tsra
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ILS Antenna Question

Thu Mar 04, 2010 6:09 am

Quick question - Can the ILS GS antenna be moved up (raised in height) and still work. For example, say the airport wants to build a hangar in the ILS crictical area, can the airport raise the antenna so the GS beam is abouve the hangar? (Assume the hangar is outside the runway safety area to the left or right of the runway)
 
vandenheuvel
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RE: ILS Antenna Question

Thu Mar 04, 2010 9:54 am

Except for an ILS downgrade, I don't think much can be done.

If the hanger is not placed on the extend of the runway centerline I don't think it has much to do with the GS antenna.

Also I am not sure if the ILS can operate if the lower radio beam is onbstructed. As both, the lower, and the higher beam are nescesarry for the aircraft to determine its position.

Maybe it's theoratically possible to change it's frequency to tighten the GS antenna's range.

Not really an answer, just some more subjects to be discussed.

[Edited 2010-03-04 01:55:34]

[Edited 2010-03-04 01:55:56]
 
KELPkid
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RE: ILS Antenna Question

Thu Mar 04, 2010 4:19 pm

Quoting TSRA (Thread starter):
Quick question - Can the ILS GS antenna be moved up (raised in height) and still work. For example, say the airport wants to build a hangar in the ILS crictical area, can the airport raise the antenna so the GS beam is abouve the hangar? (Assume the hangar is outside the runway safety area to the left or right of the runway)

If they're willing to pay the FAA to re-certify the approach (by flight testing it many, many times), create a new approach plate, and determine the new minimums for the approach, I suppose it oculd be done. In the meantime, the approach would be downgraded to a localizer approach (assuming the procedure is valid localizer only...). I was getting my instrument at a field where they lengthened the approach end of the (ILS) runway while I was flight training...it was done because the field didn't have the FAA recommended 1000' overrun area before encountering a road. At the same time, they shortened the other end of the runway by taking out the same distance on the other end (why they didn't just paint it with chevrons is beyond me). In the end, the localizer-not relocated to the new threshhold now sits in the grass, all on it's own   Looks a little goofy...
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Jetlagged
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RE: ILS Antenna Question

Thu Mar 04, 2010 6:41 pm

Quoting vandenheuvel (Reply 1):
If the hanger is not placed on the extend of the runway centerline I don't think it has much to do with the GS antenna.

The GS antenna is located to one side of the runway, alongside the touchdown point, typically 1,000 ft down the runway from the threshold. Raising it would effectively move the touchdown point further down the runway (assuming the glideslope angle remained the same). Also, reflections from the ground create the lower lobe of the beam, so proximity to the ground is important. I'm not a radio engineer, but I would suspect that raising the GS antenna onto the roof of a building is not an option. A building that close to the runway edge is not really a good idea anyway.  
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Boeing1970
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RE: ILS Antenna Question

Thu Mar 04, 2010 7:49 pm

Quoting TSRA (Thread starter):
Quick question - Can the ILS GS antenna be moved up (raised in height) and still work. For example, say the airport wants to build a hangar in the ILS crictical area, can the airport raise the antenna so the GS beam is abouve the hangar? (Assume the hangar is outside the runway safety area to the left or right of the runway)


Its not an issue of moving it up or down, or if it will work. A hanger that close to a runway is going to penetrate the lateral approach surfaces negating your ability to have a precision approach.

[Edited 2010-03-04 11:52:12]
 
tsra
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RE: ILS Antenna Question

Thu Mar 04, 2010 9:24 pm

Thanks for all the responses!

Quoting Boeing1970 (Reply 4):
A hanger that close to a runway is going to penetrate the lateral approach surfaces negating your ability to have a precision approach.

What distances from the approach end are we talking about here? I know there is the 250 ft runway safety area on both sides of a runway and the 1000 ft at the end (in some cases). There is also the 300 ft, I think, obstacle free zone. I am just curious at what options airports have in airfield expansion where room to grow is scarse, i.e. TEB. I also know LPV WAAS approaches out number the ILS and will soon be the perfered method for percision approaches negating the ILS antennas and ILS critical area.
 
Boeing1970
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RE: ILS Antenna Question

Fri Mar 05, 2010 12:46 am

Quoting TSRA (Reply 5):
What distances from the approach end are we talking about here? I know there is the 250 ft runway safety area on both sides of a runway and the 1000 ft at the end (in some cases). There is also the 300 ft, I think, obstacle free zone. I am just curious at what options airports have in airfield expansion where room to grow is scarse, i.e. TEB. I also know LPV WAAS approaches out number the ILS and will soon be the perfered method for percision approaches negating the ILS antennas and ILS critical area.

For the lateral:

Its a calculation based on the several factors and the desired minima....

http://www.faa.gov/airports/resource...ument.information/documentID/23182

Read Chapter 3. Section 306.

For starters, the distance from the runway centerline to a fixed or moveable object (NAVAIDs don't count) is 500-feet lateral from teh centerline, thats 250-feet further than the lateral component of the runway safety area. The rest depends on the the imaginary surface which is a stepped surface based on field elevation, desired precision approach capability and wingspan. They used to use a building restriction line which was typically 750-feet from the runway centerline, but they are moving to approach surface based restrictions.

Beyond the runway you can't have anything within the RPZ, which is minima based. Thats in Chapter 2.

Also, ILS isn't going anywhere soon. In fact, the FAA is spending several million right now replacing older ILS systems at major airports and some key GA airports.
 
tsra
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RE: ILS Antenna Question

Fri Mar 05, 2010 3:12 pm

Awesome! Great info, thanks!
 
FredT
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RE: ILS Antenna Question

Fri Mar 05, 2010 7:39 pm

Quoting TSRA (Thread starter):
Can the ILS GS antenna be moved up (raised in height) and still work.

No.

The heights of the antennae (there are two or three of them, depending on the type of GP installation) is calculated to give the correct signal characteristics for the GP. It is a very exact science. Moving them is a surefire way to screw the GP up horribly, but will not decrease the sensitivity to obstructions in the reflection area.

Quoting jetlagged (Reply 3):
Raising it would effectively move the touchdown point further down the runway

It is slightly more complicated than that, I'm afraid. The signal in space depends on the interaction of the emitted signals from the two or three antennae, and also the same signals reflected in the reflection plane in front of the antenna. You have a set of antennae, and a set of imaginary antennae underground due to the reflection. The combined signal pattern is what builds the GP signal, so moving the antennae up will not mean simply moving the GP upwards.

The worst job on earth is when you have to clean the imaginary antennae. Endless amounts of digging... good thing there's always The New Guy to assign to the task!
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KELPkid
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RE: ILS Antenna Question

Fri Mar 05, 2010 10:18 pm

Quoting FredT (Reply 8):
The worst job on earth is when you have to clean the imaginary antennae. Endless amounts of digging... good thing there's always The New Guy to assign to the task!

Why do I get the sense that this ranks up there along with asking the new lineboy to go to the mechanics and fetch a bucket of prop wash?   
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avt007
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RE: ILS Antenna Question

Sat Mar 06, 2010 3:17 am

Wouldn't it be easier to do it like the bad guys in the movie Diehard? Apparently you can move the GS from a computer, no digging or unbolting antennae there....   
 
FredT
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RE: ILS Antenna Question

Sat Mar 06, 2010 12:58 pm

Quoting KELPkid (Reply 9):
Why do I get the sense that this ranks up there along with asking the new lineboy to go to the mechanics and fetch a bucket of prop wash?

That's what you use to clean the imaginary antennae once you've dug them up! How did you know?! Tell'em to pick it up while they're getting the shovel... 
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KELPkid
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RE: ILS Antenna Question

Tue Mar 09, 2010 6:54 am

A little birdie (who happens to be knowledgable in these matters-maybe this birdie likes to roost on G/S antennae   ) tells me via e-mail:

Quote:
"the post about the ILS antenna specifically with a glideslope...

An Image type glideslope...radiates a signal that reflects off the ground within 1000 feet of the actual antenna. To the Aircraft receiver it appears that 3 antennas exist below ground, which creates that "centerline" and gives you a 0 DDM (difference in the depth of modulation)."
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