I'm somewhat familiar with IFF transponders because many years ago I was trained as a Hawk Pulse Acquisition Radar repairman. Are you saying that Air traffic control uses only transponders, and not regular radar as well? At the time, we used a radar system that showed transponder returns as brighter dots on the screen. If there was a transponder in the target, it was a friend. If there wasn't, it was a foe and you would shoot it down.
By today's standards that was primitive technology. But are you saying that air traffic control uses only transponders, and no accompanying radar as well that could paint the side of a plane or missile? Wouldn't that mean that if someone in a light plane, for instance, strayed into the traffic pattern, you wouldn't see it?
If you were using both radar and transponder returns, then it seems conceivable that the radar could confuse an object close to a plane but without a transponder as the plane itself.
You are NOT looking at ATC RADAR!!
There is a problem with radar in some circumstances.
Not saying this is a false report at all. I'm working on some H-Plane csc^2 antennas that are are about 12 feet long. In a vertical and accompanying horizontal one by it's side, it is very accurate. They are transmit-recieve in X-band. In circular polarity they are as accurate as linear without crappy weather. It takes a 3000 foot range to certify them. They get boresighted using a laser before hoisting them on the positioner on the other side of a canyon from the xmit source. Nice patterns.
Good to meet someone with else with experience in this field.
One more thing - they are built of composite material with a built-in heating system to keep snow and water away.