ATIS is a computer-generated voice broadcast, transmitted continuously on a dedicated VHF frequency at controlled aerodromes. It is the first port of call before you call the tower. The purpose of ATIS is simple but elegant: to reduce radio congestion by providing standard, repetitive information that does not require live air traffic controller interaction.

ATIS information never changes within the same broadcast cycle. Each time the information updates (typically every 30 minutes or when weather changes significantly), the broadcast receives a new identification letter. This identification is crucial because it allows you to tell the controller which version of the information you have heard, and the controller knows immediately whether your knowledge is current.