ILS Glideslope Antenna Positioning

[FCSoverride]

I'm working on control laws for an auto approach system. To test them, we are using a simple model of the ILS to provide inputs to the control laws and aircraft model. One issue I have noticed is that we go out of glideslope coverage before we have finished using the signal. I have some numbers for the glideslope antenna position in our model. These are 750 to 1250ft down the runway from the threshold, and offset 400 to 600ft to the side of the runway centerline. The beamwith we have assumed is 8deg azimuth (16deg total from one side of coverage to the other). If I do the trig, and also draw myself a picture, it seems that the coverage will always stop short of the threshold. Depending on antenna position, we would loose the signal in our model at heights above the ground of 149 to 223ft whilst on the extended runway centerline on a 3deg glidepath. This can't be right? I realise that the glideslope signal is not valid right down to touchdown (full autoland systems use a flare mode in the control laws for the touchdown phase). Could somebody point me to some correct glideslope antenna position information please? I'm convinced our modelled system has the antenna inappropriately positioned. Also, if anyone has some general info on where the glideslope signal cannot be used, in terms of validity and also in terms of reception, I'd be very grateful.
Cheers,

P.S. I should say more clearly that I am talking about azimuth coverage.

[matkat]

I can only talk for the B747 but guess others are the same the no. 1 antenna is situated behind the nose gear and picks up the beam as the aircraft is slightly nose up to the ground, when the gear is lowered the No. 2 antenna which is situatated on the front of the nose gear door is automatically switched on and the No1 switched off thereby leaving a direct line of sight from source to antenna and maintaining ILS beam authority.

[Blenk]

Your numbers for the GP position sound about right, with a bit of zooming in you could find some accurate (enough) real data just by using Google Earth if you need better data.

I don't think that the problem is the position of the GP in your algorithm but more the definition of azimuth you are using. Are you taking the origin of the +/-8deg from the GP itself? From memory and without consulting ICAO Annex 10 (the Bible if you like on all to do with Nav aid design, specifications & operational requirements) the +/-8degrees is from CL of the runway so the actual RF coverage of the GP antenna is much more. I've seen coverage well outside of this its obviously is a function of the antenna type.

You can use the GP signal to well past the threshold but it becomes non-linear and flare up. this is all due to how the GP guidance signal is formed, relative positions of antennas on the GP mast and the ground in front of the antenna, interesting stuff if you're really into RF.

Hope this all helps, good luck.

[FCSoverride]

Thanks for the very prompt responses! Blenk, I think you may have hit the nail on the head. I am using 8deg from the antenna location, rather than the runway centerline.

Cheers.

[NSEU]

"I can only talk for the B747 but guess others are the same the no. 1 antenna is situated behind the nose gear "

I digress, but the 747 (Classic) GS capture and track antennae (or no.1 & no.2 as you call them) are all located on the small undercarriage doors on either side of the nose gear strut (not behind the nose gear). The capture antennae are the only antennae exposed to the ILS radio beam when the gear is up (and are found on the outer skin of the small doors). The Track antennae are located on the leading edges of the small doors and are only exposed when the gear is down.

The 744 is a different story...

Rgds
NSEU

[ComJam]

From our perspective (as a flight-checker) the Glideslope must be usable from a minimum range of 10nm down to the threshold at a height of 50'.

[FE Hoppy]

From annex 10.
3.1.5.1.3 The downward extended straight portion of the ILS glide path shall pass through the ILS reference datum at a height ensuring safe guidance over obstructions and also safe and efficient use of the runway served.

3.1.5.1.4 The height of the ILS reference datum for Facility Performance Categories II and III - ILS shall be 15m (50ft). a tolerance of plus 3m (10ft) is permitted.

3.1.5.1.5 Recommendation - The height of the ILS reference datum for Facility Performance Categories I - ILS shall be 15m (50ft). a tolerance of plus 3m (10ft) is permitted.

The Reference datum (point "T") A point at a specified height located above the intersection of the runway centre line and the threshold and through which the downward extended straight portion of the ILS glide path passes.


The 8 degree sectors are described as each side of the centre line of the ILS glide path in 3.1.5.3.1

[FCSoverride]

Thanks very much for all the help. I have located myself a copy of annex 10. The model problem is now solved :)

Cheers.

[kijangnim]

Greetings

You have to include earth curvature around the outer marker :E

[safetypee]

FCSoverride have you overlooked the point that the GS consists of a reflected beam from the ground in front of the transmitter aerial?
This positions the GS angle origin slightly closer to the threshold and also results a parabolic path at low altitude (most noticeable below the threshold). The beam centreline can be biased towards the threshold, but not necessarily over it.
Also IMHO, as the beam can still be received almost to touchdown, the beam width or side lobes may be wider than you assume.

In more practical terms, as you probably realise, a flight guidance systems generally use reduced gains the closer the aircraft is to the GS transmitter. However, for even greater precision, e.g. Cat 3 autoland, the GS is progressively disregarded so that before flaring the aircraft is following an attitude, flight path angle, or similar inertial stabilised trajectory.

[EMB170]

As far as i remember, maybe comjam can confirm this, that if you use the glideslope past 200' , say for cat 2 or 3 ,the signal you use, is a integrated average from 3,5 nm to 0,5 nm and also will take inputs from Rad alt to guide you down to touchdown.

if you fly a cat 1 approach you will find that the gluide slope, will stop guiding you to touchdown at around 200', hence the papis which assist you in a "visual approach" from around decision height on cat 1 .

You will also find that if you follow the papis to touchdown that the gluide slope guideance will not give you a valid signal to touchdown.

Hope that helps. if not i'm sure there is an ICAO document explaining how it works precisely. not sure , but maybe Annex 10. again, comjam might be able to help?

[ft]

safetypee,
imaging glide path transmitters use ground reflection form a virtual antenna system comprised of the actual antennas and their mirror images. The GP beam will thus appear to emanate from the base of the GP mast and not from the reflection area. You then stagger the GP transmitter antennas laterally to distort the radiation pattern and have the GP center on the runway centerline instead of the base of the mast.

Rgds,
/Fred

[ft]

The glide path angle is measured from point A, 7500 m from the threshold, down to point B, which is 1050 meters from the threshold or about 180' on a three degree GP. Beyond this point, the GP is checked for structure but it is not uncommon to see significant flaring of the glide path signal.

On installation and categorization, the actual GP height at the threshold may be checked by measuring the last mile of the GP for CAT II and CAT III facilities. These are obviously of a higher quality. CAT I leaves a lot of room.

kijangnim,
earth curvature does have to be considered at that distance... albeit not by pilots. :)

Cheers,
/Fred