I recall reading about false lobes on the glideslope, and have seen it
firsthand doing practice approaches in VMC. I thought it was due to antenna
sidelobes, and thus the localizer might be subject to a similar phenomenon. I have
yet to see anyone reference this, so is there a technical reason why they don't exist?
One of the localizer transmitter frequencies have a different antenna pattern to
interleave the sidelobes perhaps?

-Cory

--

************************************************** ***********************
* Cory Papenfuss, Ph.D., PPSEL-IA *
* Electrical Engineering *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

wrote:
> I recall reading about false lobes on the glideslope, and have seen it
> firsthand doing practice approaches in VMC. I thought it was due to antenna
> sidelobes, and thus the localizer might be subject to a similar phenomenon. I have
> yet to see anyone reference this, so is there a technical reason why they don't exist?
> One of the localizer transmitter frequencies have a different antenna pattern to
> interleave the sidelobes perhaps?
>
> -Cory
>

If properly maintained they cancel out. However, it can be a
major problem if the facility is not maintained correctly.

The technical term is null

On Mon, 23 Oct 2006 13:42:45 +0000 (UTC),
wrote:

> I recall reading about false lobes on the glideslope, and have seen it
>firsthand doing practice approaches in VMC. I thought it was due to antenna
>sidelobes, and thus the localizer might be subject to a similar phenomenon. I have
>yet to see anyone reference this, so is there a technical reason why they don't exist?
>One of the localizer transmitter frequencies have a different antenna pattern to
>interleave the sidelobes perhaps?

I've observed these routinely while being vectored to the IAF. Unless
you're approximately on the approach course, the CDI may be pointing
anywhere. While being vectored around the airport toward the IAF, you
can expect the CDI to be flipping back and forth like a windshield
wiper. I don't notice this while flying the full ILS (ADF to LOM,
etc.) I suppose it's because in that case I'm fairly close to the
localizer. When ATC is vectoring, they usually keep me far from the
airport. That far out, anything can happen. I've always assumed it was
because the localizer is formed by an array of antennas and a stack of
antennas is always going to have multiple nulls, especially at extreme
angles. Another factor might be the mountain ridges surrounding the
airport.

The point is, you have to be at the specified point in space before
you can trust either localizer or GS.

RK Henry

WhatEver > wrote:
: wrote:
: > I recall reading about false lobes on the glideslope, and have seen it
: > firsthand doing practice approaches in VMC. I thought it was due to antenna
: > sidelobes, and thus the localizer might be subject to a similar phenomenon. I have
: > yet to see anyone reference this, so is there a technical reason why they don't exist?
: > One of the localizer transmitter frequencies have a different antenna pattern to
: > interleave the sidelobes perhaps?
: >
: > -Cory
: >

: If properly maintained they cancel out. However, it can be a
: major problem if the facility is not maintained correctly.

: The technical term is null

I've never heard of it called a null. I've only heard that referrered to
either a standing wave null, or a part of an antenna radiation pattern that has zero
radiation. Since the localizer relies on relative strength of two different
transmissions, it seems liek having overlapping sidelobes in their radiation patterns
would produce "false localizers"

-Cory

--

************************************************** ***********************
* Cory Papenfuss, Ph.D., PPSEL-IA *
* Electrical Engineering *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

: > I recall reading about false lobes on the glideslope, and have seen it
: >firsthand doing practice approaches in VMC. I thought it was due to antenna
: >sidelobes, and thus the localizer might be subject to a similar phenomenon. I have
: >yet to see anyone reference this, so is there a technical reason why they don't exist?
: >One of the localizer transmitter frequencies have a different antenna pattern to
: >interleave the sidelobes perhaps?

: I've observed these routinely while being vectored to the IAF. Unless
: you're approximately on the approach course, the CDI may be pointing
: anywhere. While being vectored around the airport toward the IAF, you
: can expect the CDI to be flipping back and forth like a windshield
: wiper. I don't notice this while flying the full ILS (ADF to LOM,
: etc.) I suppose it's because in that case I'm fairly close to the
: localizer. When ATC is vectoring, they usually keep me far from the
: airport. That far out, anything can happen. I've always assumed it was
: because the localizer is formed by an array of antennas and a stack of
: antennas is always going to have multiple nulls, especially at extreme
: angles. Another factor might be the mountain ridges surrounding the
: airport.

: The point is, you have to be at the specified point in space before
: you can trust either localizer or GS.

: RK Henry

I've definately seen it with the GS, just never with localizer.

The reason it came up is I was doing some safety-pilot with a friend. They'd
never heard of "false lobes." Of course, we're non-radar, VFR, not talking to anyone.
While flying a VOR radial towards intercepting the localizer, I said it's best to get
another reliable means (e.g. DME or another VOR radial) to get in the ballpark of the
IAF on the LOC. After getting close (but not TOO close), flipping to the LOC and
waiting for it to come in was the way to get accurately on the LOC.

In other words, two "extremes:
1. If one waits for the cross VOR radial to come it, they can be so inaccurate that
one could blow through the LOC before knowing it.
2. If one drives down a radial (possibly a number of miles) with a LOC tuned hoping to
cross it, one of the sidelobes might make you think you're establish... but you're
actually not there yet.

Reasonable summary?

-Cory

--

************************************************** ***********************
* Cory Papenfuss, Ph.D., PPSEL-IA *
* Electrical Engineering *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

wrote:
> I recall reading about false lobes on the glideslope, and have seen it
> firsthand doing practice approaches in VMC. I thought it was due to antenna
> sidelobes, and thus the localizer might be subject to a similar phenomenon. I have
> yet to see anyone reference this, so is there a technical reason why they don't exist?
> One of the localizer transmitter frequencies have a different antenna pattern to
> interleave the sidelobes perhaps?
>
> -Cory
>

There are some LOCs that are limited in the ability to provide
transitions because of false on-course indications. Seems I recall
Billings, Montana was (is) one of those.

Sam Spade wrote:

> There are some LOCs that are limited in the ability to provide
> transitions because of false on-course indications. Seems I recall
> Billings, Montana was (is) one of those.

I wonder whether this current NOTAM is an example:

!RDU 04/178 RDU 23R ILS LLZ UNUSBL BYD 20 DEGREES RIGHT OF COURSE

wrote:
> I recall reading about false lobes on the glideslope, and have seen it
> firsthand doing practice approaches in VMC. I thought it was due to antenna
> sidelobes, and thus the localizer might be subject to a similar phenomenon. I have
> yet to see anyone reference this, so is there a technical reason why they don't exist?

They exist. On the ILS 35 SGR, which calls for tracking the LOC
outbound on the missed, I once (early in my training) tracked one
outbound. It was good to about 5 nm from the station, and intermittent
to about 10 nm where it died out.

Michael

wrote:
> I recall reading about false lobes on the glideslope, and have seen it
> firsthand doing practice approaches in VMC. I thought it was due to antenna
> sidelobes, and thus the localizer might be subject to a similar phenomenon. I have
> yet to see anyone reference this, so is there a technical reason why they don't exist?
> One of the localizer transmitter frequencies have a different antenna pattern to
> interleave the sidelobes perhaps?


Yes, but not because of the same cause as the glideslope.

The false centerlines on localizer are caused by terrain
(building etc.) reflections (of course, there is always
some kind of back course, but it is often unusable).

The localizer signal is a combination of three signals
sent from the antenna array: left sideband signal, center-
line signal and right sideband signal. The left and right
sideband signals are mirror images of each other, and they
are phased 90 degrees off the centerline. The technical
terms are CSB (carrier and sidebands) for the centerline
signal and SBO (sideband only) for the left and right
signals.

The final received signal is the sum of the three signals
at the receiver antenna. The path length difference of the
CSB and SBO signals causes phase change which in turn causes
differences in received modulation depth.

The glideslope signal is built in a similar way, but there's
a problem, as the upper and centerline antennas are easy to
build into the antenna pole, but the lower sideband antenna
should be below ground level. The solution is to create the
lower side signal from a ground reflection as a mirror image
of the upper signal, but this technique brings other reflections
causing the false glideslopes above the correct one.

--

Tauno Voipio, MSEE (also CPL(A))
tauno voipio (at) iki fi

Dave Butler wrote:
> Sam Spade wrote:
>
>> There are some LOCs that are limited in the ability to provide
>> transitions because of false on-course indications. Seems I recall
>> Billings, Montana was (is) one of those.
>
>
> I wonder whether this current NOTAM is an example:
>
> !RDU 04/178 RDU 23R ILS LLZ UNUSBL BYD 20 DEGREES RIGHT OF COURSE

The most common cause is a terrain reflection which
is not easily blown away.

--

Tauno Voipio
tauno voipio (at) iki fi

Tauno Voipio wrote:
>
> The localizer signal is a combination of three signals

Whoah. Hang on. I've learned (from appropriate FAA publications) that
the localizer was made up of two signals, one modulated at 150 Hz and
the other at 90 Hz, each sent in lobes on either side of the centerline
and the localizer reciever just compares the relative strength of each
signal. Is that right, wrong, or overly simplified?

thanks,
Peter

On 23 Oct 2006 22:11:08 -0700, wrote:

>
>Tauno Voipio wrote:
>>
>> The localizer signal is a combination of three signals
>
>Whoah. Hang on. I've learned (from appropriate FAA publications) that
>the localizer was made up of two signals, one modulated at 150 Hz and
>the other at 90 Hz, each sent in lobes on either side of the centerline
>and the localizer reciever just compares the relative strength of each
>signal. Is that right, wrong, or overly simplified?
>
>thanks,
>Peter


You are exactly right. If you want to be picky, there is also a 1020
Hertz AM modulated Morse code identifier, but it does no add to the
operation of the needle.

Yes, it happens. We have that problem here in Billings. It is for that
reason that our RY28R ILS has no procedure turn. It's also why I have
two marks on my radar scope, one north and one south. The aircraft must
be at or inside these marks beore I can turn an aircraft on the
localizer and issue the clearance. If I do it too soon the aircraft may
grab the wrong localizer.



wrote:
> I recall reading about false lobes on the glideslope, and have seen it
> firsthand doing practice approaches in VMC. I thought it was due to antenna
> sidelobes, and thus the localizer might be subject to a similar phenomenon. I have
> yet to see anyone reference this, so is there a technical reason why they don't exist?
> One of the localizer transmitter frequencies have a different antenna pattern to
> interleave the sidelobes perhaps?
>
> -Cory
>

wrote:
> Tauno Voipio wrote:
>
>>The localizer signal is a combination of three signals
>
>
> Whoah. Hang on. I've learned (from appropriate FAA publications) that
> the localizer was made up of two signals, one modulated at 150 Hz and
> the other at 90 Hz, each sent in lobes on either side of the centerline
> and the localizer reciever just compares the relative strength of each
> signal. Is that right, wrong, or overly simplified?

That's right - after the three components are combined in
the receiver antenna.

All the three components carry both 90 Hz and 150 Hz modulation,
but they are all different in such a way that being off the
centerline causes an increase of the modulation depth (sound
stregth) one modulation frequency and a decrease of the other.
This is where the phase differences are needed.

--

Tauno Voipio
tauno voipio (at) iki fi

On Tue, 24 Oct 2006 17:35:41 GMT, Tauno Voipio
> wrote:

wrote:
>> Tauno Voipio wrote:
>>
>>>The localizer signal is a combination of three signals
>>
>>
>> Whoah. Hang on. I've learned (from appropriate FAA publications) that
>> the localizer was made up of two signals, one modulated at 150 Hz and
>> the other at 90 Hz, each sent in lobes on either side of the centerline
>> and the localizer reciever just compares the relative strength of each
>> signal. Is that right, wrong, or overly simplified?
>
>That's right - after the three components are combined in
>the receiver antenna.
>
>All the three components carry both 90 Hz and 150 Hz modulation,
>but they are all different in such a way that being off the
>centerline causes an increase of the modulation depth (sound
>stregth) one modulation frequency and a decrease of the other.
>This is where the phase differences are needed.

A very interesting discussion and nice to have a technical
explanation. It makes more sense as a simple signal strength change
would be likely to be unreliable.

Thanks for the explanation of the glideslope antenna. I always thought
it was the ground reflection that caused the unwanted lobe and had not
appreciated the ground was deliberately used to simulate the
underground antenna as well as introducing the extra unwanted lobe(s).