abstract:High Frequency Data Link (HFDL) is an ACARS communications media used to exchange data such as Aeronautical Operational Control (AOC) messages, Controller Pilot Data Link Communications (CPDLC) messages and Automatic Dependent Surveillance (ADS) messages between aircraft end-systems and corresponding ground-based HFDL ground stations. Using the unique propagation characteristics of high-frequency radio waves, the ground stations provide data link communications to properly equipped aircraft operating anywhere in the world.