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TRIVIA: The ILS glideslope GS is not exactly a straight line

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Interesting post!

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Better late than never.

 

It was a very slow weekend...

 

I used for this example the stock Gander CYQX runway 04. It has a 3° glide slope, with the antenna 1241 feet down the runway from the threshold and offset to the right of the centre-line by 669 feet. The runway is 496 feet asl.

 

For comparison,  a hypothetical ultra-accurate RNAV that gives a 3° approach to hit the tarmac adjacent to the glideslope antenna gives a height over threshold of sixty-five feet (561 feet asl).

 

1%20rnav%20oblique_zpsmbt4y47p.jpg

 

 

 

Starting now on the glideslope beacon, first from a top-down view, the antenna is 1410 feet from the threshold.

 

2%20gs%20topdown_zpsjhei8aa4.jpg

 

 

 

It's the cone-shaped beam from the GS antenna that actually has the three degree slope, so over the threshold the intersection with the localiser (down the runway centreline) is... seventy-four feet above the ground (569.9 feet asl).

 

3%20gs%20oblique%201_zpsum36gbtl.jpg

 

 

 

(The two upright triangles are similar so as a check, there's a simpler way of finding the new height: our GS height would be 1409.8/1241 × 65 = 73.9

 

Using Excel to calculate heights for a range of distances shows the hyperbola:

 

4%20calculated%20rnav%20v%20gs-loc_zpsuu

 

 

 

*** ***

 

 

What would have a much greater effect is the height of the glideslope beacon. I guess the cone's apex is on the middle antenna, which is about 30 feet above the ground, so the entire hyperbola is that much higher again. That would put the GS/LOC flightpath at 104 feet over the threshold, quite a difference from the original sixty-five.

 

5%20gs%20oblique2_zps5prtdoip.jpg

 

 

 

And... at the touch-down point, the GS/LOC intersection would be 35 feet higher than the antenna, so 65 feet above the ground.

 

 

 

*** ***

 

 

 

Checking in ADE I found that the entire ILS is actually set to the airport elevation and not to the middle antenna.

 

On the other hand, in FS9, to centre the NAV1 needles when abeam the antenna I had to be at 530 feet and not the calculated 531. Over the threshold I had to be at 568 feet asl, and not the calculated 569.9. It may be only that my slewing was a bit rough, or that I was a bit off in determining the position of the GS beacon (not quite the same spot as the scenery object), or the textures for the stock C172's NAV1 needles aren't quite accurate, or that I needed to reset my altimeter, or... or...

 

7%20fs9%20confirmation_zpsmn4faaeb.jpg

 

 

 

On the other other hand, the GS beam is cone-shaped and does give a hyperbolic intersection, confirmed again by slewing and keeping the needles centred.

 

8%20empirical_zpstqaoiiab.jpg

 

 

 

Back to work this morning for a bit of a rest.

D

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Great work! So the MSFS series models this effect as well. :smile:


"The problem with quotes on the Internet is that it is hard to verify their authenticity." [Abraham Lincoln]

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Great read,

 

btw, regarding reach:

 

 

  • Unlike the localizer, the glide-slope transmitter radiates signals only in the direction of the final approach on the front course. The system provides no vertical guidance for approaches on the back course. The glide path is normally 1.4° thick. At 10 NM from the point of touchdown, this represents a vertical distance of approximately 1,500 feet, narrowing to a few feet at touchdown.

 

P.S.: tested in ELITE, and also implemented :-)

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Awesome work!  Thanks for that!

 

Cheers

Edited by n4gix
Removed excessive quote and images!

- Nick

 

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The theory of the glide path taking on a hyperbolic curve when offset to either side of the transmitting antenna at close range would be correct - IF the glideslope transmitted a single beam, and IF the glideslope indication depended on the actual field strength of the received signal. But, that is not how a glideslope actually works. (In r/w at least. The way it is emulated in the sim is another matter).

 

A glideslope antenna actually transmits TWO beams, whose centers are offset toward each other in the vertical plane.

 

The upper beam is amplitude modulated by a 90 Hz audio tone, and the lower by a 150 Hz tone. The glide path is defined as the plane where the two beams intersect, where the amplitude of the two received modulating signals are identical.

 

If the aircraft is above the glide path, the amplitude of the demodulated 90Hz signal will be stronger, and if below glide path, the 150 Hz signal will be stronger.

 

Although the intensity of the electromagnetic field radiated by each individual antenna describes a hyperbolic curve if plotted in azimuth and elevation in space in relation to the antenna, the glide path itself, (the intersection of the two radiated beams, where the two demodulated signals are of equal amplitude), is linear, not curved.

 

The "thing" being compared by the glideslope receiver (and which drives the glide path indicator) is the relative amplitude of the two modulating tones - NOT the relative electromagnetic field strength of the two beams.

 

A localizer array works in similar fashion - two overlapping beams, amplitude modulated by 90 and 150 Hz waveforms.

 

The accuracy and slope of the glide path defined by the two intersecting beams does not depend on ones lateral offset left or right of the runway, any more than the shape of the localizer would be affected by the aircraft's altitude.

 

A glideslope receiver CAN give erroneous indications if the aircraft is very far above or below the true glide path, due to the fact that all directional transmitting antennas radiate "sidelobes", which are lower-amplitude signal peaks offset to either side of the main beam center.

  • Upvote 2

Jim Barrett

Licensed Airframe & Powerplant Mechanic, Avionics, Electrical & Air Data Systems Specialist. Qualified on: Falcon 900, CRJ-200, Dornier 328-100, Hawker 850XP and 1000, Lear 35, 45, 55 and 60, Gulfstream IV and 550, Embraer 135, Beech Premiere and 400A, MD-80.

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Thx Jim, excellent complement to the thread, but, from a purely geometrical interpretation, there should be a "deformation" of the glide path as the aircraft get's closer to the antena(s) and, I believe IRL it can actually be noticed in the instruments...

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Thx Jim, excellent complement to the thread, but, from a purely geometrical interpretation, there should be a "deformation" of the glide path as the aircraft get's closer to the antena(s) and, I believe IRL it can actually be noticed in the instruments...

Yes it can. When very close to the antenna (within the near field of the radiated signals), there are a variety of signal variations that can occur - mainly due to low-amplitude reflections from the ground beneath the antenna, or from the antenna support mast itself.

 

Glideslope arrays in locations subject to heavy snowfall often have elevated reflector assemblies that kind of look like a rack of metal bedsprings, mounted on the ground in front of the antenna, to help prevent the radiated beams from being skewed by snow or ice accumulations on the ground.

 

A good way to visualize how both the localizer and glideslope "beams" are formed, would be to draw two slightly overlapping circles - similar to the Venn diagrams used in the study of mathematical sets. Then, draw a straight line connecting the two vertices where the circles cross. That line represents the "on localizer" or "on glideslope" point.

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Jim Barrett

Licensed Airframe & Powerplant Mechanic, Avionics, Electrical & Air Data Systems Specialist. Qualified on: Falcon 900, CRJ-200, Dornier 328-100, Hawker 850XP and 1000, Lear 35, 45, 55 and 60, Gulfstream IV and 550, Embraer 135, Beech Premiere and 400A, MD-80.

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Thanks Jim for your excellent explanations!

 

I didn't want to get too technical in my initial post, also I was too lazy to un-dust my avionics notes. I didn't particularly like that subject. The only interesting thing out of my avionics study was that the GP is a inverted and open cone :D, the rest was all Laplace transforms which nobody understood and/or gave a damn on, haha(just joking, somewhat)

 

 

 


A good way to visualize how both the localizer and glideslope "beams" are formed, would be to draw two slightly overlapping circles - similar to the Venn diagrams used in the study of mathematical sets. Then, draw a straight line connecting the two vertices where the circles cross. That line represents the "on localizer" or "on glideslope" point.

 

Yeah, something like this!

 

 

 

uvfL2.jpg

  • Upvote 1

Jaime Beneyto

My real life aviation and flight simulation videos [English and Spanish]

System: i9 9900k OC 5.0 GHz | RTX 2080 Super | 32GB DDR4 3200MHz | Asus Z390-F

 

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A good way to visualize how both the localizer and glideslope "beams" are formed, would be to draw two slightly overlapping circles - similar to the Venn diagrams used in the study of mathematical sets. Then, draw a straight line connecting the two vertices where the circles cross. That line represents the "on localizer" or "on glideslope" point.

 

Exactly, and, apart from the other signal interferences you mentioned, that line will be slightly more above the "imaginary" glideslope the more you are shifted to the opposite side of the runway of that the antenna is installed on. So, i.e. for an  antenna installed on the left side of the rw, as you approach the rw threshold, aligned with the rw centerline, the more you get near it the more you will see the glide needle "move up".... If you are to the right of the rw centerline, the displacement will be even bigger, while if you drift towards the left off the centerline you can eventually see the needle move down as if you were above GS...


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A glideslope / localizer actually has some similarities in the way it works to the old low frequency "AN" 4-lobe radio ranges that were the primary means of aircraft navigation in the 1930's through 1950's, before VOR.

 

In the range, each leg consisted of two overlapping beams, one modulated by the morse code letter "A" ( . - ) and the other by the morse code letter "N" ( - . ) In this system, the "on course" needle was the pilot's own ears and brain.

 

When on course, the two opposite-sense morse code characters would merge into a solid tone - while if off to one side, one or the other letter would stand out in the received audio in the pilot's headset.

 

The AN ranges were definitely subject to beam distortions caused by varying conductivity of the earth's surface, (since those frequencies propagate by ground waves), along with noise from lightning, ionospheric "skip" at night etc.

 

But, the underlying principle is similar.

  • Upvote 3

Jim Barrett

Licensed Airframe & Powerplant Mechanic, Avionics, Electrical & Air Data Systems Specialist. Qualified on: Falcon 900, CRJ-200, Dornier 328-100, Hawker 850XP and 1000, Lear 35, 45, 55 and 60, Gulfstream IV and 550, Embraer 135, Beech Premiere and 400A, MD-80.

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In this system, the "on course" needle was the pilot's own ears and brain.

 

Yes! I did read and saw some videos on that! 

 

If the pilot managed to get a constant tone, then he knew he was on the right track.

 

History of flight instruments and navigation is certainly interesting!

 

BTW: I hope you're not implying that pilot's don't need a brain today anymore, hahaha

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Jaime Beneyto

My real life aviation and flight simulation videos [English and Spanish]

System: i9 9900k OC 5.0 GHz | RTX 2080 Super | 32GB DDR4 3200MHz | Asus Z390-F

 

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Although the intensity of the electromagnetic field radiated by each individual antenna describes a hyperbolic curve if plotted in azimuth and elevation in space in relation to the antenna, the glide path itself, (the intersection of the two radiated beams, where the two demodulated signals are of equal amplitude), is linear, not curved.

 

I think you reasoning is incorrect. Provided that the surface described by the glideslope is a cone (and, as you confirmed, the glidepath is linear, so in the space it should describe an arc of cone), the described effect will happen.


"The problem with quotes on the Internet is that it is hard to verify their authenticity." [Abraham Lincoln]

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