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

ILS Errors

Thu Sep 13, 2012 7:25 pm

"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
 
Horstroad
Posts: 344
Joined: Thu Apr 08, 2010 8:19 pm

RE: ILS Errors

Thu Sep 13, 2012 11:36 pm

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

Fri Sep 14, 2012 3:44 am

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: 653
Joined: Tue Dec 19, 2006 5:00 pm

RE: ILS Errors

Fri Sep 14, 2012 8:44 am

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...
 
Horstroad
Posts: 344
Joined: Thu Apr 08, 2010 8:19 pm

RE: ILS Errors

Fri Sep 14, 2012 9:28 am

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: 1141
Joined: Tue Aug 16, 2005 1:30 am

RE: ILS Errors

Fri Sep 14, 2012 12:53 pm

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

Fri Sep 14, 2012 10:46 pm

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.

http://www.moonraker.com.au/techni/horizontals.jpg

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

Sat Sep 15, 2012 9:09 pm

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

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