(CTR) A clearly defined area in the immediate vicinity of an airport and which aircraft may enter only with the authorisation/clearance of air traffic control services. It is measured from the ground to a specific altitude (Zurich 1,350 m, Geneva 1,200 m).
A Control Zone (CTR) is an aviation term that describes a volume of controlled airspace, normally around an airport, which extends from the surface to a specified upper limit, set up to protect air traffic operating to and from that airport.