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Usually, there are four lights at the sides of runways which show airplanes' altitude status:

  1. four red lights: too low
  2. two red, two white: appropriate altitude
  3. four white lights: too high.

If two airplanes with two different altitudes approach, do they see different number of white and red lights? Or are the lights set for the closer airplane?

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Here's how the PAPI lights work:

Depending on the angle of the viewer, a different color is seen: angle

Then each light is calibrated to an angle: lights

The good thing about this is that it's dead simple technology where very few things can go wrong: The lights have to be on and calibrated for the correct angle. The downside is that its usefulness decreases with lower visibility.

Some new systems appear to have solar panels as an off grid (and alternative/backup) power source which is sensible. Many systems today also have LED lights.

If two airplanes with two different altitudes approach, do they see different number of white and red lights? Or are the lights set for the closer airplane?

The system is independent and passive, in the sense that it never makes contact or note of the aircraft approaching. It lights up all to all aircraft in the beam. Hence, it 'works' for all aircraft at the same time.

Some new systems feature FAROS (Final Approach Runway Occupancy Signal) giving an indication that the runway may be occupied by another aircraft by flashing the lights.

For a little information on differences between similar systems to PAPI, see this link.

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  • $\begingroup$ A parabolic reflector like that would work well for a halogen lamp, but I'm not sure how they get a good separation with arrays of discrete LED lights. (Granted, the question isn't really concerned with the internals) $\endgroup$
    – Nick T
    Jun 1, 2014 at 18:17
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    $\begingroup$ I would like to add that some systems (at least the ones I'm familiar with, attached to an ILS) are finely tuned to the appropriate glideslope. If the box containing the lamps is tilted beyond a degree or two (hit by a mower, perhaps) it trips a switch that kills the light. $\endgroup$
    – StallSpin
    Jun 1, 2014 at 23:01
  • $\begingroup$ Perfect answer, thanks; I always used to tell myself that the PAPI is polarised somehow. $\endgroup$
    – shortstheory
    Jun 2, 2014 at 6:35
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    $\begingroup$ Re the second diagram: As the old joke goes, below four reds you see four greens. This means that the PAPIs are shining through the grass and you might want to think about pulling up. ;) $\endgroup$
    – Ed Daniel
    Jun 2, 2014 at 10:23
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    $\begingroup$ @StallSpin Note that even when there's an ILS the PAPI lights aren't really "attached" to it - They're often calibrated to match the ILS glideslope, but sometimes not (there's a note on the approach plate when this is the case - e.g. the PAPI light on runway 14 at KFRG has the same descent angle, but a different threshold crossing height (TCH) than the ILS glideslope - it puts you on the runway a little earlier). $\endgroup$
    – voretaq7
    Jun 2, 2014 at 16:14
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They use a lens system that projects the light in a narrow beam so each plane sees something different and makes the glide slope indicators a passive system.

you can see this nicely on a foggy morning:

enter image description here

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    $\begingroup$ Nice picture! +1 $\endgroup$
    – DeltaLima ♦
    Jun 1, 2014 at 12:39
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The lights you refer to are called PAPIs (Precision approach path indicators). They to do not change colour based on the altitude of the plane but are rather passive light boxes that have a different colour depending on the angle that you look at them.

enter image description here

Each light is mounted at a slightly different angle.

enter image description here

If you are at the neutral axis of such a light you will see a blend of red and white.

enter image description here

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    $\begingroup$ +1. I like the runway shape diagram. It is very important to learn the different appearances of runway and then determining the correct approach. $\endgroup$
    – Farhan ♦
    Jun 2, 2014 at 15:50
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The PAPI lights are not set for any airplane, they're unintelligent displays that are visible in different ways based on the angle of the viewer. For that reason, there can be an unlimited number of aircraft in view, and each will see the appropriate color selection.

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