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The F-22 Raptor (and other stealth aircraft) can be made more visible to radar by using radar reflectors, or in other words, radar cross section (RCS) enhancers. In the comments section of this answer to a question posted on Aviation Stack Exchange it was speculated, that the RCS enhancer used in F-22s is empty, sparking further wondering whether this was in fact possible.

So the question here stands: how can such a small device improve the visibility of a stealth aircraft sufficiently to make it a clear radar target?

The RCS enhancer on F-22 seems to be less than half a gallon (4 litres) in volume.

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The F-22 RCS enhancers size and shape (to some extent), as well as this source for example, suggest it is a Luneburg lens. The Luneburg lens is a device that reflects electromagnetic waves very efficiently back to the source, making it possible for a small device to appear as a big target on a receiver placed at same position as the transmitter (for example a radar).

According to Wikipedia:

Luneburg lenses are normally layered structures of discrete concentric shells, each of a different refractive index. These shells form a stepped refractive index profile that differs slightly from Luneburg's solution. This kind of lens is usually employed for microwave frequencies, especially to construct efficient microwave antennas and radar calibration standards.

This site further elaborates on the Luneburg lens:

The Luneberg lens is a passive radar augmentation device used to increase the radar reflectivity of a target without the use of additional energy The lens reflector is a sphere in shape, usually composed of concentric dielectric shells. By the proper selection of dielectric constants for each shell, radar energy incident on one of the faces of the lens is focused at a point on the rear surface of the lens. The rear conductive surface reflects radar energy back to the source.

Principal of Operation:

(1) Basically the Luneberg lens is made of concentric shell of polystyrene with varying reflective indices (dielectric constants) from 1 to 2, the highest refractive index being the innermost shell.

(2) A ray passing through the various concentric shell will bend towards normal and then focus at a point. If a metal reflector is placed around the focal point, the ray gets reflected and retraces the same path. This is the principal of Luneberg Lenses

(3) In order to have smooth bending of the ray theoretically, many concentric shells are required. In practice, a few concentric shell are used.

(4)The radar cross section augmentation of the lens is given by the formula( 4∏3R4/4λ )

The RCS is inversely proportional to the squire of the wave length and directly proportional to the 4th power of the radius."

[sic] Note: the correct spelling is Luneburg, after Rudolph Lüneburg

To act as a Luneburg lens, the device cannot be empty. To properly "bend" the incoming and outgoing waves, the sphere must hold some kind of inner refractive structure to make it function properly as a radar reflector over meaningful distances. A single concave spherical layer of reflective material will not achieve this.

It should be noted that the RCS enhancer on F-22 is not spherical in form, but a cylinder with rounded ends. The principle of Luneburg lens could still be applied to the device. And as always, having the inter of net as source, it is totally possible the aforementioned device operates on a different principle. Operating principles of many of the systems on F-22 are not exactly public for obvious reasons... Please feel free to overthrow this answer if better knowledge is available.

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  • $\begingroup$ What does this do that a transponder doesn't? $\endgroup$
    – Michael Hall
    Commented Dec 11, 2020 at 23:14
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    $\begingroup$ It is a passive system that works with any primary radar, when for transponder to work, you need secondary radar system (as I'm sure you know). It is fail safe and hides stealth capabilities of F-22 (stealth stealth 🙃). F-22 did not originally have transponder capability, so this thingy was especially handy then. $\endgroup$
    – Jpe61
    Commented Dec 11, 2020 at 23:28
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    $\begingroup$ Actually I am more familiar with threat radars that ATC. But I'm still not understanding why an F-22 might want to deny controllers the useful information available via transponder, but allow them a small raw "blip". What am I missing? $\endgroup$
    – Michael Hall
    Commented Dec 12, 2020 at 1:40
  • $\begingroup$ @MichaelHall -- the thing about having a passive reflector is that it will still work when the electrons go on strike :) $\endgroup$
    – UnrecognizedFallingObject
    Commented Dec 12, 2020 at 2:57
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    $\begingroup$ @MichaelHall I think the main (if not only) reason for using RCS enhancer is to hide the stealthiness of F-22. Not even the U.S. is safe from intelligence and espionage. With proper equipment would very easy to evaluate steath properties of the acft unless something was used to set them off. Flying non operational missions close to enemy makes this even more important. One good example was seen during evaluation flights of F-35 in Finland: they carried radar reflectors, I bet Russia had plenty of their ECM and other relating stuff at full use during F-35 flights in Finnish airspace. $\endgroup$
    – Jpe61
    Commented Dec 12, 2020 at 15:31
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Much of it comes down to the fact that the F-22 has such low radar visibility that you don't need a very large visible surface to drastically increase its visibility.

The number I've seen thrown around for the effective radar cross section of an F-22 is 0.0001 square meters.

With the radar cross section starting out that tiny, a reflector doesn't have to be very big to increase it by a large percentage. A 2 inch diameter piece of steel (or aluminum or other radar reflective material) would be large enough to drastically increase the visible radar cross-section.

The problem with that is that it depends heavily on the angle. It's a bit like if I'm shining a flash light around a room, and you're holding up a hand-sized mirror. If you hold the mirror at just the right angle, the light reflects back to me. But if the angle's wrong, the light reflects off some other direction (that's a lot of how low visibility designs get their low cross section to start with).

So consider instead holding up mirrored ball from a '70s disco. Now no matter what direction you hold it, as soon as I shine my light toward you, I'm pretty much guaranteed to see a reflection. And if two or three people are shining lights toward you from different directions, they'll still all see reflections.

The RCS enhancer is basically that disco ball, but tuned for radar instead of visible light. Oh, and because of how it works, it's more like half a disco ball instead of a whole one, so it covers roughly 180 degree field instead of a full 360 degrees.

But the size still comes back to the fact that the visibility starts out extremely low, so the reflector doesn't need to be terribly huge to increase it a lot. And after decades of work on radar, it doesn't have to be very big for radar to pick it up quite easily.

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  • $\begingroup$ 0.0001m^2 is 1cm^2 (plausible for F-22) , so shoud the RCS-E under F-22 work on the disco ball -principle, it would either be able to multiply RCS about 10 fold in very narrow angle window, or say, about double it in a wider range of discrete angle windows. You must have noticed a disco ball does not shine uniformly, it sparkles as it spins (moves) so getting a constant uniform reflections from a spherical multi-faceted reflector is not possible. The distances in radar surveillance are considerable, so I bet it is preferrable to get a strong reflection, just for the sake of safety alone. $\endgroup$
    – Jpe61
    Commented Dec 11, 2020 at 23:17

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