Aircraft Communications Addressing and Reporting System

What is ACARS?

ACARS (Aircraft Communications Addressing and Reporting System) is the digital messaging backbone of airline operations. Developed in 1978 by ARINC, it replaced the need for pilots to relay routine operational messages by voice, instead transmitting short digital messages between the aircraft and the airline's operations center. An ACARS message might contain a position report, a weather update request, a gate assignment change, a maintenance fault code, or a fuel status update — all sent automatically or by crew input.

ACARS operates over three different media: VHF (line-of-sight, used over land), HF (long-range, used over oceans and remote areas), and satellite (SATCOM, global coverage). The system automatically selects the best available link based on aircraft position. VHF ACARS uses a network of ground stations operated by providers like ARINC (now Collins Aerospace) and SITA. Satellite ACARS uses the Inmarsat or Iridium constellations. Each message is typically under 200 characters — this is a low-bandwidth, high-reliability system, not an internet connection.

Modern ACARS has evolved significantly from its original specification. FANS (Future Air Navigation System) uses ACARS as the transport layer for Controller-Pilot Data Link Communications (CPDLC), enabling text-based ATC clearances over oceanic airspace. ADS-C (Automatic Dependent Surveillance — Contract) also rides on the ACARS network, providing position reports at intervals agreed between the aircraft and ATC. These applications have made ACARS essential infrastructure for oceanic and remote airspace operations where radar coverage does not exist.

Why It Matters for Airspace Risk

ACARS provides an independent data channel that continues to function when GPS-based systems are compromised. In GPS-denied environments — where jamming or spoofing degrades ADS-B surveillance and navigation accuracy — ACARS-based position reports (via ADS-C or manual crew entries) provide an alternative means of tracking aircraft. The MH370 investigation demonstrated the forensic value of ACARS satellite handshakes in reconstructing aircraft positions when all other tracking was lost.

For airspace risk monitoring, ACARS data is a valuable signal source. Airlines can use ACARS to relay real-time GPS interference reports from cockpits to their operations centers, enabling rapid assessment of jamming or spoofing events. Some operators have implemented automated ACARS triggers that fire when the aircraft's navigation system detects GPS anomalies, creating a near-real-time picture of interference patterns across their fleet. This operational data, aggregated across multiple airlines, forms one of the inputs used to map active interference zones. ACARS also carries the OOOI events (Out of gate, Off the ground, On the ground, Into the gate) that enable precise flight tracking independent of ADS-B.

Key Facts

• •ACARS operates over VHF, HF, and satellite (SATCOM), automatically selecting the best available link.
• •FANS/CPDLC and ADS-C ride on the ACARS network, enabling datalink ATC and surveillance over oceanic airspace.
• •ACARS satellite handshakes were key evidence in determining MH370's flight path across the Indian Ocean.
• •Messages are typically under 200 characters — a low-bandwidth, high-reliability system designed for operational data.
• •Airlines can use ACARS to relay GPS interference reports from aircraft to operations centers in near-real-time.

Related Terms