Silly Radar Question
OK, this is probably going to be one of those questions to which the answer is so obvious that people stopped discussing it years ago but it seems that one of the issues that plagues the current ATC system is being unable to tell at what altitude an unidentified aircraft is flying, thereby not knowing if he's in, under, or over a particular piece of airspace. Hence Mode C with altitude encoding.
Here's my question. If airborne radar exists which can track fast moving targets from a fast moving platform, and sort out everything about everyones' positions and vectors to within a gnat's whisker, why can't ATC radar do the same? In other words, why does ATC radar need the aircraft to tell it what altitude it's at? I would have thought radar could be much more accurate than steam-driven altimeters anyway.
Am I missing something here?