Which organization assigns the designator codes used at airports?

The organization responsible for assigning designator codes used at airports is the International Civil Aviation Organization (ICAO). These designator codes are part of the broader framework for international aviation established by ICAO, which sets standards and regulations for the safety, security, efficiency, and environmental sustainability of civil aviation.

ICAO assigns a unique four-letter alphanumeric code to each airport, ensuring that every airport around the world has a standardized identity recognized internationally. This system helps in facilitating communication and coordination in air travel, especially in flight operations and navigation.

In contrast, the International Air Transport Association (IATA), while also involved in the aviation industry, primarily deals with airline passenger and cargo services and assigns three-letter codes that differ from ICAO codes. The Federal Aviation Administration (FAA) is a national authority responsible for regulating civil aviation in the United States but does not issue international designator codes. Air Traffic Control (ATC) is focused on the safe and orderly flow of air traffic but does not assign airport codes themselves. Therefore, the correct answer is the International Civil Aviation Organization (ICAO).