How do Luneberg lenses hide a stealth aircraft's real signature?

Question

I thought aircraft was supposed to be stealth and not show up on radar under the right conditions. This article says

Removable Luneburg lens type radar reflectors are sometimes attached to military aircraft in order to make stealth aircraft visible during training operations, or to conceal their true radar signature.

So even with Luneberg lenses, will there be 2 blips on the radar screen, one from the reflector, and a weaker blip from the aircraft signature?

If I am potential adversary observing training involving stealth aircraft, is it possible to filter data around the "visible aircraft" to get its true signature? How is this difficult?

Answer 1

Since the attached reflector and the rest of the aircraft are at the same position and velocity, they form a single target. There will be no second blip. They're one and the same.

When a target has moving parts, like exposed engine blades, they distort the signal, producing a radar signature. This signature will be obscured by the much stronger reflection from the Luneberg lens.

This is a small side effect of the attached reflector's purpose: to allow allies to relatively easily track and target the aircraft. However, such a handicap also keeps them from experiencing stealth as it is and developing tactics to deal with it.

Even when high-res radar imaging is used, spatial separation alone is generally not precise enough to separate the signatures of other airframe parts from the removable reflector on its own. Engine signatures (JEM) can be separated through Doppler shift. When the engines are concealed, there's not much in the way of distinguishing the airframe's static signature from the reflector's.

For more technical information, look into the subject of NCTR (non-cooperative target recognition).
Some ACARS papers can be easy to digest.
Pages 177-184 (28-1) make for a good starting points on the basics, 97-110 (8-3) address reflectors and their effect on the radar signature more specifically.

Answer 2

There is no "signature" that identifies an aircraft by name. There is only a radar reflection. In most cases, a radar cannot tell the difference between an F-35 and a suitably sized, supersonic toaster.

Let's assume the radar uses a radio frequency of 3 to 11 GHz. Thanks to the wave nature of radio waves, the measurement will be limited to around 20 cm resolution. That's in the direction of the radar beam; in the other directions it will be a lot worse. Getting that ~20 cm resolution is going to require some expensive electronics, but militaries can afford it.

The Luneberg lens is only about 2 cm from the aircraft, so the chances of finding the aircraft's radar cross section from a direct measurement are very small.

Even if you could distinguish the radar reflection of the lens from the radar reflection of the aircraft, your radar can easily be defeated by adding a few more lenses.

In practice, a military would find it much more convenient to use Google Image Search to find some pictures of the aircraft. Then they use a supercomputer to estimate the radar cross section based on the shape.

If it's not cloudy, you could skip the radar and just use a bunch of binoculars.