posted on May, 15 2010 @ 11:09 PM
Let's assume you're trying to describe active cancellation but not describing it properly.
Active cancellation does not "affect the incoming radio signal so that it reflects poorly making the airplane invisible to radar".
What it does is emit a conjugate return so that the spoof signal and the aircraft's reflection cancel at the receiving antenna, like I was
describing. It doesn't do diddly squat to the incoming radio signal - it can't. It doesn't even really do squat to the outgoing reflection -
can't do that either. What you can do is try to establish a superposition at the receiving antenna so that at the receiving antenna, the sum of the
echo and the spoof are as close to zero as possible.
This doesn't make either signal go away in flight, it just sums them to something close to zero at the receiving end, sort of the same way your FM
radio in your car fades in and out as you drive in a city where multipath echoes from the buildings can sometimes sum to near zero and leave "dead
There are a lot of problems in doing this, as I said. One way that was proposed to do it was to have an active coating that was basically a plastic
sheet of little wideband transmitters that would emit a cancellation wave as they picked up the reflection from the plane and re-radiated it dinked in
phase as it came out of the sheet. That patent is Navy.
The problem comes in that your impinging radar wave can hit at any angle, and you don't know what the generating radar is at first, so you don't
know the characteristics of the antenna on the receive end. The angle it hits at will cause a return from the plane that is complex in terms of phases
and amplitudes, since it will hit different structures first, and the amount of return you get from each structure will vary also on the angle of
impingement. So what you have to return as far as a cancellation function is nasty and varies with the radar that's scanning you, the angle it's
hitting you from, the distance to the radar, and where in the pulse you are - you have to generate an attack, sustain, and decay phase set.
If the radar that's hitting you is chirping, you not only have to deal with the leading edge echo set being different from the sustain echo set, you
have a sort of non-stop leading edge problem with the frequency changes that takes a LOT of computation, you also have to change the return constantly
based on the changing aspect you are presenting to the radar as you maneuver or just fly away from it, which changes the angle and distance parts by
All that is multiplied in spades if you're getting hit from more than one radar, to the point that you just about can't do it effectively if you're
being imaged in more than one direction. AESA can do single radar active cancellation, there are a few other add-on boxes that can, ACES/ASPIT do not,
that I am aware of.
Even with all that supercomputer number crunching to get this to work, it still doesn't make you invisible, btw, because you can never do it
perfectly, and you have to identify the transmitting set before you can do a good job of it. On top of which, systems that do randomization in pulse
and chirp characteristics make it nearly impossible, and UWB isn't cancellable. Active cancellation is a lot more effective on lower frequency sets,
too, not so much on high frequency stuff.
edit: AESA can do it for most of the forward hemisphere, but not the rear of the plane
second edit: the distance mostly comes in as a "am I fairly close" issue. Once you're "far away" it doesn't matter so much. The problem is, if
you're far away, you'll be hit with what is a nice approximation of a plane wave, that is, the conical section that the main lobe is putting out
will be essentially flat by the time it gets to you. That simplifies things A LOT. If you're closer, the leading edge of the wave may look spherical,
which complicates the returns, badly. Most radars aren't capable of resolving the signature on the leading edge of the return, but if you're
spoofing for a plane wave and you're close, the summed leading edge of your return will be a noise burst because your plane wave conjugate spoof
return will be wrong for a semi-spherical main lobe.
A very few of the newest radars can analyze the first few hundred ns of leading edge features and make a good guess at what sort of plane it is, and
what angle it's presenting.
[edit on 15-5-2010 by Bedlam]
[edit on 16-5-2010 by Bedlam]