Any large object will suffice, especially objects that are naturally reflective. Thus, birds and bottle rockets won't do, but other planes and missles will. I can assure you the PUSSAR software will filter out the reflections occuring off identified objects. Low altitude is where everything essentially gets filtered out by the software.
Your postulation is why radar does not work at low altitude. Above 1000 ft, where are the reflections occuring? Only off other planes. If you are identifying all the other planes, you got the problem half solved.
Do you realize how small and non-radar significant a non-radar guided surface to air missile is? Believe me, you aren't going to bounce an IFF transponder transmission off of a missile that is less than 3 inches in diameter and moving at twice the speed of sound. Nor will you find a surface to air missile with a 50 second time of flight (the length of time the ghost image appears). And if it did have a 50 second time of flight, at supersonic speeds it would move across LA during that time, instead of loitering around AA612. Conversely, if it was an aircraft, it would be extremely difficult for it to maneuver from 1500ft to 7500ft in 50 seconds, unless it was a fighter. And if it were a fighter sized target, it would show up as a raw return on LAX ATC radar.
"Your postulation is why radar does not work at low altitude. "
But it absolutely does work at low altitude. Mine worked just fine at 300ft and it could easily pick up targets operating at tree level when I was at 20,000ft.