Aviation operations involve a large team of people at a given airport who work to ensure that aircraft can land, take off, and move around quickly and safely. Air safety is an important concern of Aviation operations. A major part of runway maintenance is airfield rubber removal, which helps maintain friction levels and ward off birds that might otherwise seek to nest at the airport. Sometimes environmental concerns are also addressed to by aviation operations personnel. Aviation operations personnel include the controllers of an air traffic control (ATC) system who direct aircraft movements via radio or other communications links. Ground control personnel are responsible for directing ground traffic, whereas tower control personnel control the aircraft in the sky. Airport guidance signs provide direction and information to taxiing aircraft and airport vehicles. Lighting and weather operations also form a part of overall aircraft operations. A job on the operations staff of an airport requires a four-year college degree or the equivalent, along with technical expertise. Demonstrated competency is also sometimes taken into consideration.