novice
Topic Author
Posts: 90
Joined: Tue Aug 07, 2012 2:55 pm

### ILS Errors

"False glide slope. The ground transmitter creates a mirror 150-Hz side lobe that overlaps the top of the 90-Hz main lobe to produce a false glide slope at approximately twice the angle of incidence above the real glide slope (e.g., 3 degrees real glide slope, 6 degrees false glide slope)."

Can anyone help me understand this error what it means by it creates a mirror 150-HZ side lobe that overlaps the top of the 90-Hz main lobe, i'm not aware of what lobes are

Thanks

Posts: 421
Joined: Thu Apr 08, 2010 8:19 pm

### RE: ILS Errors

the transmitted frequencies have the shape of a lobe. there are 4 lobes, 2 for glide slope, 2 for localizer, each a pair of 90Hz and 150Hz modulated.

 Quote:[...] One is modulated at 90 Hz, the other at 150 Hz and these are transmitted from separate but co-located antennas. Each antenna transmits a narrow beam, one slightly to the left of the runway centerline, the other to the right. The localizer receiver on the aircraft measures the difference in the depth of modulation (DDM) of the 90 Hz and 150 Hz signals. The difference between the two signals varies depending on the position of the approaching aircraft from the centerline. If there is a predominance of either 90 Hz or 150 Hz modulation, the aircraft is off the centerline.

similar principle for the glide slope.

if there is a second mirrored 150Hz lobe overlapping the 90Hz lobe, another area without a predominance of either signal, so to say another glide slope, is created.

KELPkid
Posts: 5247
Joined: Wed Nov 02, 2005 5:33 am

### RE: ILS Errors

This is why most localizer-only approaches OR LDA approaches OR localizer back course approaches that share the localizer with an ILS tell you to disregard the glideslope
Celebrating the birth of KELPkidJR on August 5, 2009 :-)

9VSIO
Posts: 654
Joined: Tue Dec 19, 2006 5:00 pm

### RE: ILS Errors

Would this be why you always intercept the GS from below? To avoid the false GS?
Me: (Lining up on final) I shall now select an aiming point. || Instructor: Well, I hope it's the runway...

Posts: 421
Joined: Thu Apr 08, 2010 8:19 pm

### RE: ILS Errors

 Quoting 9VSIO (Reply 3):Would this be why you always intercept the GS from below? To avoid the false GS?

you intercept from below because overshooting above the glide slope is not as bad as overshooting below it. also you have more potential energy (and probably more kinetic energy as well) to kill when you come from above, so the chance to overspeed is higher.

Fabo
Posts: 1154
Joined: Tue Aug 16, 2005 1:30 am

### RE: ILS Errors

Besides, you dont ALWAYS intercept from below, at least not anymore. In some airports (hello London) it is relatively common to intercept from above. You have to know how to handle it though. Level portion before intercept from below is great for slowing down, but there is no level portion on CDA with intercept from above, so you need to configure your plane with this in mind.

False intercept is a risk when intercepting from above, but the fact that you would capture it at more than 5000ft AAL at about 7.5-8 miles out (instead of 2500) and that your descend would be about two times faster (easily over 1500fpm) should make it relatively easy to prevent/recognize.
(small rule of thumb for final descent, anything above 1000fpm at a reasonable speed for a jet airliner should make you question yourself)
The light at the end of tunnel turn out to be a lighted sing saying NO EXIT

mmedford
Posts: 449
Joined: Mon Nov 05, 2007 5:54 pm

### RE: ILS Errors

 Quoting horstroad (Reply 1):the transmitted frequencies have the shape of a lobe. there are 4 lobes, 2 for glide slope, 2 for localizer, each a pair of 90Hz and 150Hz modulated.

Not quite;

They are referring to the propagation of the array; which creates multiple lobes in the pattern, but we are only concerned about the one that creates the glidepath we are looking for.

The idea of ILS is concentrated around space modulation and phasing; we radiate multiple signals in each antenna of the array the create the effect we are looking for. This happens with localizers and glideslopes.

I deal with mostly whats known as capture effect; image type glideslopes...it has a bit of extra protection built into them; to prevent CFIT.
ILS = It'll Land Somewhere

novice
Topic Author
Posts: 90
Joined: Tue Aug 07, 2012 2:55 pm

### RE: ILS Errors

Thanks for your helps guys, i understand it a bit better now

### Who is online

Users browsing this forum: n757kw and 30 guests

### Popular Searches On Airliners.net

Top Photos of Last:   24 Hours  •  48 Hours  •  7 Days  •  30 Days  •  180 Days  •  365 Days  •  All Time

Military Aircraft Every type from fighters to helicopters from air forces around the globe

Classic Airliners Props and jets from the good old days

Flight Decks Views from inside the cockpit

Aircraft Cabins Passenger cabin shots showing seat arrangements as well as cargo aircraft interior

Cargo Aircraft Pictures of great freighter aircraft

Government Aircraft Aircraft flying government officials

Helicopters Our large helicopter section. Both military and civil versions

Blimps / Airships Everything from the Goodyear blimp to the Zeppelin

Night Photos Beautiful shots taken while the sun is below the horizon

Accidents Accident, incident and crash related photos

Air to Air Photos taken by airborne photographers of airborne aircraft

Special Paint Schemes Aircraft painted in beautiful and original liveries

Airport Overviews Airport overviews from the air or ground

Tails and Winglets Tail and Winglet closeups with beautiful airline logos