As mentioned, ATC radar cannot see directly above itself. It simply isn't designed to be able to do that. In the above example, all but airplane #4 are detected by radar site A. Aircraft 1 and 2 represent low-altitude aircraft, 3 is a slightly higher aircraft, and 4 & 5 are much higher altitude aircraft. Also, before we move on, notice that the cone-of-silence is very small near the radar, and it gets wider the higher we go. That's why they call it a cone.
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