# What is the ultraviolet signature, if any, of aircraft and jet engines?

The Wikipedia article on the FIM-92 B variant of the Stinger Missile says:

In this version, the infrared seeker head was replaced by a combined IR/UV seeker that utilized rosette scanning. This resulted in achieving significantly higher resistance to enemy countermeasures (Flares) and natural disturbances. Production ran from 1981 to 1987, a total of 600 missiles were produced.

I'm assuming "IR/UV" means "infrared/ultraviolet."

Do aircraft (or jet engines) really have a strong ultraviolet signature? If so, how is it formed? Is it only evident at high airspeeds?

(Also, what on Earth could it be talking about by "natural disturbances"?)

• In addition of the black body UV radiation which is very limited, UV are produced in the engine plume (especially afterburners) by fluorescence, and UV trom the Sun are reflected by the aircraft. An UV sensor sees all of them and try to contrast the aircraft UV and the background which is full of UV from the Sun. On the other hand flares do not produce, reflect or transmit UV. While IR analysis may fail to contrast targets and flares, UV analysis can distinguish between them. IR and UV sensors are usually combined in the analysis.
– mins
Nov 3, 2015 at 9:38
• @NathanTuggy Why was the "ultralight" tag removed? What is "ultralight" if not ultraviolet light? Nov 4, 2015 at 6:33
• @DrZ214: An ultralight is a type of slow, low-flying, generally VFR-only aircraft, often made similarly to a hang-glider or parafoil. Ultraviolet light is something entirely different; it's never called ultralight in any context, but especially not aviation. Nov 4, 2015 at 7:08
• @NathanTuggy Do you mean ultralight as in ultra-not-heavy? If so, how do we edit the description of that tag? It had no description at all when I used it. Nov 4, 2015 at 7:10
• @DrZ214: You can if you want, but in English aviation terminology, "ultralight" is just the conventional name of that class of aircraft. Nov 4, 2015 at 7:17

## 4 Answers

Your assumption is correct - it is indeed about infrared (wavelength between 750 nm and 10,000 nm) and ultraviolet (wavelength between 100 nm and 400 nm) radiation. The sensor detected two specific wavelengths, one from each range.

Every body has an electromagnetic radiation which is consistent with its temperature. This radiation has an intensity distribution which shifts to shorter wavelengths with temperature. By comparing the intensity of the IR and UV parts of this radiation, the sensor can much better distinguish between radiation sources of different temperature. Flares are normally hotter than engine exhausts, so by detecting an excess of UV radiation, the sensor can distinguish between flares and exhaust pipes.

"Wiens law" by 4C - Own work, based on JPG version. Licensed under CC BY-SA 3.0 via Commons.

Natural disturbances means reflected sunlight off a polished aircraft surface or canopy, or off lakes or windows on the ground. Even after filtering by the upper atmosphere, sunlight will contain much more UV radiation than your average exhaust pipe, and glares were a big distraction to early thermal seekers. Sunlight would correspond to the green line in the diagram above, while the exhaust would be even below the dark red line.

Note that the absolute intensity also goes up with temperature, but would not help in identifying the temperature of a target if only one wavelength is observed. A more distant, hotter target would cause the same intensity than a colder, closer one. By sensing two wavelengths, the sensor could much better tell an exhaust pipe from other heat sources.

To answer the question directly: The UV signature of the aircraft itself is practically zero except for the reflection of the UV part of sunlight, and the active UV emission of a jet engine is extremely low and concentrated around its exhaust when seen from behind. Lighting the afterburner will increase the UV radiation, but it will still be much less relative to the IR radiation that that of, say, burning magnesium.

• If I'm reading this correctly, then IR/UV missiles will not work at night? Mar 19, 2016 at 15:29
• @DrZ214: Why would you think so? At night you have no distraction from sunlight, so an IR seeker has even less trouble to find its target. The radiation from the hot exhaust pipe and flares would still be the same, so the seeker will work at night as it does during daylight. Mar 19, 2016 at 17:17
• I'm talking about the UV part. The UV signature of the aircraft itself is practically zero except for the reflection of the UV part of sunlight, and the active UV emission of a jet engine is extremely low So it sounds like the UV tracking won't work at night? Mar 20, 2016 at 1:38
• @DrZ214: Tracking is still done by IR, and UV is used to classify the IR source. No sun reflections just means less false positives that need to be weeded out. IR tracking at night is easier. Again, at night you have no distraction from sunlight, so an IR seeker has even less trouble to find its target. Mar 20, 2016 at 6:23

What is the ultraviolet signature, if any, of aircraft and jet engines?

The exhaust plume of the jet engine that has radiation which flickers within the Ultraviolet Range and the ultraviolet light radiation that exists as a result of a pressure wave on the tip or leading edge of the aircraft that is moving fast enough to create a sonic pressure wave. Several different constituents of a jet engine – accretion flow; stellar magnetosphere; winds and inner parts of the accretion disk radiate in the Ultraviolet spectrum.

I'm assuming "IR/UV" means "infrared/ultraviolet."

Correct

Also, what on Earth could it be talking about by "natural disturbances"?

In this context, what it means is that Infra-Red (IR) is the main method of tracking the target. Ultraviolet(UV) is used in addition to discriminate the target from flares and "natural disturbances" like direct sunlight or sunlight reflecting from clouds as they usually have totally different amount of UV radiation compared to the target. Basically they are referring to the Proportional Guidance system of the Stinger MANPAD. Since the primary job of MANPADS like Stinger is to stay locked on, the heat seekers’ fields of view are usually small to prevent distraction from competing sources.Seeker optics magnify IR/UV signatures emitted by distant aircraft,keeping track of the target aircraft is another matter altogether

• "Ultraviolet(UV) is used in addition to discriminate the target from flares" does this means that there is no UV decoys/flares ? Nov 3, 2015 at 14:45
• @ManuH, no, it means that most decoys/flares produce too much UV and the seeker can filter them out based on that. Nov 3, 2015 at 15:24
• @JanHudec That seems odd. If it makes too much UV, then it should be able to blind the sensor (prevent it from seeing anything else such as the real target). Nov 3, 2015 at 21:37
• @DrZ214, not too much as to blind the sensor (that would take quite a lot of energy). Just the ratio of brightness in UV and IR is much higher. Nov 3, 2015 at 22:59

You're correct that IR/UV means Infra red/Ultra Violet.

In general, the aircraft (or its exhaust) does not have any appreciable UV emission.

MANPADS such as Stinger using a IR detector can be confused by launching flares that have the same (or similar) IR signature.

The reason the UV sensor was added was that that it can be used to discriminate the target by detecting the 'UV shadows' cast by an aircraft in a background sky radiance of atmospherically scattered solar UV. These will be different for the actual aircraft and the flares.

Some Soviet missiles achieved the same effect by using IR sensors in two different wavelengths, while most modern missiles have sensors of sufficiently high resolution to discriminate the target.

• If this is true, then IR/UV missiles will not work at night? Mar 19, 2016 at 15:28
• @DrZ214 - they work fine at night. They don't rely on IR/UV reflected off the a/c---they rely on IR/UV emitted by the target a/c. Jun 4, 2017 at 21:18

Contrary to the suggestions in replies to this question the UV seeker of the FIM-92 Stinger looks for the "shadow" created by an aircraft against the relatively bright UV background of the sky. The aiming and arming procedure of an FIM-92 with a dual-frequency seeker involves, amongst other things, aiming the seeker at a patch of empty sky for calibration purposes.

At night one imagines it reverts to the more basic modes alluded to above.

UV seeking mode mentioned here: https://bit.ly/2Hm59w5

Still working on where I discovered the "point seeker at empty sky" calibration procedure.

• armyrecognition.com/…
– CJPN
Jun 6, 2018 at 13:11
• Not at all. Presently dredging brain for where I sourced the aiming and arming procedure.
– CJPN
Jun 6, 2018 at 13:14