Secondary RADAR antenna on top, Primary RADAR dish below
This site is typical of a Terminal Approach RADAR (TAR) where two
types of RADAR are present:
Primary RADAR is the traditional form of RADAR where pulses are
transmitted, and the receiver detects reflections from any aircraft
(or other things!).
Secondary RADAR transmits a signal, which is received by
suitably equipped aircraft. The aircraft then transmits an
identifying code (as well as other data) in return. This allows
aircraft to be identified, rather than just being a blob on the
screen as with primary RADAR systems. A combination of both
provides awareness of all aircraft near major airports.
The system is due to be replaced, so there will probably be some
activity here soon (I noticed some suspicious looking crates here
when I placed the cache!)