial is fixed in space (until the VOR is realigned).

If there's actually anyone left in the world trying to learn NDB navigation, they'll need to understand the difference. Interchanging the terms will only cause confusion.
 
MRC01 said:
Regarding reverse sensing, here's a way to think about it differently. If the needle is not centered, then it's swinging left or right. If you look at the radial numbers around the dial, following any of them on the same side as the needle, will take you to radial that is dialed in at the top. "Follow any of them" means turn so that your course matches that number. It doesn't matter whether the flag says TO or FROM.

Put differently: dial in the radial you want to be on, at the top. Then turn to any heading that is on the same side of the dial as the needle. Of course, listen to the station ID first so you don't fly out to Timbuktu following a VOR that is out of service.
Click to expand...
I don't think that is correct. If you are going to intercept and fly outbound from the VOR on the Radial then yes, you put it at the top. If you are going to intercept and fly inbound to the VOR you want the reciprocal at the top. Easiest way to do that is put the Radial at the bottom. Don't require no math
 
luvflyin said:
I don't think that is correct. If you are going to intercept and fly outbound from the VOR on the Radial then yes, you put it at the top. If you are going to intercept and fly inbound to the VOR you want the reciprocal at the top. Easiest way to do that is put the Radial at the bottom. Don't require no math
Click to expand...
It is correct. It doesn't matter which radial you put on top. For example, 45 or 225. One way, the needle swings left, the other way, it swings right, but in both cases, the headings you need to turn to are on the same side the needle swings. This method eliminates the bugaboo of "reverse sensing" and it doesn't matter whether the CDI says "to" or "from".
 
MRC01 said:
It is correct. It doesn't matter which radial you put on top. For example, 45 or 225. One way, the needle swings left, the other way, it swings right, but in both cases, the headings you need to turn to are on the same side the needle swings.
Click to expand...
If that's what your gonna do then remember to fly away from the needle instead of to it if you are going to be flying To the VOR and you put the Radial at the top
 
luvflyin said:
If that's what your gonna do then remember to fly away from the needle instead of to it if you are going to be flying To the VOR and you put the Radial at the top
Click to expand...
It's not about "away" or "toward". It's not about "left" or "right". It's about fly a heading on the side the needle swings toward. Think of it as, the needle is pointing toward a heading you need to fly to intercept whatever is at the top of the dial.
 
Larry in TN said:
If there's actually anyone left in the world trying to learn NDB navigation, they'll need to understand the difference.
Click to expand...
I'm not worried about that. :)

Nobody will ever use ADFs again since GPS is dominant, but I always thought that NDB approaches were a great test of situational awareness, and a perfectly flown NDB approach will constantly have the aircraft on "radial" from the station for the entire approach. Radials are just like imaginary spokes on a wheel, with the station as a hub. It's identical to a VOR station in that respect. You're either on the radial or you're not. True, the terminology is a little different but the concept is the same.

I hate to quote myself, but this is all I said:
dbahn said:
I'd suggest that an NDB has radials as well. You just have to do more work to find which one you're on. ;)
Click to expand...
 
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