The antenna system of an Airport Surveillance Radar-9 with primary radar (<i><a href="https://en.wikipedia.org/wiki/parabolic_antenna" class="extiw" title="en:parabolic antenna">parabolic antenna</a></i>) and <a href="https://en.wikipedia.org/wiki/secondary_radar" class="extiw" title="en:secondary radar">secondary radar</a> (<i>rectangular antenna, top</i>).  The secondary radar interrogates the transponder on the aircraft, which sends back a coded signal indentifying the craft and giving its altitude.
Unknown author<span style="display: none;">Unknown author</span>
FAA ASR-9/Mode S Service Life Extension Program Photogaller, transfered from English Wikipedia where it was uploaded originally by <a href="https://en.wikipedia.org/wiki/en:User:Dmcdevit" class="extiw" title="w:en:User:Dmcdevit">w:en:User:Dmcdevit</a>