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I know that an ILS transmits not only the correct glideslope but also signals that can be interpreted as a false glide path. But I'm not familiar with the details of these erroneous indications. What causes "side lobe" or "mirror" false glideslopes? How many are there, where are they located, and what sensing do they cause in the aircraft? What is the typical behavior of a glideslope signal outside its usable range, or is it completely unpredictable?

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3 Answers 3

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The glideslope system is an analog system, and as such, it's subject to aliasing, resonances, heterodyning, and eight other technical terms I pulled out of my ass.

Maybe a diagram will help:

enter image description here

false glide slopes

In plain English, above the "real" glideslope, there are false glide slopes caused by your equipment locking onto the wrong phase of the signals. If you lock onto the wrong lobe, it will still take you to the runway, but at a much higher rate of descent than you planned for. Maybe steeper than you can manage safely.

Assuming the correct lobe is the bottom-most one in the system (see diagram), you'll be ok as long as you intercept it from below. If you intercept from above, you could lock onto the wrong one, and then you're gonna have a Real Bad Day.

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    $\begingroup$ I would +1 for a great answer but not in favor of the cuss word. So no +1. $\endgroup$
    – wbeard52
    Commented Feb 20, 2017 at 1:40
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    $\begingroup$ I think the answer is not complete and the diagram is confusing, but I totally appreciate the cuss word part. I love it when people get honestly retrospective. There's way too much BS taken way too serious as it is. So here's 1 UV from YT $\endgroup$
    – user55607
    Commented Mar 12, 2022 at 13:25
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    $\begingroup$ Upvoting for the cuss(?) word. $\endgroup$
    – Jack Deeth
    Commented Mar 12, 2022 at 23:16
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Side lobes generally are unwanted radiations in a given direction. Smaller the antenna, smaller are the side lobes and vise versa.

  1. Easiest way to find out false GP is ROD (rate of descent) readings. If you're on a GP and the ROD is way higher than usual, then, you're on a false GP.

  2. To be sure, I've used the following method in my training. You receive GS signal at an altitude; to find out whether the GP angle is more or less than 3deg, take your Altitude (in feet) and divide by your DME (in Nm) x 100. Mental math

Ex: your altimeter reads 3000' and you're 5Nm to the station, and you are on the GP, then your angle of approach is 3000'/500= 6deg, therefore you're getting false GS signal. Basically Altitude divided by Range multiplied to 100, gives you approx Glide Angle.

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Once you are established on ILS, it will direct you to the center of the desired glide path. If you are above the center, ILS will direct you down and if you are below it, ILS will guide you up, but only to a certain extend.

If you are way too high or way too low, the ILS direction will flip and show you the opposite of what you need to know. So if you're way too high, it tells you to go even higher and if you're way too low, it tells you to go even lower. In that case you need your other instruments to tell you your ILS readout can't be right, unless you have one of those ILS readers that do their own math and warn you if you are no longer established.

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