unreliable VOR navigation
David Doshay
I was headed towards the Gorman VOR from the NorthWest, and
noticed that my VOR indicator needles were swinging back and forth
about 10 or 15 degrees. I saw a building cumulus cloud over the
mountians near the VOR, and guessed that electrical activity in the
cloud was causing the problem. It seemed that the cloud was blowing to
the east of my path, so I continued following the "center" of the
swinging needles. After a few minutes I realized that I was not where
I should have been, and started flying towards where i thought I
should be. That would have put me under the cloud, so I went around
it, and then continued east. All in all, I figured out where I really
was, and flew on to where I was going, but I was shocked to find that
I was, at one point, almost 20 miles west of my intended path. That
meant crossing the mountian range in a place that was VERY rugged,
with few emergency landing spots.
On my way back North, on Sunday, I noticed the same problem once I was
North West of the Gorman VOR, and called Flight Watch to report the
problem. They asked about my readings from other VORs, and said they
had no other pireps about the Gorman VOR. Right after my transmission,
another pilot called to verify my observation of swinging needles,
again 10 to 15 degrees. The next VOR was steady again, so I called
again to let them know that it was not flakey radios in my plane.
Any net.experiance similar to this? Any "Well, hell, what I would have
done ...." ?
I have never been anything remotely like lost before, but the rugged
terrian and a sudden sense of confusion almost had me calling on the
radio with a sheepish "where am I?" I decided to give myself 5
minutes, and in about 2 was sure about my location. My ground track
must have been interesting, because the controller from the Burbank
ARSA called to ask me just what my heading was. Instead of going
direct towards my destination I was heading east towards better
emergency landing sites.
OK, I've asked for it, turn the flames on.
David dos...@soma.arc.nasa.gov
The thought police insist I tell you:
my thoughts, not NASA's
Gregory R. Travis
some positively wacky VOR indications when between VORS that share
a common frequency. Best example that can think of is an area from
about 80 miles south of the Grand Rapids VOR (GRR) to 70 miles north of
the Hoosier VOR (OOM). Both share 110.2 as their frequency (they are
separated by over 200NM).
In this "bermuda" zone the VOR needle will constantly swing back
and forth just like a windshield wiper. Kinda fun. Kinda disconcerting.
And, yes, the behavior happens only well outside the published service
volumes of both VORs (they are low-altitude VORs) so there is
nothing to really complain about; it's just that I'm "used" to getting
60-100 mile range out of most VORs at reasonable altitudes. They
do put out 250 watts, after all!
greg
--
Gregory Reed Travis D P S I
Data Parallel Systems Incorporated gr...@dpsi.com (For MX mailers only!)
Bloomington, IN gr...@indiana.edu (For the others)
Skip Guild
>Subject: unreliable VOR navigation
>On Friday I was flying from the SF Bay area into the LA basin.
>I was headed towards the Gorman VOR from the NorthWest, and
>noticed that my VOR indicator needles were swinging back and forth
>about 10 or 15 degrees...
>Any net.experiance similar to this?
>OK, I've asked for it, turn the flames on.
No flames here. I had the same experience going from Providence, RI to
Albany, NY over relatively flat land. Providence (Ocean) Approach gave me
a clearance "direct Barnes" when I was well within the 40 nm service volume
of the VOR. The OBS was swinging +/- 8 degrees as we tried to intercept
it. I was flying at over 5,000 feet so altitude shouldn't have been a
problem. Informed Approach who then amended the clearance eventually to a
vector to intercept V-130 from Bradley VOR which worked fine.
David Doshay
|> Is it possible that you were too far from the station? ...
|>
|> In this "bermuda" zone the VOR needle will constantly swing back
|> and forth just like a windshield wiper. Kinda fun. Kinda disconcerting.
I was within 40 miles of the station, and close to the station it was
so bad that I did not end up flying anywhere near the station. I almost always
see the station, and this time I was certianly far away.
Jim Schinnerer
"dancing needles syndrome", too. It happens alot when I am relatively
low with respect to the station. Distance matters a little, but altitude
is the prominant factor.
Gorman is on the top of a mountain, so even if you are high over the
Central Valley, you are low over the VOR.
I have been told that VOR's are not designed to work if you are below
them. This could explain why I never used to get Woodside on the
ground at RHV. Of course, I have really bad radios...
---------------------------------------------------------
Jim Schinnerer - PP-ASEL-IA | Hewlett Packard
Mooney - 350X - "Buster" | Vancouver, WA
email - schi...@vcd.hp.com | (206) 944-3324
---------------------------------------------------------
Leads Network News
>"dancing needles syndrome", too. It happens alot when I am relatively
>low with respect to the station. Distance matters a little, but altitude
From: cla...@leadse9.UUCP (Claude Goldsmith)
Path: leadse9!claude
Signal "scalloping" is a constant problem with VOR navigation. A
major contributor to this is "multi-path" propagation. That is, the
signal arrives at a single point in space by 2 or more paths of
different lengths.
An insensitive receiver can complicate the problem by increasing the
amount of "noise" that accompanies the signal. Some designs are
better than others in separating signal from noise. The effect of
noise on the needle can sometimes be similar to multi-path.
Multi-path is caused by reflections of the signal off of terrain and
structures. As a result of those reflections, the strongest signal
that you receive may not actually be the signal for the radial that
you are on. Since the geometry of your position relative to the
station and the reflecting surfaces is constantly changing, so are the
signals that you are receiving. The result is your CDI wandering back
and forth. Multi-path is not a function of the nav receiver. It is a
function of the environment. It is more apparent on some some nav
receivers because the CDI is less damped.
The manufacturer of the equipment knows that wandering CDI's, whether
caused by noise on the signal or multi-path, are annoying to the pilot
and a problem for some autopilots. They include circuitry to
integrate the signal over time and display the results. On those
displays the needle seems to be steady or only move slowly.
Some manufacturers feel that seeing the needle move around gives the
pilot important information about the quality of the received signal;
so they include less damping. It then becomes the pilots
responsibility/task to smooth out (average) the course information.
This same kind of needle dance can also be observed when flying the
LOC/ILS.
Claude Goldsmith, CFII <cla...@sunfse.ese.lmsc.lockheed.com>
twisted-pair (408) 742-7514 | Opinions expressed are not
on the air - WB6UOO | necessarily those of my
through the air - Mooney N6416U | employer.
Pierre Collet
>In article <greg.711685423@octopus>, gr...@octopus.dpsi.com (Gregory R. Travis) writes:
>|> Is it possible that you were too far from the station? ...
>
>so bad that I did not end up flying anywhere near the station. I almost always
>see the station, and this time I was certianly far away.
>
The rule of thumb for VOR range is the following ;
Operating range ~= 1.23 sqrt(altitude above the VOR station)
nm ft
Now 40 = 1.23 sqrt(1060), and I guess you were above 1000ft AGL, so
you very probably were within the range of that VOR.
Blue skies,
Pete.
Bill Hopkins
>
>Signal "scalloping" is a constant problem with VOR navigation. A
>major contributor to this is "multi-path" propagation. That is, the
>signal arrives at a single point in space by 2 or more paths of
>different lengths.
>Multi-path is caused by reflections of the signal off of terrain and
>structures. As a result of those reflections, the strongest signal
>that you receive may not actually be the signal for the radial that
>wandering back and forth. [...] It is more apparent on some some nav
>receivers because the CDI is less damped.
>
>The manufacturer of the equipment knows that wandering CDI's, whether
>caused by noise on the signal or multi-path, are annoying to the pilot
>and a problem for some autopilots. They include circuitry to
>integrate the signal over time and display the results. On those
>displays the needle seems to be steady or only move slowly.
Hang on a minute, here! Typically, multipath propagation will give
one correct signal and one or more incorrect signals. Averaging
(damping) in these cases will guarantee an incorrect indication!
This doesn't seem to be such a great idea...
>Some manufacturers feel that seeing the needle move around gives the
>pilot important information about the quality of the received signal;
>so they include less damping. It then becomes the pilots
>responsibility/task to smooth out (average) the course information.
There's a lot of appeal in this approach. In addition to a quality
indicator, it gives you an opportunity to pick the right reading. In
particular, if you're tracking a stable radial, and the needle starts
oscillating to one side, it's a fair bet that the good signal is
still centered and a bogus one is showing up to cause the deviation.
Centering the oscillation should require a substantial correction,
more at larger distances from the VOR, and that suggests GIGO (garbage
begets garbage) to me. Wouldn't it make sense to keep the oscillation
where it appears (if it can be done without abrupt heading changes)?
This assumes that (a) you're not close to the VOR, (b) the damping
isn't interfering, (c) you're paying enough attention to identify
the good indication, and (d) nothing else is going on that might
cause CDI excursions. This is *only* speculation, of course, but
it *is* testable if you know your position via GPS, pilotage, or
other arcane technology, when scalloping appears.
Bill (isn't this what experimental aircraft are for?) Hopkins
------------------------------------------------------------------------------
hop...@GVL.Unisys.Com Paramax Systems Corporation (A Unisys Company)
215-648-2854 or 363-7464 Great Valley Labs, PO Box 517, Paoli, PA 19301
MXE 016 radial, 8 nmi Opening my mouth is not company policy.
alex france
>In article <1992Jul21....@news.arc.nasa.gov> dos...@ursa.arc.nasa.gov (David Doshay) writes:
>The rule of thumb for VOR range is the following ;
>
>Operating range ~= 1.23 sqrt(altitude above the VOR station)
> nm ft
I've always found it easier to remember as :-
sqrt(1 and 1/2 times ht. in feet)
since this is a bit easier to do in your head. I had a nice practical
confirmation of this last weekend flying Cranfield to Skegness,
tracking the 026 radial of CFD. Flying at 2500 feet it started to
dither about then dropped out over Boston (Lincolnshire, not Mass.!)
which was on the 60 nm mark. On the way back though, I had to be a
good 10 nm miles nearer before it would lock in again - presumably
there is some hysteresis in the sensitivity which causes it to hang
onto a signal that has been locked in, as opposed to picking up a new
weak one.
# Alex France, Crosfield Electronics, | Phone: +44 442 230000 xt.3541 #
# Three Cherry Trees Lane, | Fax: +44 442 232301 #
# Hemel Hempstead, HP2 7RH, England. | Email: a...@crosfield.co.uk #
Bruce G. Bostwick
[lots of stuff about VOR oscillation deleted]
>An insensitive receiver can complicate the problem by increasing the
>amount of "noise" that accompanies the signal. Some designs are
>better than others in separating signal from noise. The effect of
>noise on the needle can sometimes be similar to multi-path.
>
>Multi-path is caused by reflections of the signal off of terrain and
>structures. As a result of those reflections, the strongest signal
>that you receive may not actually be the signal for the radial that
>you are on. Since the geometry of your position relative to the
>station and the reflecting surfaces is constantly changing, so are the
>signals that you are receiving. The result is your CDI wandering back
>and forth. Multi-path is not a function of the nav receiver. It is a
>function of the environment. It is more apparent on some some nav
>receivers because the CDI is less damped.
Don't forget that at certain engine speeds the prop blades can cause
this oscillation -- the interruption frequency of a two-blade (or a
slow three-blade) prop can be rather close to the subcarrier
modulation of the VOR signal (the wavering whistle you hear on the
IDENT mode along with the Morse identifier). At the right combination
of relative bearing to the VOR and engine speed this oscillation can
be almost full-scale. Yes, it's extremely bothersome.
--
<BGB>
li...@ccwf.cc.utexas.edu / "I can't complain,
(really Bruce Bostwick) / but sometimes I still do"
from the great state of TEXAS / --Joe Walsh--