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?