To quote John Oliver:

"Yemen, home to 26 million Yemeni people, and 27 million American drones."

While these numbers may be slightly exaggerated I started to wonder. As far as I know, military drones are used primarily for survaillance, meaning that they circle a potential target for hours on end to gather intelligence.

I immediately thought, that they would be spotted quickly, and any illegal activity would cease. Since they seem to be effective though, this can't really be true.

So my question is: Given typical operational parameters during survaillance, can military drones, like the Predator, be spotted with the naked eye if they are circling above you?

I tried to find an answer with some quick calculations of my own.

I can see quite a lot of airliners flying above me, and I can see their bodies quite easily. Airliners are larger though.

Some sizes I could gather are:

A320 Source

Length: 37.57m
Wingspan: 34.1m
Body width: 3.96m

Predator Drone Source

Length: 8.23m
Wingspan: 14.84m
Body width: 0.83m (estimated from picture)

When I look at just the length of the aircraft, I can calculate the angular size of the object in the sky if I know the altitude. I assume the A320 flies at around 32000 feet, or 9753m. For the drone I have no idea. But it can fly at 7620m and I would assume it stays a bit below, so I assume 6000m.

enter image description here

The angle of the object is then:

Angle = ArcTan(Length/Altitude)

The human eye has a resolution of 0.5-1 arc minutes, or 0.0125°.

For the given aircraft I come to these values:

Angle = Atan(37.57/9753) = 0.22°

Angle = Atan(8.23/6000) = 0.08°

So from the length, both aircraft should be visible. However, that doesn't seem to make sense, because they need to be wide enough as well. So for the body width's I get:

Angle = Atan(3.96/9753) = 0.024°

Angle = Atan(0.83/6000) = 0.008°

So the A320 should be visible quite comfortably, while the drone should be invisible to the naked eye.


This is all theoretical. I don't know the operation of the predator, especially how high it flies. I also don't know what really determines visibility. Does the wingspan alter the numbers?

Can anyone share any actual insight?

  • $\begingroup$ I think those drone figures are not even an exaggeration. Exaggerations are based in fact; John Oliver has no idea how many American drones are in Yemen. By comparison, I understand that fewer than 1 million people have registered their hobby drones in the US so far. $\endgroup$ – J Walters Mar 13 '16 at 17:00
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    $\begingroup$ This seems to be a problem of optics, not aviation. That said the resolution is dependent on the the diameter of the lens. The larger the lens, the better the resolution, but there are limits. See this question on Space.SE: Were the Apollo lunar activities observed from Earth? $\endgroup$ – mins Mar 13 '16 at 17:50
  • $\begingroup$ Thank you for your input @mins. The resolution of the eye is quite clear, given at 0.5-1 arc minute. I thought about Physics SE, but given that the question contains some parts about operating conditions of drones that I am unsure of and shape of aircraft (wings/body, instead of spherical approximation), I found it to be a better fit on Aviation. It's a corner case between the two in my opinion. $\endgroup$ – Jens Mar 13 '16 at 17:59
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    $\begingroup$ You may focus on the drone aspect here, and post the resolution question on Physics.SE. You may edit your question, which is about to be closed. $\endgroup$ – mins Mar 13 '16 at 18:02
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    $\begingroup$ I'm pretty sure you can't see them but according to journalist Jere Van Dyk you can hear a Predator drone overhead. $\endgroup$ – TomMcW Mar 14 '16 at 4:32