This is when the ATC facility can accept more traffic or users; that is, airlines cancel flight plans, thus causing openings.

Pilots are required to establish and maintain two-way radio communications with the ATC facility providing air traffic control services prior to entering the airspace.

Some airbases are co-located with civilian airports, sharing the same ATC facilities, runways, taxiways and emergency services, but with separate terminals, parking areas and hangars.

Returns from both radars at the ground station are transmitted to the ATC facility using a microwave link, a coaxial link, or (with newer radars) a digitizer and a modem.

It provides the aircraft's pressure altitude to the transponder, so that it may relay the information to the ATC facility.

The altitude transmitted is pressure altitude, and corrected for altimeter setting at the ATC facility.

This information is usually broadcast on the ATIS at ATC facilities, as well as over VOLMET stations.

The system is activated with four "key clicks" on the VHF radio to contact the appropriate ATC facility or six "key strokes" to contact the FSS.

The FAA was given total authority over American airspace, including military activity, and as procedures and ATC facilities were modernized, air collisions gradually subsided.

A Terminal Radar Approach Control (TRACON) is an ATC facility usually located within the vicinity of a large airport.