VOR and reverse sensing

Robert Perkins writes:

Regarding position, it's true that one VOR won't tell you your exact
position. But if you combine pilotage from a chart with the relative
position data ("southeast of the station", "west of the station",
whatever.), you will have your position.

You get an approximate position fix from a single VOR transmitter by
flying perpendicular to the radial and timing how long it takes you to
get to a different radial (say, 10 degrees off). It's not all that
practical or accurate in real-life, of course, but it does sort-of
work.

Apologies in advance if I've missed the point of the thread and
duplicated a previous posting.


All the best,


David

--
David Megginson, , http://www.megginson.com/

"gross_arrow" wrote in message
om...


if the vor system is properly explained
to begin with, it is not all that hard to grasp. learning anything
"wrong" would, imho, only lead to subsequent confusion and necessitate
eventual re-learning the correct way.

It's for this EXACT reason that I posted about what Duniho was saying, and
it has nothing to do with the simple 101 fact that a VOR bearing has nothing
to do with aircraft heading. Any pre cross country student should know this.
The problem with what he was saying is that he was STRESSING beyond what was
normal under the circumstances of the discussion, that a single VOR bearing
from a single VOR is sufficient enough to be of great value in estimating a
geographical fix ,which it absolutely is not. Every student from day one
should be taught that a single VOR bearing is just step one in obtaining a
"fix". If a single VOR is all that's available in the aircraft, then a
second VOR should be
selected to establish the cross reference necessary
to establish a "fix". The point I'm making here is a simple one. In teaching
navigation to students, it's my opinion that it's just not appropriate for
ANYONE to stress the importance of a single VOR bearing as sufficient in any
way whatsoever in establishing a fix.
This is a student group; not rec.aviation.pilots. People are learning to fly
over here. There's a right way and a wrong way to approach the issues that
are discussed on this particular group. I have no problem with Duniho per
se'. I don't post to him at all if it's possible to avoid it. If I see
something from ANYONE that peaks my interest as a flight instructor, I
usually post a response. It's not meant to be personal, although in Duniho's
individual case, I'll admit freely that I just don't have any use for the
man. There is much in what he says at times that is both true and useful.
This occasion wasn't one of those times; in my professional opinion anyway.
Dudley Henriques
International Fighter Pilots Fellowship
Commercial Pilot/CFI
Retired

"Dudley Henriques" wrote in message hlink.net...

you are absolutely right about one VOR radial ident NOT being a
positive"position" fix. True, having a directional bearing from a single VOR
is better than having nothing at all, but even if only a single VOR is
involved, it is ALWAYS recommended that a SECOND VOR be chosen, and a cross
check made before considering what you have as a "fix" When discussing a
position FIX, TWO is the magic number, NOT one! The second of the two can be
DME, an ADF bearing, or a second VOR positive radial ident, but when
discussing a position fix, ESPECIALLY on a student newsgroup, using a single
VOR radial as an example of a geographical fix is wrong in my opinion, and I
would urge all students NOT to begin thinking of a positive geographical fix
on these terms.....PERIOD!!!!

Dudley, I know you're aware of this, but since this is a student
newsgroup I think it should be mentioned that it *is* possible to
obtain a reasonable position estimate using a single VOR.

These days, I suppose a pilot is more likely to be in a cockpit
with 2 GPS and no VOR than to be in a cockpit with no GPS and 1 VOR,
but let's suppose that's the situation. Only 1 VOR station can
be received, for some reason.

Of course you're correct that a single VOR simply places you on a line.
So to determine position, we need additional information.

We all know (or can see, if we think about it) that the distance between
VOR radials varies with distance from the station by a function of 1 in
60. At 60 nm from the station, it's roughly 1 nm between radials,
30 nm it's roughly 0.5 nm between radials. IOW, we use the distance
between radials to estimate our distance from the station.

How to do so practically? We use time and our knowledge of our plane's
approximate groundspeed. One centers a radial, then turns the OBS to
deflect the needle 10 degrees, notes the time, and flies until the
needle centers. Note the time again.

Let's say one is flying a typical GA plane with a groundspeed of
about 2 nm/min. Let's say it took 3 minutes to fly 10 degrees.
We calculate our distance as 60 * (2 nm/min * 3 min)/10
or 36 nm from the VOR.

To boil it down:
*know your airplane's approx. speed in nm/min
*calculate 6 * speed in nm/min * time in min to fly 10 deg

Of course, for accuracy, one would need to correct for wind
either by estimating groundspeed, or by flying in both directions
and averaging the time.

I mention it because I was taught this technique as a student, but
many don't seem to be. As I said, Dudley, I'm sure you're aware
of it, and I don't mean to contradict what I see as your basic point
that a single VOR radial by itself does not, in fact, give position
but rather only 1/2 of the information needed to derive position.

The practical aspect of the above method is that awareness of
the distance between radials as a function of distance from the
station helps smooth VOR intercepts, something I need to remind
myself of as I use GPS far more often than VOR these days.

Best,
Sydney

"Snowbird" wrote in message
om...
"Dudley Henriques" wrote in message
hlink.net...

you are absolutely right about one VOR radial ident NOT being a
positive"position" fix. True, having a directional bearing from a single
VOR
is better than having nothing at all, but even if only a single VOR is
involved, it is ALWAYS recommended that a SECOND VOR be chosen, and a
cross
check made before considering what you have as a "fix" When discussing a
position FIX, TWO is the magic number, NOT one! The second of the two
can be
DME, an ADF bearing, or a second VOR positive radial ident, but when
discussing a position fix, ESPECIALLY on a student newsgroup, using a
single
VOR radial as an example of a geographical fix is wrong in my opinion,
and I
would urge all students NOT to begin thinking of a positive geographical
fix
on these terms.....PERIOD!!!!

Dudley, I know you're aware of this, but since this is a student
newsgroup I think it should be mentioned that it *is* possible to
obtain a reasonable position estimate using a single VOR.

These days, I suppose a pilot is more likely to be in a cockpit
with 2 GPS and no VOR than to be in a cockpit with no GPS and 1 VOR,
but let's suppose that's the situation. Only 1 VOR station can
be received, for some reason.

Of course you're correct that a single VOR simply places you on a line.
So to determine position, we need additional information.

We all know (or can see, if we think about it) that the distance between
VOR radials varies with distance from the station by a function of 1 in
60. At 60 nm from the station, it's roughly 1 nm between radials,
30 nm it's roughly 0.5 nm between radials. IOW, we use the distance
between radials to estimate our distance from the station.

How to do so practically? We use time and our knowledge of our plane's
approximate groundspeed. One centers a radial, then turns the OBS to
deflect the needle 10 degrees, notes the time, and flies until the
needle centers. Note the time again.

Let's say one is flying a typical GA plane with a groundspeed of
about 2 nm/min. Let's say it took 3 minutes to fly 10 degrees.
We calculate our distance as 60 * (2 nm/min * 3 min)/10
or 36 nm from the VOR.

To boil it down:
*know your airplane's approx. speed in nm/min
*calculate 6 * speed in nm/min * time in min to fly 10 deg

Of course, for accuracy, one would need to correct for wind
either by estimating groundspeed, or by flying in both directions
and averaging the time.

I mention it because I was taught this technique as a student, but
many don't seem to be. As I said, Dudley, I'm sure you're aware
of it, and I don't mean to contradict what I see as your basic point
that a single VOR radial by itself does not, in fact, give position
but rather only 1/2 of the information needed to derive position.

The practical aspect of the above method is that awareness of
the distance between radials as a function of distance from the
station helps smooth VOR intercepts, something I need to remind
myself of as I use GPS far more often than VOR these days.

Best,
Sydney


What you are saying here is completely accurate. However, if you read back
through Duniho's posts on this thread, there is not ONE instance where he
even comes close to mentioning what you and I, and I assume Duniho knows as
well, is the correct procedure for establishing a reasonable distance check
from a single VOR using the procedure you have outlined. His entire context
deals directly with the premise that a single VOR bearing used alone is
valuable POSITIONAL information. This is not true at all. He does mention
that additional information is "helpful", but he is STRESSING the point that
a single bearing alone is valuable positional information.
The reason I entered a reply was because he was "correcting" someone who had
correctly stated that a single bearing from a VOR was only that....a single
bearing, and NOT a position fix, which is absolutely correct. In fact,
Duniho's opening statement was
"How is this not a position fix?"
As I said, this is a student group. Pilots who post here should be keenly
aware of that. In aviation training, we work fairly hard to get student
pilots to understand that TWO sources are required for position fixes, not
ONE! That second source can indeed be a second VOR bearing, an ADF bearing,
a DME readout, or indeed, a distance check on a single radial as you have
mentioned. If Duniho or anyone else for that matter wants to post here that
a single VOR bearing is valuable positional information, they should include
the need for that second source as PARAMOUNT to obtaining a position fix,
and not simply mention a second source as being desirable, but not
necessary, as did Duniho in his explanations on this thread.
That's my opinion, and as a professional flight instructor, I'll go with
that.
Dudley Henriques
International Fighter Pilots Fellowship
Commercial Pilot/CFI
Retired

Dudley, I know you're aware of this, but since this is a student
newsgroup I think it should be mentioned that it *is* possible to
obtain a reasonable position estimate using a single VOR.

No offense intended, but you might have missed a crucial point in
earlier posts in this thread. There was a contention that position could be
derived from a single VOR simply by knowing which radial the airplane was on
at a given moment.

Of course you're correct that a single VOR simply places you on a line.
So to determine position, we need additional information.

Which is exactly my original point...

...that a single VOR radial by itself does not, in fact, give position
but rather only 1/2 of the information needed to derive position.

Thanks, Sydney

On 20 Aug 2003 04:33:13 -0700, (Snowbird)
wrote:

. . . One centers a radial, then turns the OBS to
deflect the needle 10 degrees, notes the time, and flies until the
needle centers. Note the time again.

1) I think you mean that you offset the OBS by ten degrees from what
it reads when the needle is centered. (The needle itself not being
conspicuously calibrated in degrees.)

2) Don't you then need to fly a course at right angles to the bearing
TO/FROM the station, in the direction that will intercept the new OBS
setting? For example, if your track coincided with the bearing TO the
station, the needle would only center when you were (nearly) over the
station. Your distance from the station would then be (essentially)
zero, regardless of flight time. (There's an even more trivial case
if you fly outbound on the radial -- you eventually fly off the
chart.)

I'm not an instructor, but my inclination would be to tell a student
to save this cute stuff for when he/she's bored on a long
cross-country. If you really need to know where you are, don't screw
around. Use multiple VOR stations and triangulate.

Don

On 20 Aug 2003 04:33:13 -0700, Snowbird wrote:

[snip]

How to do so practically? We use time and our knowledge of our plane's
approximate groundspeed. One centers a radial, then turns the OBS to
deflect the needle 10 degrees, notes the time, and flies until the
needle centers. Note the time again.

[double-snip]

I mention it because I was taught this technique as a student, but
many don't seem to be.

FWIW, this technique is the subject of a couple of questions in the
IFR written test question bank. It's an interesting academic exercise,
but I can't see much practical use for it, especially IFR.


Morris

On Thu, 21 Aug 2003 02:19:32 -0000,
(journeyman) wrote:


FWIW, this technique is the subject of a couple of questions in the
IFR written test question bank. It's an interesting academic exercise,
but I can't see much practical use for it, especially IFR.

I believe it's a variation on a technique in *American Practical
Navigator* (Bowditch) for determining how far a sailing vessel is off
a lee-shore headland. It was probably known to Odysseus.

Don

"Casey Wilson" wrote in message .. .

Dudley, I know you're aware of this, but since this is a student
newsgroup I think it should be mentioned that it *is* possible to
obtain a reasonable position estimate using a single VOR.

No offense intended, but you might have missed a crucial point in
earlier posts in this thread. There was a contention that position could be
derived from a single VOR simply by knowing which radial the airplane was on
at a given moment.

No offense taken, and I didn't miss that point at all.

I thought I said so, but, it was probably too well hidden by too
many other words

My intention was only to explain an additional technique.

Perhaps a clearer way to make my point would have been to say,
while I agree two of *something* is needed, a 2nd VOR or ADF is
not needed; a 2nd radial from the same VOR, and a method of determining
the distance between them, may substitute.

Cheers,
Sydney

Don Tuite wrote in message . ..

On Thu, 21 Aug 2003 02:19:32 -0000,
(journeyman) wrote:
FWIW, this technique is the subject of a couple of questions in the
IFR written test question bank. It's an interesting academic exercise,
but I can't see much practical use for it, especially IFR.

Morris, as I think I mentioned, for the technique itself I concur.

The IFR written questions, though, are just geared towards seeing
if the pilot is aware of the relationship between distance from
the VOR vs. distance between radials.

I do think there's some practical use for that awareness. For
one thing, it allows one to understand what the error may be on
a VOR approach where the VOR is 20-30 nm off the field (there
are a number of those around here). For another, it is helpful
in understanding how to intercept a radial smoothly at varying
distances from the station, something at which I could improve.

I believe it's a variation on a technique in *American Practical
Navigator* (Bowditch) for determining how far a sailing vessel is off
a lee-shore headland. It was probably known to Odysseus.

Interesting, Don. I suppose it's off-topic, but could you explain?
The reference to Odysseus implies that it's very low technology.

It may have been known to Odysseus, but my impression is, it's
not known to many of today's pilots . We just had our #2 OBS
repaired, and our avionics repairman opined that 9 out of 10
pilots couldn't find our airport from a nearby VOR with 2 working
NAVS, 2 working OBS, 2 hands, and a flashlight. He might have a point.

Thanks,
Sydney