Controller–pilot data link communications

One of the major problems with voice radio communications used in this manner is that all pilots being handled by a particular controller are tuned to the same frequency.

This includes a set of clearance/information/request message elements which correspond to voice phraseology employed by air traffic control procedures.

The sequence of messages between the controller and a pilot relating to a particular transaction (for example request and receipt of a clearance) is termed a ‘dialogue’.

The net result of this decrease in voice channel occupancy is increased flight safety and efficiency through more effective communications.

"[2] There are two main implementations of CPDLC: The following Area Control Centres (ACCs) offer CPDLC services: Following the PETAL I and II (Preliminary Eurocontrol Trial Air Ground Data link) trials in 1995 including NEAN (VDL Mode 4), today both ATN (VDL Mode 2) and FANS 1/A services are supported.

ATC clearance (ACL), aircraft communication messages (ACM), and check mike (AMC) services are supported, including the automatic uplink of the SSR transponder code into the cockpit.

CPDLC will probably be a major enabler for following on projects as monitor message, route clearance uplink, 2-4 D trajectories, continuous descent approaches, and constraint coordination also.

A consequence of these requirements is that CPDLC implementations, both on aircraft and at ATC centres, must have access to an accurate clock (to within 1 second of UTC).