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I was watching a timelapse video yesterday of the approaches on a busy airport (I don't remember which one), and I saw that about 3 or 4 planes were lined up and descending at the same time (they were far from each other) and that got me thinking:

  • Is it possible (from a technical perspective) for more than one airplane to intercept the ILS signals (glideslope and localizer) at the same time?
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  • $\begingroup$ Basically the ILS is a radio signal emitted by ground facilities that does not care about whose receiving it and does not react to receiver's action (like TV signals, except it is directional). Thus the only limitation on the number of users/receivers is possible interference. $\endgroup$ – Manu H Nov 11 '15 at 11:28
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    $\begingroup$ The question is not as simple as it seems, reflection will occur on preceding aircraft, and Doppler shift too. Reflection may change the relative strength of the dual frequency beams, but Doppler may allow to exclude reflected signal them from reception. I really don't know the impact, but not simple. On the other hand, it is done. $\endgroup$ – mins Nov 11 '15 at 11:31
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    $\begingroup$ @abelenky You're mostly right, but it is possible that an aircraft in front could be blocking and/or reflecting the signal. Just like normal radio stations, the waves propagate in a (more or less) straight line away from the transmitter and they don't travel through an aluminum aircraft very well (and they do reflect off of aluminum aircraft.) This is why we have to keep planes on the ground from taxiing in front of the localizer or glideslope transmitters while another aircraft is using the ILS for an approach. $\endgroup$ – reirab Nov 11 '15 at 15:36
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    $\begingroup$ @reirab: I think the analogy is very strong. Just as with the FM radio stations, some listeners may be blocked, or may hear reflections or interference. At extreme distances, the signal may be weak. For an FM listener, its annoying. For an airliner, its a safety hazard. $\endgroup$ – abelenky Nov 11 '15 at 15:42
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    $\begingroup$ @abelenky Oh, yes, I agree in that regard. Certainly, the signals propagate and are subject to interference in (mostly) the same ways. There are a few forms a interference that are different with AM vs. FM (amplitude distortion doesn't matter with FM, but it does with AM and ILS is AM,) but it's mostly the same. The biggest difference between this scenario and the scenario of a normal radio station is that the 'listeners' in the case of an ILS are all lined up with each one between the transmitter and the 'listener' behind them... and that all of the 'listeners' are giant pieces of aluminum. $\endgroup$ – reirab Nov 11 '15 at 15:56
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Yes, since the ILS is just a set of radio signals emitted and received by aircraft, there can be more than one aircraft established on localizer and glidepath. The ILS does not lock or tune onto one single aircraft, it continuously broadcasts the localizer and glidepath signal.

Since the localizer and glidepath antennas are located at the end of runway for the localizer and at the side of the runway for the glidepath, you will only need to worry about interference by aircraft on the ground, where they are close to the antennas and can deflect the signal due to proximity. This is why there are protection areas and aircraft need to hold at certain holding points, e.g. CAT II holding points when CAT II approaches are in use.

Aircraft in the air can also interfere and deflect the signal, however the emitted signals spread in a cone, so other aircraft still can receive the correct signals.

To quote KeithS from one of the comments:

[...] basically, the localizer is projected from the far end of the runway, and the glideslope is projected from the side of the runway at or near the touchdown target. Thus, aircraft in line to touch down on the ideal target will have good line-of-sight to both transmitter pairs (unless there's an aircraft in the ILS Critical Area)

GS+LOC
(Image Source: www.aopa.org)

http://cdn.londonreconnections.com/assets/planes.jpg

(Image Source: nustyR AirTeamImages (found via LondonReconnections))

FR24 Lined up Aircraft

(Image Source: When is an aircraft cleared to land?)

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  • $\begingroup$ Is that top image real? Those airplanes look way too close to each other, unless there's some weird zooming illusions going on here. It doesn't look like there would be any way for the one in front to clear the runway before the next one is at DH. It doesn't even look like the one in front would be on the runway before the next one gets to DH. $\endgroup$ – reirab Nov 11 '15 at 15:30
  • $\begingroup$ I think the image is from LHR, where they use pretty closely packed spacing, especially in a headwind (nats.aero/newsbrief/time-based-separation-heathrow-world-first) Here is a video, that shows much the same spacing as the photo - youtube.com/watch?feature=player_embedded&v=fgHjVvqLXV8 And speculation, I don't know for sure -- but LHR has two close, parallel runways, so perhaps the following plane goes to the other runway in case of difficulties? $\endgroup$ – Gremlin Nov 11 '15 at 16:01
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    $\begingroup$ This is a good answer, but it doesn't explain why aircraft lined up right behind each other on the glideslope can still receive the signal with minimal interference. Some information about the placement of the transmitters on the runway complex might be useful; basically, the localizer is projected from the far end of the runway, and the glideslope is projected from the side of the runway at or near the touchdown target. Thus, aircraft in line to touch down on the ideal target will have good line-of-sight to both transmitter pairs (unless there's an aircraft in the ILS Critical Area). $\endgroup$ – KeithS Nov 11 '15 at 19:39
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    $\begingroup$ @BrockAdams Correct, the 3 NM are ATC requirements, not ILS requirements. $\endgroup$ – SentryRaven Nov 12 '15 at 9:40
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    $\begingroup$ @reirab, most likely it's real. A long telephoto lens will compress distance quite a bit, and it's likely this was shot with a 600mm or longer lens. $\endgroup$ – FreeMan Nov 12 '15 at 17:37
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If you look at the following picture, you'll see that a typical Localizer can giving reliable signals up-to 18NM from its position.

Localizer

On the other hand, a typical Glideslope can give reliable signals up-to 10NM.

enter image description here

That said, it is absolutely possible to have multiple-aircraft conducting the approach right behind each other as long as all aircraft receive positive guidance.

Needless to say, traffic separation has to be maintained according to the regulations.

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    $\begingroup$ This, I think, is the better answer to the OP as it illustrates that the ILS signal is not broadcast from the spot on the runway where the planes are aiming (which is a common misconception). The glideslope component is projected from the side while the localizer is broadcast from the opposite end of the runway, and on approach you typically have good line of sight to both points even with a plane in front of you. $\endgroup$ – KeithS Nov 11 '15 at 19:42
  • $\begingroup$ @KeithS that's right. To be more precise, the Glideslope is located in a building 750-1250ft from the approach end of the runway and 250-650ft from the centerline, and the localizer is located 1000ft behind the departure end of the runway. $\endgroup$ – RaajTram Nov 11 '15 at 21:59
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An ILS is a beam of radio energy pointed down the approach path, there's no limit to how many aircraft can be using it from a technology point of view. In practice you will only have a few descending on the ILS at the same time due to separation requirements.

As you could see the 4 airplanes on approach it means that conditions were visual, so the airplanes weren't necessarily using ILS for approach. It's probable they were all approaching visually.

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    $\begingroup$ As mentioned by casey here the SOP often requires that if ILS is available, it must be used even for visual approach. $\endgroup$ – Jan Hudec Nov 11 '15 at 12:54
  • $\begingroup$ "there's no limit to how many aircraft...": For the sake of accuracy, there are limits that can be ignored up to a certain level, the DDM (difference of depth of modulation) must remain within ICAO limit. Outside airfield protected/critical ILS areas, wind generators, preceding and departing aircraft all create a multipath reception which affect DDM. $\endgroup$ – mins Nov 11 '15 at 15:58

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