HELP WITH VOR !!
tiffa...@hotmail.com
After fininshing gound school and about 30 hours total, I can't seem to
understand how to use a VOR...I read it over and over again, had my
instructor explain, but I still am not sure..
Are there any instructors that could explain it to me one last time.
Thank you all
Tiffany Crow
less...@aol.com
Well, let's see. We know you are a 30 hour student pilot and you do not know
how to use a VOR. Sounds like you are right on track to me. Seriously, when
I was a 30 hour student pilot I too did not know how to use a VOR. The
difference between you and me is I was not smart enough to know I did not
know how to use a VOR. I had to come to this realization the hard way.
I will take a stab at explaining the VOR. I have a student that is currently
having the same difficulty. This will give me a chance to think through my
explanation. At least if my explanation does not help you understand the VOR
any better, I will not feel guilty for taking your money and not teaching you
anything. ;)
Let's start by dividing the discussion into two sections. Theory and
Application. From your post, it sounds like you are having problems with the
application of the VOR. The "how to use it" type of information. But since
I am not really sure, let's start from the beginning with a little verbiage
about the theory of the VOR.
I will assume that you are able to identify VOR's on the sectional chart.
You know what they look like. You know how to find the frequency of a given
VOR. You understand that the compass rose (blue circle drawn around the VOR
station with compass indications on it) of the VOR is aligned with magnetic
North.
Theory: With respect to this discussion, theory refers to how the VOR is
supposed to work and what information it will and will not give us. I will
not go into the design theory of the VOR transmitter or receiver.
Get out an old sectional chart. One that you do not mind drawing on. Locate
your favorite VOR on the sectional chart. Any VOR will do, but the less
congestion on the chart in the area of the VOR the better. Now use your
plotter to draw a strait line through the VOR station. The direction of the
line is not important at this point, but try to draw it on a nice readable
compass heading. Let's say you drew the line through 300 on the compass
rose, through the station, and through 120 on the other side of the compass
rose. Be sure the line extends six to eight inches either side of the VOR
station. The length of the line is not really important either, but it makes
the line easier to see.
The VOR's basic function is to tell you that you are somewhere on a line.
The celestial navigation term for this line is Line Of Position (LOP). I was
a sailor for many years, old habits die hard. The LOP tells us that our
position is somewhere on the line, but at this point we know not where. When
the needle on the OBS is centered, we know we are on a line that goes through
the center of the VOR station and through what ever compass heading is under
the pointer at the top of the OBS head. Let's say the OBS is indicating 300.
At this point we do not know which half of this line we are on. That is,
are we on the 300 side of the station or are we on the 120 side of the
station. This is where the "To/From" flag comes in. The To/From indication
is with respect to the heading under the pointer at the top of the OBS. This
is an important point, so let me try to phrase it in another way. The
"To/From" indicator tells us which direction we would be traveling with
respect to the VOR station if we flew a magnetic course of whatever the OBS
is indicating. If the OBS reads 300 and there is a "To" indication, then the
airplane is somewhere on our LOP that would allow us to fly TO the VOR
station if we flew a magnetic course of 300 degrees. OK, now look at the
line on your sectional. Where would we be if we were flying a magnetic
course of 300 along that line AND heading TO the VOR station? Right, we
would be on the South East side of the station.
If the OBS reads 300 and there is a "From" indication, we would be on the
North West side of the station (on your line, moving along a magnetic course
of 300, traveling FROM the station).
Up until now we have assumed the needle was always centered. What is the
needle telling you if it starts to move one way or the other? The needle is
telling us the LOP is in the direction it is deflecting. If the needle moves
right, our LOP is to the right if we are flying the heading indicated by the
OBS. If the OBS indicates 300 degrees, the DG or magnetic compass indicates
we are indeed heading 300 degrees, and the needle starts moving to the left,
then we need to turn to the left slightly to fly back to our LOP.
Notice I stipulated the needle indication with "if we are flying the heading
indicated by the OBS." Does is make a difference? Well for the sake of this
discussion, let's say we are now flying a helicopter. We are hovering just
above our LOP South East of the VOR. We dial in 300 degrees in the OBS. The
needle centers and we get a "To" indication. We glance at out superbly
synchronized DG and see that we are indeed heading 300 degrees. While
maintaining our hover, we turn the helicopter around to a heading of 120
degrees and observe the OBS. It does not change. You see it does not matter
which direction the aircraft is pointing. So what does matter? Let's turn
the helicopter back around to a heading of 300 degrees. Now let's fly the
helicopter one mile off of the LOP to the North East. The needle is now
deflected to the left. The OBS indicates 300 & "To", and the DG indicates
300 degrees. This tells us LOP is to the left. Now let's hold this position
but turn the helicopter around to 120 degrees. The needle is still deflected
to the left, but the LOP is now on our right side. So you see what matters
is if we are trying to get back to the LOP, the needle indicates the
direction to turn when the aircraft is heading toward the compass indication
in the OBS. If the aircraft is heading away from the compass indication in
the OBS, the needle is said to be "reverse sensing."
Until now we have only known what side of the VOR station we were on. How do
we use our VOR to find our position? We could find our LOP as above and then
look out the window to see if we can identify anything on the ground that
appears along our LOP. Another way is to use triangulation. That is, locate
your LOP on one VOR and then do the same with another nearby VOR. Where the
two line cross is your position.
Application: Flying along near XYZ VOR, look on the sectional for the
frequency. Enter the frequency into the VOR receiver. Turn up the volume of
the VOR receiver to 'Ident" the station. Use the Morse code listed on the
chart to confirm you have indeed tuned the correct station. Turn the OBS
until the needle is centered and you have a FROM flag. Read the OBS heading.
Draw a line on your sectional, mentally or with a pencil, from the station
out through the heading on the compass rose. You are somewhere out on that
line.
Now let's assume you wish to fly to XYZ VOR. Tune and ident the VOR. Turn
the OBS until the needle is centered and you have a "To" flag. Read the OBS
heading. Turn the airplane to this magnetic heading. Our magnetic heading
and our OBS heading are roughly the same, thus the needle will indicate which
way to correct our course as we fly along toward the VOR. We will know we
are at the VOR when the flag changes to "From."
I have left out several of the finer points of using the VOR, but for now
understanding the basic use of the VOR is more important. I hope this helps.
--
Les Sparks
less...@aol.com
http://members.aol.com/woodglider/
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Sam Trask
my advice with a grain of salt. That said, in addition to this newsgroup,
you might want to try rec.aviation.ifr. Also, I'm reading an awesome book
right now called "Van Sickles Modern Airmanship" which I found at my local
library. Very informative, explains everything from the physics of flight
to flying supersonic aircraft. Hope that helps!
Sam
http://magma.mines.edu/students/s/strask
Roger Halstead
N833R World's oldest Debonair? S# CD-2
http://members.tm.net/rdhalste
tiffa...@hotmail.com wrote in message ...
>Hi People,
>
>After fininshing gound school and about 30 hours total, I can't seem to
>understand how to use a VOR...I read it over and over again, had my
>instructor explain, but I still am not sure..
I'm not an instructor, but I have been using VORs for many years so possibly
I may be able to help.
First I have a couple of questions and then a short explanation.
Do you have a problem with the VOR concept, or is it in a particular area?
As to a simplified explanation of VORs:
A VOR is simply a receiver that *knows* where it is located as far as
direction from a particular radio station.
So... without going into the "How it works", just think of having a receiver
that lets you turn a dial that will tell you which heading to fly *TO* and
VOR, or what Heading you would fly to go straight *FROM* the radio station.
The only difference is when you turn the knob on the indicator to center the
needle whether you get a "TO", or "FROM" flag in the window of the
indicator.
Draw a small circle in the center of a piece of paper. Then draw lines for
North, South, East, and West. You know that N is 360, E is 90, S is 180, and
W is 270. These are called radials and indicate the direction FROM the radio
station. You could draw in a line for each degree if you wanted but the
four should be enough.
Now put a circle on one of the lines. It doesn't matter which one. Think
of that circle as being your airplane. If you were in that airplane and
turned the VOR indicator until it was centered It would either indicate the
direction TO or FROM the station. Think of that information as being
contained in the radio signal being received. That being the case the
direction the aircraft is pointed has no impact on the indication. It only
indicates where you are in relation to the transmitting radio station.
Now lets say you you are on the 090 line (which is East) and you want to fly
straight away from the station. The line and the heading are 090 away from
the station. So when the needle is centered with the FROM flag showing the
indicator should have 090 at the top. If you drift to the right of the
course say to 092 the needle will move to the LEFT indicating YOU have to
turn a bit to the LEFT to get back on course.
They say fly TO the needle.
Now if you want to go to the station and you already know that you fly a
heading of 090 to go away then you have to make a 180 degree turn to go West
TO the station. That will be a heading of 270 degrees ( you should already
know that West is 270 degrees and a 180 degree turn 090 + 180 just happens
to equal 270) and with the needle centered in the VOR indicator you will see
270 at the top with the TO flag showing.
As in when you were flying away from the station if you drift to the RIGHT
the needle will move to the LEFT indicating you have to turn a bit to the
left to get back on course. You can think of it as flying TOWARD the needle
to get back on course.
What happens if you get the indicator set backwards (which is what most
often confuses students)? Well remember you have it set to indicate TO when
you are going AWAY/FROM the station or FROM when you are going TO the
station. Think. You have it set backwards so it will indicate backwards.
When it is set backwards and you drift to the right the indication is
backwards to what it would have been. So... the needle drifts RIGHT as well.
That means if every thing else is backwards you do the opposit of what you
would do with it set properly. The needle goes RIGHT when YOU drift RIGHT
so you trun a to the LEFT, AWAY FROM the needle to get back on course. The
proper term is reverse sensing at this point.
Just think of a VOR in the simplest terms (it knows which way to the
station, it knows not which way you are pointed.). Once you remember how to
set the indicator, and which way to fly to get on course THEN it becomes
very simple. Then you can start dealing with those questions about course
headings and possibly intercept courses, or calculating how far you are from
the station by how long it takes to get from one radial to another. Just
learn the simple stuff first. Take it one step at a time.
I hope this is of some help.
Roger (Nitpickers I did say this was a simplified explanation)
Snowbird
> After fininshing gound school and about 30 hours total, I can't seem
> to understand how to use a VOR (....)
> Are there any instructors that could explain it to me one last time.
I'm not an instructor, but I'll try to share a few of my favorite
tips if you like. I'm gonna assume you've had your head stuffed
with details like principles of operation and how to tune and ident
and what VORs look like on a sectional chart so I'm going to try
to lay out the Big Picture.
My favorite general description is in Rod Machado's Private Pilot
Manual. I have to keep buying more copies so I can reread it,
because friends who are interested in flying keep swiping mine.
Give it a try if you haven't, you might find it makes more sense
to you.
First principle. A VOR is simply designed to tell you where
you are relative to the VOR (but not, of itself, how far away).
Nothing more, nothing less. It doesn't care where the nose of
your plane is pointed, it doesn't care which direction you're
flying, it just tells you very specifically where you are
relative to the VOR. In which direction someone seated on the
VOR would point at your plane.
Second principle. You can use that information to guide you,
quite accurately, either towards the VOR, or to a specific
location away from the VOR or relative to the VOR.
Third principle. Same Side Safe. Read on for explanation.
Howtadoit. First off, next time you're flying, tune in and
ident a VOR. Turn the knob of the OBS until 0 is at the top.
Now. How do you tell where you are relative to the station?
The flag will either say "to" (on the top) or "from" (on the
bottom). The needle will either point right or left. Brainless
approach: look at the headings opposite the needle and opposite
the flag. ie if the needle is to the left and the flag is "from",
look at numbers in the top right quadrant. That's where you are,
NE of the station. If the needle is to the right and the flag
is "To", look at numbers in the bottom left quadrant. You're
SW of the VOR.
OK, if you're NE of the VOR and you wanna fly towards it,
which way are you gonna fly? Forget it's a VOR and answer as
if you were being asked how to get to your home airport if
you're NE of it.
You fly SW, right? So start turning the OBS until SW headings
are at the top. The needle will center and the flag flip to "To"
on one of the numbers. Turn to that heading, and fly. If the needle
drifts off to the left, turn left slightly until it stops moving (the
difference between the heading where it stops, and the number at the top
of the dial is the approximate wind correction you'll need to hold
to stay on course). Now turn left a little more until the needle
starts moving back toward the center. When it centers, turn
to your wind correction heading. If the needle moves right, turn
to the right. As you approach the VOR, the needle will become
very sensitive. Just hold course. As you cross the VOR, the
flag will flip to "From". Voila, you've just tracked a VOR--
flown a straight line from your current location to the little
beast. Draw lines from VOR to VOR like that and next thing you've
flown from Chicago to Miami without needing to look out the window.
Pretty cinchy. What's the catch? The catch is, for every
location about the VOR, there are two numbers which will
center the needle. One is the heading you would put at the top
of your DG to fly towards the VOR or the direction you'd hold
out your arm to point at the VOR. This is called your 'bearing'
to the station. When that's dialed in, the "To" flag shows.
The other is the direction someone sitting on the VOR would
point (or fly) in order to locate your plane. This is called
your 'radial' from the station. When that's dialed in, the
"From" flag shows. If you put that number at the top of your
DG and fly in that direction, keeping the needle centered with
small course corrections, you will be flying directly away
from the VOR.
The hassle arises if you're trying to fly *towards* the station,
but you have a radial dialed in. Then attempts to center the
needle by flying towards it will have the opposite effect of
putting you off course. How to guard against this? Well, first
off, start by figuring out where you are relative to the VOR
and which way you logically want to fly to get there. If you
determine (using the brainless approach described above) that
you're NE and you need to fly SW to get there, you better have
a SW heading on both your DG and at the top of your OBS. And
you're going TO the station, so the flag should say TO.
If your intent is to fly directly away from the VOR and you
are NE, then you better have a NE heading on your DG, a NE
heading at the top of the OBS, and since you're flying FROM
the station, the flag should say "FROM".
Lastly, use the "same side safe" rule to check course corrections.
If you turn towards the needle, your DG should show a heading
which is on the OBS on the "same side" as the needle.
Follow these rules, and you'll never get messed up with this
'reverse sensing' stuff (or you'll catch it straight off).
OK, what else is a VOR good for? Well, we've determined
that you can locate your position FROM the station -- the
specific direction someone sitting on the VOR would point
a laser to target your plane. But that doesn't tell you
exactly where you are, it just lets you draw a line from
the station and say "I'm somewhere on this line". Now, what
if you repeat the same exercise with a second VOR? Two
lines, they intersect somewhere, and you're at the intersection.
Remember, you want your position FROM the VOR for this, so the
flag has to say FROM.
Suppose you can only receive one VOR? Well, the distance
between radials varies depending on how far you are from the
station. 1 degree is .1 nm when you're 6 miles away, 1 nm when
you're 60 miles away. So suppose you figure out what radial
you're on, turn 90 degrees and fly until you're on a radial
10 degrees away, and time how long that takes. You know about
what distance your plane flies in a minute, so you can get
a reasonable idea how far away from the VOR you are. There's a
good description of this technique in Gene Whitt's material
(available from him, email gwh...@ix.netcom.com if you don't
already have it).
So a VOR can get you un-lost as well as guide you either
directly towards it, or directly away from it. What else?
Well, you can haul out a chart and draw a line from a VOR
to some other point you'd like to identify -- say, a strange
airport. Look at the radial FROM the VOR towards that airport.
Let's say it's 330, the airport is NW of the VOR. Tune in and
ident the VOR, and be sure you know where it is relative to your
current position as described above. Let's say it's SW, and
you're flying W to reach it. You can use the VOR to tell you
when you're there. Dial in the 330 radial, make sure the flag
says "FROM". The second application of the "Same Side Safe"
rule says, if the VOR is off to your left (SW) and the needle
is off to the *same side*, you haven't crossed that radial yet.
As soon as the needle crosses to the right, you've crossed
that radial and missed the airport. It might be N of you,
and it might be S of you, but you know it's gotta be somewhere
along that radial. If landmarks don't come to your aid, you
could always track either inbound on the bearing which
corresponds (radial + 180) or outbound on the radial, and
find the airport.
There are more fun and games to be played w/ VORs, but
this has already gotten long. It's just a different way than
I've usually heard VORs explained, so I thought it might help.
Snowbird
wild...@newpiper.com
>Hi People,
>
>After fininshing gound school and about 30 hours total, I can't seem to
>understand how to use a VOR...I read it over and over again, had my
>instructor explain, but I still am not sure..
>
>Are there any instructors that could explain it to me one last time.
>
>Thank you all
>
>Tiffany Crow
AHA!
Finally one of the skills you CAN improve with PC flight simulators!
I will not add to the excelent reply already here, but i would
recommend that this is something you might want to play with MS Flight
Sim (if you have the appropriate hardware).
Even if you go buy the software retail- its less than you'd spend for
an hour of flight time.
One question though, Im not surprised that you haven;t mastered VOR
after only 30 hours of flight time, but I am surprised that you have
completed ground school without nailing it down....
If you are still having trouble visualizing the concept- consider one
of the several video courses available out there (they might even be
at your local public library!).
Hungry
problems are you having.
James M. Knox
>Hi People,
>
>After fininshing gound school and about 30 hours total, I can't seem to
>understand how to use a VOR...I read it over and over again, had my
>instructor explain, but I still am not sure..
You should have been at a talk I gave a while back. We covered how VOR's and
Loran and GPS and ADF all work. Complete with "visual aids" -- had folks
running around the room with balloons and flashlights and stopwatches and
throwing darts through hoops... May not have been the most "mathematical"
talk I ever gave, but I bet it was remembered longer. <G>
As for the VOR transmitter itself: Think of a old lighthouse, with one
modification -- there is a BIG standard lightbulb (the GE "I've got an idea"
type) mounted on top, as well as the usual one that turns around and around.
Now, assume three pieces of information. 1) The rotating spotlight takes six
minutes to make a complete circle (360 seconds), 2) Every time the rotating
light points to magnetic north, the big bulb on top flashes briefly, and 3)
you remembered to wind the wristwatch on your arm!
So you stand there on the beach and watch the lighthouse. You see the big
bulb flash, and you note the time. After a while you see the rotating light
flash past you and you note that it has been 40 seconds since the big bulb
flashed. Where are you?
Well, you know that the spotlight is turning at one degree per second (360
seconds to make one full 360 degree turn), and you know that when the big bulb
flashed it was pointing 0 degrees, so you are standing **on the 40 degree
radial FROM** the lighthouse. Continue away FROM the lighthouse and you are
traveling on a course of 40 degrees magnetic. Turn and walk TO the lighthouse
(obviously 180 degrees difference) and you are inbound on the 220 degrees
magnetic radial TO the station.. err, lighthouse.
Want to know more about how the actual VOR CDI knobs do their thing (and how
you can use them to find out other useful information about where you are -
and even how fast you are traveling)? Let me know...
jmk
ras
less...@aol.com wrote in article <6o3jla$v06$1...@nnrp1.dejanews.com>...
> I will take a stab at explaining the VOR.
Thanks, Les, that's going into the permanent save file!
ras
Mike Carter
As you know, a VOR (VHF Omni Range) is a radio aid to navigation. Here's a
picture of one:
http://www.cyberair.com/tower/af/af7.jpg
I feel it's important to know how the VOR works before you learn how to use
it. :) Here's a little introduction:
If you've ever seen a lighthouse, you can create a good mental picture of how
a VOR works. Like the lighthouse's light, the VOR revolves an electrical
signal through 360 degrees. Picture that lighthouse's light revolving around.
Additionally, the VOR has a reference signal which "points" north, just as if
the lighthouse owner were standing atop his lighthouse pointing a flashlight
due north.
When the VORs electrical "light" is lined up with it's reference signal, the
two are said to be in phase--in line with each other, to simplify the concept.
At any other time, the two signals are out of phase. This is important because
your VOR receiver is measuring this differene in phase to determine the
bearing to the station. This translates into what "radial" you are currently
on. We'll get into those concepts in a minute.
Say you were in a boat out in the ocean. You can visually see the lighthouse
in the distance. You see the sweeping beam of the lighthouse light, as well as
the flashlight beam the owner is pointing north. To determine your bearing
from the lighthouse, you would watch the revolving beam. When it passes the
owner's reference beam (the flashlight pointing north) you begin counting
seconds and stop when the revolving beam from the lighthouse flashes you in
the eyes. What you have now is the angular distance (phase) between the
reference beam and the revolving one. If you know how quickly the revolving
beam is revolving in seconds, it's an easy calculation to determine your
bearing from the lighthouse!
For example, if you counted 10 seconds between when the lights were in phase
and when the revolving beam hit your eyes, and you found out it takes 60
seconds for the beam to make one revolution, you are 10 degrees from
reference, or north (1 rev. per minute equals 1 degree per second).
The VOR receiver in the airplane is making the same calculations, except that
the VOR's electric signal is revolving much faster. The VOR receiver
calculates the phase difference from the station and displays it on your VOR indicator.
To use the VOR, first you tune in and identify the VOR station. Each VOR has a
3-letter identifier, and will identify itself every 30 seconds in Morse code.
One of our local VORs here in California, the San Francisco VOR, taps out
'SFO' to identify itself. That looks like this in Morse code: "ooo oo-o ---".
If you aren't familiar with Morse code, the sectional chart displays what the
Morse looks like. It's important to identify the VOR every time you use it! If
the FAA needs to service the VOR, or if the VOR fails, it will remove the
normal ID and send 'TEST' instead (- o ooo -), or send nothing at all! If you
try to use a VOR when it is down or being serviced, you may get erroneous information.
Once you've identified it, you can start using it. The simplest way to use a
VOR is to set your indicator to tell you what radial you are currently on. To
so this, rotate the OBS until the needle on the indicator centers with a TO
indication. Now read the heading under the pointer at the top of the
indicator. Since the TO flag is showing, the heading you read is the heading
that will take you directly to the VOR!
Say we tuned in the SFO VOR and identified it. Next we rotate the OBS until
the TO flag appears and the needle centers. We look under the index pointer at
the top of the indicator and see 315 degrees. If we are flying 270 degrees, we
would have to turn the plane right 45 degrees to fly towards the station. Try
drawing a compass rose on a piece of paper. Draw the four points of the
compass, 360, 90, 180 and 270 degrees, and the VOR in the center of the rose.
Put a dot on the rose where the VOR indicator is pointing, at 315 degrees,
which is halfway between 270 and 360. Now, measure the difference between our
magnetic flight heading (270 degrees) and the heading the VOR indicator
reported to us (315 degrees). See where 45 degrees is derived? It's the
angular difference between the two. Rotate your compass rose so that our
magnetic flight heading is at the top, since that's the way we are flying. You
will see immediately that the mark you made for the VOR is to the right of our
route of flight, hence the need to turn right to get to the VOR.
Once you turn, all you need to do is keep the needle centered to fly directly
to the station. Because of wind and the distance your plane moved during your
turn to get to 315 degrees, you may need to take up a heading of, say, 317
degrees to keep the needle centered. This is normal. If we lived in a windless
atmosphere or could turn the plane on a dime you wouldn't have to make any
corrections. As long as you keep the needle centered you are flying on the
indicated radial, no matter what heading the plane in on.
Now for a little more terminology. This is where it can get a little
confusing. When we look at a VOR on a chart, we see it as a little hexagon
symbol with a compass rose around it. If you were to take a ruler and lay it
down over the VOR and draw a line outward, you would have drawn a "radial", a
line designating a particular direction from a VOR. Although there are an
infinite number of them in reality, we only use one radial for each degree,
which makes 360 radials per VOR.
All radials go outward from the VOR, and are named with the magnetic degree
they point to. For example, a radial drawn from the center of the VOR that
passed through the "18" on the compass rose on your chart would be the 180
degree radial since it points to 180 degrees.
You use radials to describe your position from a VOR. In our example above, we
are flying towards the VOR on a heading of 315 degrees. Which radial are we
on? If you think we're on the 315 degree radial, you'd be wrong! This is were
a lot of students get confused. Although we are flying 315 degrees to get TO
the VOR, we are really flying inbound on the 45 degree radial! Well, where is
the 315 degree radial then? On the compass rose you drew earlier, draw line
outward from the center out through the mark you made for the VOR at 315
degrees. Continue it out a little bit beyond the compass rose. This is the 315
degree radial. Write "315" at the end of the line.
Now draw our plane. Since we aren't at the VOR yet, but are flying on a
heading of 315 degrees to get there, we'll be on a line extending from the
center of the compass rose back 180 degrees the other way from the first line
you drew. Draw a dotted line going from the center of the rose back *away*
from the VOR mark. The line you just drew--which is the line our plane is on
now--is the 135 degree radial. Notice it's exaclty 180 degrees different.
Remember, all radials point *outward* from a VOR. This is probably the most
important mental picture you can create about VORs. Just imagine you are
standing on the VOR with a fishing pole. If you cast your line due north, you
just cast out the 360 degree radial! So, in our example, although we were
flying 315 degrees to get TO the VOR, and haven't gotten to the VOR yet, we
were flying inbound on the 135 degree radial! If I was on the VOR with my
fishing pole and pointed towards your approaching plane, I'd be facing 135
degrees. When I cast my line out towards your plane, I'd be casting out the
135 degree radial.
So here we are back in the plane flying towards the VOR on a heading of 90
degrees, which we now know puts us on the 135 degree radial from the VOR. What
happens when we pass over the VOR? If you've done this before, you know that
the little flag in the VOR indicator--called the ambiguity flag--will do a
flip flop and change from a TO indication to a FROM indication! That flip-flop
indicated station passage, or that you've passed the VOR.
Here's another tricky question for you. If you continue flying straight ahead,
with a magnetic heading of 315 degrees after passing the VOR, what radial are
you on? Let's look at our drawing and move our finger along the radials we've
drawn out. Once your finger passes the middle of the compass rose you've
passed the station. You should be following that first line we drew, which we
determined was the 315 degree radial.
Here's an interesting point to note. If you are flying a straight line through
a VOR, you will be flying on one radial inbound, and another radial outbound!
The two radials will be 180 degrees apart (as long as you keep your flight
path a straight line!).
Finally, here's a little tip that will help you. Whenever the ambiguity flag
says FROM, the number you read at the top index on the VOR indicator is the
radial you are on. If the flag reads TO, then the heading under the BOTTOM
index on the VOR indicator tells you what radial you are on. Just remember, to
determine the radial you are on, use the phrase: FROM the top TO the bottom.
Flying using VORs is a great way to maintain your positional awareness. If you
ever get lost while flying, tune in any VORs in the area. When you get one, ID
it, center the needle and write down the bearing from the station. Now find
another if you can and do the smae thing. If you draw the radials on your
sectional chart, your position will be where the two lines intersect.
I hope this has helped you understand VORs better. If you have any questions,
feel free to email me!
-Mike Carter-
mca...@netopia.com
tiffa...@hotmail.com wrote:
>
> Hi People,
>
> After fininshing gound school and about 30 hours total, I can't seem to
> understand how to use a VOR...I read it over and over again, had my
> instructor explain, but I still am not sure..
>
Eric W. Seelig
technical questions, though.
First, how fast does a VOR's directional signal actually rotate?
I assume (for some reason...could be totally wrong) it must be
somewhere between 1/min and 1/sec.
Second, how wide (in degrees) is the directional signal? Is it
actually less than 1 degree?
Mike Carter wrote:
< NICE EXPLANATION SNIPPED >
> For example, if you counted 10 seconds between when the lights were in phase
> and when the revolving beam hit your eyes, and you found out it takes 60
> seconds for the beam to make one revolution, you are 10 degrees from
> reference, or north (1 rev. per minute equals 1 degree per second).
Hmmm...might want to re-check that math, Mike! <g>
Seriously, though, thanks for the explanation!
Eric
Alan Sanderson
>
> So that's how it works! It's just a timing thing. A couple of
> technical questions, though.
>
> First, how fast does a VOR's directional signal actually rotate?
> I assume (for some reason...could be totally wrong) it must be
> somewhere between 1/min and 1/sec.
The lighthouse explanation was a good description of the principles
involved. The newer VOR's transmit a reference signal and
electronically phase shifted signals to describe the angle. The
receiver at its location detects the two signals and determines the
phase difference between them to determine the radial. The VOR receiver
displays the angle to the nearest degree.
The signal is transmitted as a 9960HZ reference amplitude modulated at
30HZ. At one time this was done with a spinning antenna.
For another analogy, drop two pebbles in a pool of water, one shortly
after the other. You will see two sets of waves. The phase angle is the
distance between the wave crests from the two pebbles.
>
> Second, how wide (in degrees) is the directional signal? Is it
> actually less than 1 degree?
The width of the signal is not important - the information comes from
the phase difference between two signals. A new style VOR transmitter
puts out a 360 degree signal all at once.
DctrHappy
Never heard of a rotating signal before.
Jay
CP-ASMEL/IA
-------------------------------------------------------------------------
-=>DIRECT ALL REPLIES TO: jsla...@concentric.net
Greg Bullough
shine. Microsoft FS 98 or Sierra Pro Pilot work very well for
learning how to dial in, intercept, and fly a radial.
ProPilot has the edge in documentation, and can be had for
forty bucks or so at the Megastores. It's not as 'mainstream'
as FS, so there aren't as many add-ons. But it does come with a
ground-mapping GPS, so you can learn that too. The flight-manual
for ProPilot is almost like old ASA or Jepp course materials.
But your ground-school books will work with either.
FS98 is main-stream, and you can find GPS add-ons (no mappers yet)
at www.flightsim.com, along with more add-on airplanes than you can
stand.
They're both also decent for other types of learning how instruments
react.
You can buy either for less than the cost of an hour of flight time.
Also, it's one of the few ways you can safely (as a student) leave
your head down looking at the needles to see how they behave, since
you need not worry about outside traffic.
By the way, there seems to be a bundle deal on a good joy-stick and
FS98 going on.
Greg
--
Greg Bullough | AFM Local 1000 AFL/CIO
g...@eclipse.net | K2GWB
| PP-ASEL
www.eclipse.net/~gwb for Compass Rogues & NY Chantey Sings
Roy Smith
>> and when the revolving beam hit your eyes, and you found out it takes 60
>> seconds for the beam to make one revolution, you are 10 degrees from
>> reference, or north (1 rev. per minute equals 1 degree per second).
"Eric W. Seelig" <ese...@nwu.edu> wrote:
> First, how fast does a VOR's directional signal actually rotate?
> I assume (for some reason...could be totally wrong) it must be
> somewhere between 1/min and 1/sec.
The problem with the rotating beam explanation is that it's, well, wrong :-)
It's a good analogy, but it's only an analogy. The VOR works by comparing
phases of two signals, and while an electrical engineer might draw a
phasor diagram to describe the signal waveforms and talks about the
rotating phasors, that's just a mathematical abstraction. Unfortunately,
I'm not sure I can come up with a better explanation which would make
sense without a good grounding in AC signal analysis and/or vector
algebra.
> Second, how wide (in degrees) is the directional signal? Is it
> actually less than 1 degree?
There is no specific width. Conventionaly, we talk about a VOR producing
360 radials, but it is a continious signal and there is no good
theoretical reason why you can't measure azimuth using the VOR signal to
any arbitrary precision you want.
--
Roy Smith <r...@popmail.med.nyu.edu>
New York University School of Medicine
Dave Mould
> So that's how it works! It's just a timing thing. A couple of
> technical questions, though.
>
Welllll .... the lighthouse thing is a good analogy, but that's all it is - an
analogy. Like most analogies, you cannot thrash it to death! It's purely for
ease of concept.
> First, how fast does a VOR's directional signal actually rotate?
> I assume (for some reason...could be totally wrong) it must be
> somewhere between 1/min and 1/sec.
>
If you *really* want to thrash it, the rotational speed is 30 times per
*second*, although this is pushing the analogy a lot, because the phase
difference could be measured instantaneously and used to work the radial as
often as you like. The electronics that do it do not really work this way,
however - the phase difference is sampled continuously and integrated over a
time period considerably longer than 30 times per second (but less than 1 time
per second).
> Second, how wide (in degrees) is the directional signal? Is it
> actually less than 1 degree?
There is no width as such. For additional "thrashing" assume that the lights do
not flash at all, instead they both get progressively stronger and weaker in a
continuous cycle of bright/dim/dark/dim/bright/dim ... etc. Due North, both
lights change from bright to dark in step. Due South, one is brightest whilst
the other is darkest. West, one is brightest while the other is halfway between
bright and dark but becoming brighter, whilst East the same is the case except
that the half-intensity light is becoming dimmer. Thus the difference between
the light/dark cycle ("phase") of the two lights may be used *continuously* to
give the radial. The "width" is proportional to how well the receiver is able
to resolve the "relative intensities" (phase angle). The analogy is now
becoming more accurate, but less conceptual! This is the difficulty with almost
all analogies :-(
Thrash this one, though, and there are even more considerations to take into
account - the next step is really to learn radio modulation theory and (for
modern VORs) how pseudo-doppler shift works! Neither is difficult to understand
BTW if you are interested in the subject - I'll have a go via e-mail if anyone
wants to get in any deeper.
==========
Dave Mould
==========
Michael Hoffman
>shine. Microsoft FS 98 or Sierra Pro Pilot work very well for
>learning how to dial in, intercept, and fly a radial.
I completely second that!
My wife took groundschool with me - but without taking actual flying lessons
as I did (she starts this Sunday). And VOR/ADF navigation was giving her
lots of trouble, seeing as she'd just enver "experienced" them in the
cockpit.
So, a few days before the Written, we sat down with our sectional and FS98
and, while I did the "flying" and pushing of keys, she navigated around our
airspace, watching the needles until it just went *click*.
Cheers
Mike
Eric W. Seelig
enough understanding of electroncs/wave mechanics that I
should be able to understand most of what you throw at
me (and if I don't, I can go back to my textbooks). Or
maybe it'd be easier if you just give me a good reference
to read it.
Anyway, let me start with a few questions/comments.
Say you have two 30Hz signals...I can think of two ways to
generate a phase shift between them. The first is to simply
generate them out of phase and originate them from the same
position...but this is NOT direction-sensitive. The second is
to generate them from different positions...the nodes/anti-nodes
thing. But again, the phase shift is not directionally
dependent...well, it kinda is, but you can get the same phase
shift on many different directions depending on distance.
So what is the actual physical setup? Maybe if you just
describe how it is set up, I can figure out on my own how to
get directional information...
Actually, I guess it's not really a pure 30Hz signal...more like
a 10x.x signal modulated at 30Hz....Now I'm more confused than
I was when this all started!
Eric
highflyer
A VOR transmits both an AM and an FM modulated sine wave. One is
fixed and one electronically rotates in such a way that the phase
difference between the two signals if equal to your bearing from
the station. Originally rotating signal was transmitted from an
antenna that actually rotated. Now it is an array of small antennas
around the flat roof that surrounds the upside down ice cream cone
that is a VOR. The antennas are fired in such a way that the signal
is electronically rotated at the appropriate rate. I do believe it
is 30 Hz.
The lighthouse is a pretty close analogy. That is exactly how
the VOR determines your bearing.
Roy Smith
> The lighthouse is a pretty close analogy.
Actually, the really funny thing about the lighthouse analogy is not that
it doesn't really describe exactly how a VOR works (which is OK, since
it's just an analogy), but that it doesn't even describe how a lighthouse
works!
I missed the beginning of this thread, but the analogy is usually given
something like "In a lighthouse, there's two lights. One blinks and the
other rotates. By timing the difference between when you see the two, you
can figure out your bearing from the lighthouse". Something like that is
very clever, and would actually be kind of useful. The only problem is
that in all my years on the water, I've never seen nor heard of a
lighthouse which had two lights like that!
James M. Knox
>highflyer <high...@alt.net> wrote:
>> The lighthouse is a pretty close analogy.
>
>Actually, the really funny thing about the lighthouse analogy is not that
>it doesn't really describe exactly how a VOR works (which is OK, since
>it's just an analogy), but that it doesn't even describe how a lighthouse
>works!
Hey... you should have seem my ADF demo, complete with hoops and balloons!!!
Or the Loran demo with guys running around the room with stopwatches and
slips of paper.<G>
>I missed the beginning of this thread, but the analogy is usually given
>something like "In a lighthouse, there's two lights.
Actually, it was more like "Suppose you had this lighthouse, but with another
big light mounted on top (like the GE "I have an idea" kind of lightbulb)..."
>The only problem is
>that in all my years on the water, I've never seen nor heard of a
>lighthouse which had two lights like that!
Interestingly enough, it would *almost* work! The problem is that the
vertical separation between the two lights would have to be quite large to
differentiate them - especially in fog which tends to diffuse the light
anyway. Oh, and the non-directional light at top would have to be VERY
powerful to have anywhere near the range of the rotating one.
Actually, the biggest maritime problem with lighthouses (other than their
expense <G>) is that there is no "morse identifier"! Ships have grounded
because they thought one lighthouse was actually another.
jmk
Dave Mould
> Actually, the really funny thing about the lighthouse analogy is not that
> it doesn't really describe exactly how a VOR works (which is OK, since
> it's just an analogy), but that it doesn't even describe how a lighthouse
> works!
This is true :-) However, there are those clever DF *radio* "lighthouses"
that *do* work this way. Just more difficult to explain. All you need is a
cheap AM radio and you can get your radial. Ever come across one?
Dave
Dave Mould
> >The only problem is
> >that in all my years on the water, I've never seen nor heard of a
> >lighthouse which had two lights like that!
>
> Interestingly enough, it would *almost* work! The problem is that the
> vertical separation between the two lights would have to be quite large to
> differentiate them - especially in fog which tends to diffuse the light
> anyway.
No, you could easily separate the lights *horizontally* as well!
> Oh, and the non-directional light at top would have to be VERY
> powerful to have anywhere near the range of the rotating one.
Well, it could be semi-directional (would only need to shine out to sea). I would think
that with modern equipment, a pulsed light (maybe a xenon tube or something) would do
the trick.
>
> Actually, the biggest maritime problem with lighthouses (other than their
> expense <G>) is that there is no "morse identifier"! Ships have grounded
> because they thought one lighthouse was actually another.
>
All the lighthouses along the coasts that I sailed *were* coded. Not with Morse, but
with a distinct flash pattern that was marked on the chart, complete with
timing information. Very easy to see the difference between "Group flashing 3" and
"Group flashing 2", and easy (even without a watch) to tell a 2 second flash from a
5 second flash. The lighthouses around Southern Africa are placed very well - just
as one starts to fade away astern, the next one along becomes visible ahead, except in
heavy fog :-( I would think that the groundings are for the same reason people fly
the wrong VOR - they assume instead of correctly identifying!
The worst was around Florida, when it was just about impossible to tell the lighthouse
apart from all the other flashing lights on shore. I timed and so identified the flashes
of a lighthouse - only to see as I got closer that it spelt out the words "Coca Cola"!
The actual lighthouse was *far* weaker than those crazy beacons sent out (for some
extraordinary reason) from the many used-car lots.
Dave
Dave Mould
Dave Mould
Chris Stratton
>
> A VOR transmits both an AM and an FM modulated sine wave. One is
> fixed and one electronically rotates in such a way that the phase
> difference between the two signals if equal to your bearing from
> the station. Originally rotating signal was transmitted from an
> antenna that actually rotated. Now it is an array of small antennas
> around the flat roof that surrounds the upside down ice cream cone
> that is a VOR. The antennas are fired in such a way that the signal
> is electronically rotated at the appropriate rate. I do believe it
> is 30 Hz.
>
Actually, the signal fed to the antenna switcher is only amplitude
modulated. The fact that the transmitting antenna seems to rotate
brings the doppler effect into play. When it is on the side coming
towards you, the frequency rises, and when it is going away from you it
drops. This produces a sinusoidal frequency modulation with a phase
that depends on the radial you are at from the station.
This also works for direction finding if you rotate the receiving
antenna, often by switching between a small array of 4 or 8 antennas.
Such doppler RDF receivers can sometimes be seen mounted on the roof of
cars, and are popular as they are relatively easy to operate and don't
succomb to the same types of false readings (front/back confusion, side
loabs, signal overload) common with directional antennas.
Chris
> The lighthouse is a pretty close analogy. That is exactly how
> the VOR determines your bearing.
--
Christopher C. Stratton
Engineering Consultant, Horn Player, and Brass Instrument Maker
http://www.mdc.net/~stratton or http://bounce.to/stratton
344 Boston Avenue Medford, MA 02155 (781) 393-0034
Frank Maier
Jim Weir wrote in message <35af85bd...@news.gv.net>...
...explanation snipped...
>Did this help?
>
>Jim
>Jim Weir (A&P, CFI, and other good alphabet soup)
>VP Engineering, RST
>Web address: www.rst-engr.com
Jim,
I wanna personally thank you for this, and all your other, posts in this
newsgroup. Your expertise is most appreciated.
I look forward to reading your responses here and your articles in aviation
periodicals. Plus I'm thinking about ordering a couple of your headsets for
my girls. Those purple, fuschia, whatever color choices sound totally cool!
Frank (I didn't understand half of it and I don't really care; but DAMN!
it's nice to have somebody I have confidence in spell it out for me.)
Jim Weir
shared these priceless pearls of wisdom:
->Actually, I would like to go a little deeper. I have good
->enough understanding of electroncs/wave mechanics that I
->should be able to understand most of what you throw at
->me (and if I don't, I can go back to my textbooks). Or
->maybe it'd be easier if you just give me a good reference
->to read it.
Actually it works like this. Consider a directional antenna that has one
broad peak and one broad null. Rotate this antenna (and there are at least
half a dozen electronic ways to do it) at a rate of 30 rotations per
second. Think of how an AM receiver will view this rotation. It will go
through a peak (top of a sine wave) then a null (bottom of a sine wave)
then another peak...and so on. Net result to an AM receiver will be as
though the carrier wave on the antenna was modulated with a 30 Hz. sine
wave.
Note that another observer at a different direction from the rotating
antenna will also see a sine wave, but if the two of you compare notes, the
other receiver will appear to be phase shifted from yours. If you think
about it, the amount of phase shift will be exactly the difference in angle
between you and the other observer. If you are at exactly due north of the
rotating antenna and you compare your sine wave with your buddy at due
east, you will find that his sine wave lags yours by exactly 90d.
Now consider that I've split the RF wave going to the rotating antenna into
two parts. One still goes to the rotating antenna, but now I put a 9960hz.
modulation on the second half of the RF signal. I now FM modulate the 9960
subcarrier with an unvarying 30 Hz. sine wave and transmit this second half
of the signal in all directions. If I am careful about how I get that 30
Hz. FM signal, I can arrange it so that when you detect the FM "subcarrier"
30 Hz. signal and compare it in phase to the rotating antenna AM signal,
when you are due north of the rotating antenna, the two 30 Hz. signals will
be exactly in phase. If you then move due east of the rotating antenna,
the "reference" subcarrier will not change in phase, but the "variable"
signal from the antenna will be phase shifted 90d. South? 180d. West?
270d. And so on.
Chris Stratton
>
> Actually it works like this. Consider a directional antenna that has one
> broad peak and one broad null. Rotate this antenna (and there are at least
> half a dozen electronic ways to do it) at a rate of 30 rotations per
>Now consider that I've split the RF wave going to the rotating antenna
>into
>two parts. One still goes to the rotating antenna, but now I put a
>9960hz.
>modulation on the second half of the RF signal. I now FM modulate the
>9960
>subcarrier with an unvarying 30 Hz. sine wave and transmit this second
>half
>of the signal in all directions. If I am careful about how I get that
>30
>Hz. FM signal, I can arrange it so that when you detect the FM
>"subcarrier"
>30 Hz. signal and compare it in phase to the rotating antenna AM
>signal,
>when you are due north of the rotating antenna, the two 30 Hz. signals
>will
>be exactly in phase.
Actually, this doesn't work (AM giving radial phase, FM as reference).
The minute you start rotating the antenna you introduce an FM modulation
due to the doppler effect that has a phase which depends on your radial
from the VOR. The AM signal is the fixed-phase reference.
The beauty of this is that no directional antennas are needed - which is
great because directional antennas are bulky, complicated, misleading
beasts. It's far easier to rotate a simple vertical dipole (old) or
electronically swith between a circular array of dipoles or whips over a
common groundplane (new).
Jim Weir
shared these priceless pearls of wisdom:
->Actually, this doesn't work (AM giving radial phase, FM as reference).
->The minute you start rotating the antenna you introduce an FM modulation
->due to the doppler effect that has a phase which depends on your radial
->from the VOR. The AM signal is the fixed-phase reference.
If I were you, sir, I'd stick to making brass instruments as your sig
indicates. I cut my teeth on this stuff in the '50s and haven't stopped
working on it since. You are south of the station, sir, 180 degrees out of
phase.
Roger Halstead
--
Roger Halstead K8RI and EAA Chapter 1093 Historian
N833R World's oldest Debonair? S# CD-2
http://members.tm.net/rdhalste
Roy Smith wrote in message ...
>highflyer <high...@alt.net> wrote:
>> The lighthouse is a pretty close analogy.
>
>Actually, the really funny thing about the lighthouse analogy is not that
>it doesn't really describe exactly how a VOR works (which is OK, since
>it's just an analogy), but that it doesn't even describe how a lighthouse
>works!
One thing about all these analogies getting into *how* the VOR works. I
believe the original question was from a particle standpoint and not a
technical.
Also when I took my check ride oh so many years ago the DE asked that
question and I started to explain *how* a VOR worked. She stopped me as
said "Lets start again. I don't want a technical explanation. I want to
know how you use one to navigate. Explain it like I know nothing about VORs
and yet the explanation will be good enough for me to be able to use one
properly".
That, is what a DE will most likely ask, and it is what a student needs to
know. Not the technical end except where the students curiosity requires
it..
There is no need to complicate *how* a VOR works explanation with telling
them *how* it works. When it comes to a *How* does a VOR work, they are
really asking how do I use this damn thing.
I see many technical explanations on how things work, which is fine for
those who really want to know, but this is a student group. I think that
strict technical definitions and explanations do more to confuse the already
confused student than just a simple "This is how you use the damn thing",
explanation. <G>
Keep it simple for the students unless they really ask for a technical
explanation. they are already struggling with the how to use, let alone how
it works.
I had 26 years working with electronics, instrumentation, and
communications. Then I finished up a degree in Computer science with a minor
in math. I never had any trouble with the technical aspect of either the
avionics or the flight physics, but never saw any need for giving it to
students.
Roger.
Chris Stratton
>
> Chris Stratton <stra...@netway-dot-com.nospam>
> shared these priceless pearls of wisdom:
>
> ->Actually, this doesn't work (AM giving radial phase, FM as reference).
> ->The minute you start rotating the antenna you introduce an FM modulation
> ->due to the doppler effect that has a phase which depends on your radial
> ->from the VOR. The AM signal is the fixed-phase reference.
>
> If I were you, sir, I'd stick to making brass instruments as your sig
> indicates. I cut my teeth on this stuff in the '50s and haven't stopped
> working on it since. You are south of the station, sir, 180 degrees out of
> phase.
>
Look, I may only have 5.3 hours of flight time, but I do have a degree
in electrical engineering and and advanced class amateur radio license.
You cannot transmit a signal through a rotating antenna without FM
modulating it in the process - check any radio communications textbook
(ARRL Handbook recommended for those who dislike complex numbers).
Could you FM modulate the signal at a higher frequency in addition?
Sure, but why bother when rotating the antenna does it for you?
It makes logical sense to use the free FM modulation for your phase
and either AM modulation or a second FM signal for your reference. The
system you describe may not be impossible, but it is far more
complicated and lower performance than the one I describe.
But then it is a _goverenment_ installation built with 50's technology.
Chris
Chris Stratton
>
> Chris Stratton <stra...@netway-dot-com.nospam>
> shared these priceless pearls of wisdom:
>
> ->Actually, this doesn't work (AM giving radial phase, FM as reference).
> ->The minute you start rotating the antenna you introduce an FM modulation
> ->due to the doppler effect that has a phase which depends on your radial
> ->from the VOR. The AM signal is the fixed-phase reference.
>
> If I were you, sir, I'd stick to making brass instruments as your sig
> indicates. I cut my teeth on this stuff in the '50s and haven't stopped
> working on it since. You are south of the station, sir, 180 degrees out of
> phase.
>
Mr. Weir does in fact describe the only system which _could_ be built
with 50's technology, and is in fact correct. Although the system
I assumed was in use (based on standard RF techinuqes rather than
avaition-specific references) and explained above would be simpler,
cheaper, and higher performance, it requires a much higher rotation rate
on the order of tens of thousands of RPM, which is only possible with
electronic antenna switching. That was not economical in the vaccum
tube era.
Sorry for the error,
Jim Weir
shared these priceless pearls of wisdom:
->Actually, the signal fed to the antenna switcher is only amplitude
->modulated. The fact that the transmitting antenna seems to rotate
->brings the doppler effect into play. When it is on the side coming
->towards you, the frequency rises, and when it is going away from you it
->drops. This produces a sinusoidal frequency modulation with a phase
->that depends on the radial you are at from the station.
That's an interesting heretical abomination of modulation theory. That
wasn't an earthquake, just Marconi rolling over in his grave again...
Andrew M. Sarangan
>I see many technical explanations on how things work, which is fine for
>those who really want to know, but this is a student group. I think that
>strict technical definitions and explanations do more to confuse the already
>confused student than just a simple "This is how you use the damn thing",
>explanation. <G>
>
>Keep it simple for the students unless they really ask for a technical
>explanation. they are already struggling with the how to use, let alone how
>it works.
>
This really depends on how the student learns. For me, it helps a great
deal to undertand some technical details of how something works. For others
that might not be the case. Since the majority of posters here have some
kind of technical background (I assume) it makes some sense that they
are inclined towrads the technical aspects.
--
Andrew Sarangan
PP-ASEL-IA http://lights.chtm.unm.edu/~sarangan/aviation/
Jim Weir
shared these priceless pearls of wisdom:
->Look, I may only have 5.3 hours of flight time, but I do have a degree
->in electrical engineering and and advanced class amateur radio license.
->You cannot transmit a signal through a rotating antenna without FM
->modulating it in the process - check any radio communications textbook
->(ARRL Handbook recommended for those who dislike complex numbers).
Look, sir, I don't give a hoot how much flight time you have. As for your
degrees and ham ticket, I 'spect I've had mine a wee bit longer than you..
Degree was in '67; ham ticket was in '59. MSEE if I ever get around to
finishing my thesis...but at my age, who gives a royal damn?
Neither Krause nor Jasik (two of the most respected antenna books in print)
make any mention of FM modulation by means of a rotating antenna, nor does
Hamsher in his "bible" on radio communications. Neither does the ARRL
Handbook nor any of the half-dozen ARRL Antenna books -- and even if they
did, ARRL publications are widely noted for their lack of technical
accuracy. Perhaps you could give us chapter and verse for your source of
information? Author, title, and chapter/page reference?
The advantage I have is that I've been actively employed in the field of
avionics since I was seventeen -- that's coming up on 40 years in the biz.
Five of them with the airlines and the last 25 years as the VP Eng. of my
own company. I've designed VOR receivers, VOR testers, and VOR simulators.
I *think* I've got the book larnin' as well as the practical experience --
besides teaching the stuff at the local community college (including the
avionics courses in the A&P program) for the last 25 years. Two antenna
books written and published, two antenna patents in rotating antennas
(phased array)...and about ten thousand aircraft flying around with my
antenna designs in them.
So I say it again. It just ain't so, and I'm not going to carry on a
running debate with you. Lay out your credentials and let the folks
believe who they wish. Me? I'm putting the airplane back together after
annual and getting ready for OSHKOSH!!! (My antenna forum at The Show is
Saturday morning for anybody that wants to keep this silly thread going.)
Chris Stratton
>
> Chris Stratton <stra...@netway-dot-com.nospam>
> shared these priceless pearls of wisdom:
>
> ->modulated. The fact that the transmitting antenna seems to rotate
> ->brings the doppler effect into play. When it is on the side coming
> ->towards you, the frequency rises, and when it is going away from you it
> ->drops. This produces a sinusoidal frequency modulation with a phase
> ->that depends on the radial you are at from the station.
>
> That's an interesting heretical abomination of modulation theory. That
> wasn't an earthquake, just Marconi rolling over in his grave again...
>
Actually nothing heretical, it's a pretty standard technique widely
used in RDF applications (look up "doppler RDF") But it is not
apparently used in VOR's.
Too bad Marconi didn't really understand modulation.
details in private email,
Mark Mallory
: The minute you start rotating the antenna you introduce an FM modulation
: due to the doppler effect that has a phase which depends on your radial
: from the VOR.
Not true. The center of radiation of the antenna coincides with the axis
of rotation, which results in *zero* doppler shift as the antenna is
rotated.
Chris Stratton
Perhaps for the original installations (single rotating antenna).
Multiple antenna installations might or might not act this way,
depending on if they are directional antennas driven one at a time
(doppler present) or several at a time (phased array) with a
possibly constant center of rotation (no doppler). Apparently any 30hz
modulation which results is enough seperated from the interesting
signals in the passband to be unimportant.
Dave Mould
> ->Actually, this doesn't work (AM giving radial phase, FM as reference).
> ->The minute you start rotating the antenna you introduce an FM modulation
> ->due to the doppler effect that has a phase which depends on your radial
> ->from the VOR. The AM signal is the fixed-phase reference.
>
> If I were you, sir, I'd stick to making brass instruments as your sig
> indicates. I cut my teeth on this stuff in the '50s and haven't stopped
> working on it since. You are south of the station, sir, 180 degrees out of
> phase.
ISTM that the confusion (and confrontation) is arising because there are 2
different types of VOR transmitter design. The early VORs used a fixed FM
reference, and a rotating AM component (the AM part being induced by
directional antenna rotating on its axis an so having no doppler shift).
Current designs use a fixed AM reference with a rotating FM component (the FM
part being induced by pseudo-rotating a circular array of antenna and so
creating a sinusoidal frequency change due to doppler effect).
A single antenna rotating on its axis will not exhibit any doppler shift. An
antenna rotating in a circle around a central point distant to the antenna
will. Any signal source, be it a light, an antenna or a train whistle
will exhibit a doppler shift if it is moving either away or toward the
receiving device.
The receiver cannot tell the difference between the two methods of producing
a VOR signal. There are differences in how the receiver behaves in the
presence of interfering reflections.
Dave Mould
Jim Weir
shared these priceless pearls of wisdom:
->ISTM that the confusion (and confrontation) is arising because there are
2
->different types of VOR transmitter design. The early VORs used a fixed
FM
->reference, and a rotating AM component (the AM part being induced by
->directional antenna rotating on its axis an so having no doppler shift).
->Current designs use a fixed AM reference with a rotating FM component
(the FM
->part being induced by pseudo-rotating a circular array of antenna and so
->creating a sinusoidal frequency change due to doppler effect).
Would you be so kind as to give me just one example (location and
identifier) of a VOR in the USA that uses this method of modulation? I'm
not talking about TACAN, the thread is VOR. Just one. A civilian VOR
using an AM reference. There ain't some.
Mark Mallory
: Dave Mould <da...@airstrip.demon.co.uk>
: ->Current designs use a fixed AM reference with a rotating FM component
: ->(the FM part being induced by pseudo-rotating a circular array of
: ->antenna and so creating a sinusoidal frequency change due to doppler
: ->effect).
: Would you be so kind as to give me just one example (location and
: identifier) of a VOR in the USA that uses this method of modulation? I'm
: not talking about TACAN, the thread is VOR. Just one. A civilian VOR
: using an AM reference. There ain't some.
There's one just down the road from where I live. The San Jose, CA (SJC)
VOR uses a large number of radiating elements (looks like about 64) arranged
in about a 50 foot diameter circle. The whole thing is mounted on top of
a large counterpoise structure maybe 150 feet in diameter and 35 feet
high or so. It's located just south of highway 101 along De La Cruz st,
just west of the north end of the SJC airport.
It's my understanding that this type of VOR transmitter is preferred at
congested locations, where objects in the vicinity of the transmitter can
cause multipath problems with conventional VOR's. The much larger
"effective aperture" of this type of antenna provides increased immunity
to multipath.
Dave Mould
> ->Current designs use a fixed AM reference with a rotating FM component
> (the FM
> ->part being induced by pseudo-rotating a circular array of antenna and so
> ->creating a sinusoidal frequency change due to doppler effect).
>
> Would you be so kind as to give me just one example (location and
> identifier) of a VOR in the USA that uses this method of modulation? I'm
> not talking about TACAN, the thread is VOR. Just one. A civilian VOR
> using an AM reference. There ain't some.
Well, two avionics textbooks that I have read describe these exactly as I
have posted. The photograph in one looks a lot like *all* of the VORs I fly
over here in the UK. These are the type with a large circular groundplane
with multiple vertical antennas all around its circumference. I've not seen
any USA ground installations, so can't comment specifically. If this type do
*not* work in the way my books describe, please explain how these type of
VORs *do* work. E-mail is probably more appropriate if it's highly technical
(we can post the conclusions later). Go into as much detail as you like - I
do have an appropriate technical background to be able to understand. OTOH
avionics is not my business, so I've only read about it out of interest, (I
like to know how things work as well as how they operate), and I suppose
text-books have been known to have been wrong before.
If you really need confirmation, email & I'll dig out one of the books and
scan in the relevant pages in GIF or TIF. I can assure you that I did not
come up with this out of thin air.
Dave Mould
Rob Warnock
+---------------
| Jim Weir (j...@rst-engr.com) wrote:
| : Would you be so kind as to give me just one example (location and
| : identifier) of a VOR in the USA that uses this method of modulation?
|
| There's one just down the road from where I live. The San Jose, CA (SJC)
| VOR uses a large number of radiating elements (looks like about 64) arranged
| in about a 50 foot diameter circle.
According to Scott's '93 article, that'd be "52 Alford loops around
the 44 foot roof perimeter".
Whereas I think the SAC (Sacremento) VOR may be the older kind --
looks like a plain white cone sticking up out of a field.
-Rob
-----
Rob Warnock, 7L-551 rp...@sgi.com http://reality.sgi.com/rpw3/
Silicon Graphics, Inc. Phone: 650-933-1673
2011 N. Shoreline Blvd. FAX: 650-933-4392
Mountain View, CA 94043 PP-ASEL-IA
Rob Warnock
+---------------
| Look, I may only have 5.3 hours of flight time, but I do have a degree
+---------------
Yes, yes, theoretically, of course, but...
+---------------
| Could you FM modulate the signal at a higher frequency in addition?
| Sure, but why bother when rotating the antenna does it for you?
+---------------
Because (1) you don't get *enough* frequency deviation, by a large factor
(the old antennas weren't very large, and the Doppler depends on the linear
velocity, which here is the product of length and angular velocity), and
(2) it doesn't do it *independently* of the rotation angle, which after all
is the whole point of the VOR scheme in the first place. Fortunately, in fact,
the Doppler FM is small enough to be negligible, and the VOR's modulation
method cuts even *that* way down [see below].
Here's a good description Chris Scott posted back in 1993 [annotated/extended
slightly in square brackets by yours truly]:
The Vhf Omni Range became the US standard in 1946 and replaced
(gradually) the four course range; The signal is produced by an
omnidirectional transmitter (horizontally polarized) that contains
modulation (AM) at 9960 hz. This 9960 hz "tone" [sub-carrier] is
itself FM'ed or frequency "warbled" at 30 cycles per second, 480 Hz
deviation. This forms the [azimuth-independent] phase reference.
Four Alford Loops radiate a cardioid horizontally polarized signal
that is "rotated" by a motor driven goniometer at 30 cycles per second.
[This forms the azimuth-dependent phase information. The depth of
the cardioid "notch" is deliberately *not* maximized, so that the
FM limiter (below) can ignore the amplitude variations.]
A standard AM receiver is used [which when tuned on-freq will for
all practical purposes ignore any small amounts of Doppler FM in
the RF carrier freq]. This has the effect of producing in the VOR
receiver, two signals, not counting the 1020 Hz Morse: a 9960 Hz
signal [the sub-carrier] which is then FM-detected [which due to
the limiter pretty much ignores the AM component] to recover the
reference phase, and an [AM-detected from the sub-carrier] 30 Hz
signal, which is then fed to the wiper on the OBS in the airplane.
The pilot turns the knob which rotates the wiper, with the meter
needle indicating (essentially) phase shift. When centered, zero
phase shift is indicated, meaning that you are located on the "tip
of the peak" of the cardioid pattern.
Another system for transmitting is the Standard Elektrik Lorenz-
manufactured variety which did actually involve a rotating horizontal
dipole [and thus, yes, in theory, some small amount of Doppler FM].
Problematic terrain around the VOR site led to the development of the
Doppler VOR, using 52 Alford loops around the 44 foot roof perimeter.
Using this system, which I believe now uses solid-state commutation
(no moving parts) attaining .5 degree accuracy is routine, and .1
degree is theoretically possible. The current spec is max error of
1 degree.
Note that the only incidental Doppler FM that could make it into the
recovered signals due to a mechanically-rotating antenna would be the
*difference* between the Doppler FM experienced by the main VOR carrier
and the Doppler FM of the upper and lower sidebands carrying the 9660 Hz
signal (because that's the only thing that could change the frequency of
the recovered 9660 Hz sub-carrier!). Since the main carrier of the VOR
is ~118 MHz, whatever Doppler FM there *was* [*already* very low because
of the low "spinning" rate of the antenna] will be cut to the ratio
9660/118000 or ~8% of its original value. Probably negligible...
-Rob
p.s. However (and I'm not sure about this), the diameter of the modern
"Doppler VOR" referred to at the end of the quote does, I believe, *is*
large enough to exploit the Doppler FM of the "apparent motion" of the
"virtual antenna" as it wheels around the circle (electronically commuted)
at 30 Hz. And instead of transmitting an azimuth-independent FM on the
9660 Hz sub-carrier and an azimuth-dependent AM on the sub-carrier, I
believe it transmits an azimuth-*independent* AM (transmit power constant
regardless of which antenna) on the sub-carrier, and an azimuth-*dependent*
FM via the Doppler effect of the "moving" virtual antenna, which has to move
in the *opposite* direction to the old mechanically-rotating VORs so that
the relative phase differences between the AM & FM sub-carrier modulations
AS RECEIVED AT THE AIRPLANE remain the same as before. (As I said, I'm a
bit shaky on this part. Corrections welcomed.)
Mark Mallory
: p.s. However (and I'm not sure about this), the diameter of the modern
: "Doppler VOR" referred to at the end of the quote does, I believe, *is*
: large enough to exploit the Doppler FM of the "apparent motion" of the
: "virtual antenna" as it wheels around the circle (electronically commuted)
: at 30 Hz. And instead of transmitting an azimuth-independent FM on the
: 9660 Hz sub-carrier and an azimuth-dependent AM on the sub-carrier, I
: believe it transmits an azimuth-*independent* AM (transmit power constant
: regardless of which antenna) on the sub-carrier, and an azimuth-*dependent*
: FM via the Doppler effect of the "moving" virtual antenna, which has to move
: in the *opposite* direction to the old mechanically-rotating VORs so that
: the relative phase differences between the AM & FM sub-carrier modulations
: AS RECEIVED AT THE AIRPLANE remain the same as before. (As I said, I'm a
: bit shaky on this part. Corrections welcomed.)
The azimuth-independent signal is transmitted as 30 Hz AM modulation of
the main RF carrier, and is radiated from an omnidirectional antenna at the
center of the array. This signal also carries the ident and voice
modulation.
The azimuth-dependent signal is an unmodulated RF carrier at a frequency
9960 Hz above that of the main carrier, which is radiated from a circular
array of omnidirectional antennas through an electronic commutation
system. This simulates a single antenna moving around a circular path
30 times per second, thus producing azimuth-dependent doppler-fm
modulation of the carrier. A path radius of 2.55 wavelengths (about 23 ft
at 108 MHz) is needed for the required frequency deviation of 480 Hz.
The power level of the azimuth-dependent signal is about 1/16 that of the
main carrier, which produces "beat-modulation" at the AM receiver with a
modulation percentage of about 25%. The 9960 Hz subcarrier is thus
equivalent to single-sideband modulation of the main carrier, rather than
double-sideband as in a conventional VOR signal.
Chris Stratton
>Neither Krause nor Jasik (two of the most respected antenna books in
>print)
>make any mention of FM modulation by means of a rotating antenna, nor
>does
>Hamsher in his "bible" on radio communications. Neither does the ARRL
>Handbook nor any of the half-dozen ARRL Antenna books -- and even if
>they
>did, ARRL publications are widely noted for their lack of technical
>accuracy. Perhaps you could give us chapter and verse for your source
>of
>information? Author, title, and chapter/page reference?
Quickest reference at hand: ARRL Handbook, 1992 ed, page 38-19
or look up "doppler shift direction finders" in the index of
your edition. I expect an apology when you finish reading what
you say is not contained in that volume.
I find it very suprising that someone with your credentials
and experience (especially in phased arrays) would be ignorant
of this effect which arrises whenever an antenna is rotated
(physcially or electrically) around a point which does not coincide with
its center of radiation. The effect becomes significant when the
rotation rate and radius produce a high enough tangential velocity
relative to the carrier frequency to produce meaningfull deviation.
It's also nice to know that I'm not completely wacko, as
others have explained that modern VOR installations do indeed
work as I described.
Chris
Jim Weir
shared these priceless pearls of wisdom:
->>information? Author, title, and chapter/page reference?
->
->Quickest reference at hand: ARRL Handbook, 1992 ed, page 38-19
->or look up "doppler shift direction finders" in the index of
->your edition. I expect an apology when you finish reading what
->you say is not contained in that volume.
As I stated last time, the week before Oshkosh isn't the time to be doing
research. I'll look it up when I get home.
->
->I find it very suprising that someone with your credentials
->and experience (especially in phased arrays) would be ignorant
->of this effect which arrises whenever an antenna is rotated
->(physcially or electrically) around a point which does not coincide with
->its center of radiation.
That's the point, the whole system was intended to mimic the old goniometer
that WAS centered at a point. Ignorant of doppler shift for RDFing? Only
for the last 20 years or so when good PIN diodes became available to do the
process easily.
->It's also nice to know that I'm not completely wacko
That's not my field of expertise. You will have to get another opinion.
, as
->others have explained that modern VOR installations do indeed
->work as I described.
All of them from the UK, and all of them prefacing their statements, "I've
never really done this, but my book says...".
Chris Stratton
> ->others have explained that modern VOR installations do indeed
> ->work as I described.
>
> All of them from the UK, and all of them prefacing their statements, "I've
> never really done this, but my book says...".
Well look what I found on the FAA web site of all places!
http://www.faa.gov/and/and700/and740/dvor.htm
>Doppler VOR (DVOR) Conversion Kits
>
> The DVOR Kit is installed in an existing VOR when some of
> the radials in its service volume are unusable due to natural
> obstructions (trees, etc.) urban encroachment, towered
> structures, and power lines.
Looks like the US FAA does indeed use doppler-based VORs in circumstances where they provide
an advantage. I'd be very suprised if they didn't work in the manner I described, only
rotating in the opposte direction (as others pointed out) to be compatible with aircraft
that expect the opposite relationship between the fixed and rotating signals.
My guess is wholesale conversion is only delayed by the cardinal economic rule of "if it ain't broke
don't fix it" and that any new installations are usually DVOR - but with government procurement you
never can tell. It wouldn't be at all suprising if other countries have a greater installed
DVOR base, hence it's greater familiarity to oversees contributors.
Rob Warnock
| : bit shaky on this part. Corrections welcomed.)
|
| The azimuth-independent signal is transmitted as 30 Hz AM modulation of
| carrier at a frequency 9960 Hz above that of the main carrier... which
| produces "beat-modulation" at the AM receiver... 9960 Hz subcarrier is thus
| double-sideband as in a conventional VOR signal.
Cool! That's even more clever than I thought. Thanks for the correction &
additional details.
-Rob
Mike Wei
did countless instrument approaches in FS98 during my instrument
training. I'm not otherwise a big fan of Microsoft, but this simulator
propably saved me hundreds of bucks.
--Mike
PPASEL-IA
Michael Hoffman wrote:
>
> >Learning VOR usage is one area where the PC simulators really
> >shine. Microsoft FS 98 or Sierra Pro Pilot work very well for
> >learning how to dial in, intercept, and fly a radial.
>
> I completely second that!
>
> My wife took groundschool with me - but without taking actual flying lessons
> as I did (she starts this Sunday). And VOR/ADF navigation was giving her
> lots of trouble, seeing as she'd just enver "experienced" them in the
> cockpit.
>
> So, a few days before the Written, we sat down with our sectional and FS98
> and, while I did the "flying" and pushing of keys, she navigated around our
> airspace, watching the needles until it just went *click*.
>
> Cheers
> Mike
--
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Mike Wei