Field Guide · protocol

Also known as: CPDLC

CPDLC (Controller-Pilot Data Link Communications) is an aviation protocol that lets air traffic controllers and flight crews exchange routine clearances, requests, and reports as text messages instead of voice. It rides over air-ground data links such as VDL Mode 2, HFDL, and satellite communications, and in older deployments over character-oriented ACARS.1 By moving structured, repetitive traffic off the congested voice channels, CPDLC reduces frequency loading and read-back errors, especially on oceanic and remote routes.

ATC ground datalink aircraft "CLIMB TO FL350" / "REQUEST DIRECT" — structured text, not voice
CPDLC carries structured clearances and requests as text between controllers and aircraft over VHF, HF, or satellite data links.

Overview

A CPDLC session pairs one aircraft with the controlling authority (ATC unit) that currently owns it. Controllers send uplink messages drawn from a standardised set (clearances, instructions, requests for information, negotiations), and crews reply with downlink messages (requests, confirmations, WILCO/UNABLE responses). Message elements are defined by ICAO so that the same intent renders consistently regardless of the underlying bearer. Two message-standard families coexist: the older FANS-1/A, built on ACARS and used widely over oceans, and the newer ATN Baseline 1 (B1), used in European continental airspace.

Technical characteristics

Property Value
Message set ICAO Doc 4444 / DO-258A element library
Bearers VDL Mode 2, HFDL, Inmarsat/Iridium satcom, legacy ACARS
Families FANS-1/A (ACARS-based), ATN B1
Session One controlling ATC unit at a time, with hand-off
Content Clearances, requests, reports, free text

History

CPDLC emerged from the ICAO Future Air Navigation System (FANS) effort in the 1990s, with FANS-1/A pioneered by Boeing and Airbus for oceanic operations where voice HF is poor. The ATN variant, standardised by RTCA and EUROCAE and coordinated through ICAO, followed for high-density continental airspace under programmes such as Europe’s Data Link Services mandate.

Deployment

CPDLC is now routine on oceanic tracks (North Atlantic, Pacific) and in European en-route sectors, typically alongside ADS-C position contracts. Airliners carry it in their communication management units; general aviation rarely does.

Decoding it with GopherTrunk

GopherTrunk is a land-mobile trunking scanner and does not decode CPDLC. The protocol sits above bearers that GT does not currently demodulate, and much of its interest is in higher-layer messaging rather than the RF/trunking control plane GT targets. Hobbyists who want to see CPDLC typically decode the underlying VDL Mode 2 or ACARS link with purpose-built aviation tools.

Sources

  1. Controller-pilot data link communications — Wikipedia, for the CPDLC message concept, FANS-1/A and ATN families, and the VHF/HF/satcom bearers it runs over. ↩

See also