Posted on Jan 1, 1

Introduction to Aviation Datalink: ACARS

This article is a gentle introduction to the ACARS system used for aircraft communication.

ACARS is a global short message (like SMS) system for aircrafts. Example messages include but are not limited to: digital clearance (PDC), digital ATIS (D-ATIS), digital position reporting (ADS-C), digital controller commands (CPDLC), airline messages from the AOC, automated weather updates (METAR / TAF updates), and automated diagnoses messages from various parts of the aircraft.

Initially deployed in 1970s, ACARS was originally meant to replace the manual block time reporting on voice, so the airlines can automatically and precisely track how many hours did the crews work and pay them accordingly (probably meant to pay them less). The original transmission reuses the existing VHF airband (118MHz to 138MHz).

After decades, more and more voice communication were replaced by ACARS messages. The lengthy IFR clearance and ATIS recordings are common ones. Enroute controller commands are also largely replaced by digital CPDLC, so the pilots won’t got distracted by the annoying ATC commands enroute which seldomly comes and interrupts their chat (LOL). Nowadays, millions and millions of ACARS messages are getting transported every month across the globe, and almost all modern airlines use it.

The ACARS system evoloved from using a single VHF protocol to different medias, enabling a global coverage. Besides the original VHF ACARS protocol (now called Plain-old-ACARS, PoA), a new VHF protocol called VDL2 was built, which has much greater transmission speed. Also, HFDL and Satcom are also in-use to provide ACARS coverage on remote areas like the polar or over the oceans.

Even though the ACARS system is so advanced and widely-used, barriers still exist, preventing enthuisasts from understanding how the system works. Luckily, there are already lots of community work done in this field. Software like acarsdec (PoA demodulator), dumpvdl2 (VDL2 demodulator), dumphfdl (HFDL demodulator), JAERO (Inmarsat demodulator) exist, allowing us to easily listen to the datalink at home. People also built Airframes.io, a community-feeded database like Flightradar24, but for datalink.

Unfortunately, with all these community work, ACARS is still mysterious in many aspects. Tons of messages are airline-custom or some proprietary formats used in some avionics, while the others are all protected under the extremely expensive ARINC standards (costing more than 500 US dollars each PDF). The Airframes community has an on-going reverse engineer work that tries to document as many messages as possible, but the ACARS world is too unstandardized and too proprietary.

Here comes this article. I am not trying to understand and explain how each bit is modulated in VDL2, nor am I going to master every airline-specific ACARS message and parse any engine diagnose data from them. Instead, this article is a comprehensive overview to the whole ACARS system, from the application layer to link layer, hoping to give newcomers a fresh start on these topics.

Dis-ambiguous

If you try to lookup “ACARS” online, you will see the following topics:

  • ACARS VHF signals
  • The ACARS system
  • ACARS messages
  • CPDLC commands
  • VDL2 signals

and so on.

Confusion raises. What is the difference between ACARS and CPDLC (they are usually used together)? Is ACARS specific to a couple of 13xMHz and 12xMHz frequencies? Is ACARS a protocol, a system, or some kinds of CPDLC format?

To clarify on the confusion, we must learn a little bit on the history of ACARS and its modern architecture, since most of the confusion is a result of evolution within decades.

History of ACARS

ACARS begin in the 1970s. Before that, all aeronautical communications were in VHF voice. This yields in a siginificant problem

  • What is ACARS, a bit of history, voice, Telex, ARINC, SITA
  • ACARS architecture (Application layer, network layer, link layer)
  • Application layer
    • FANS-1/A CPDLC
    • FANS-1/A ADS-C
    • Flight services (AOC, ATIS, METAR)
    • Other messages (Diagnoses, OOOI)
  • Network layer (ACARS)
    • Addressing
    • Message format
    • ACK
    • Squitter
  • Link layer
    • Plain-old-ACARS (AM-MSK)
    • VDL2 (Raw-PSK, XID, I / S / U frames, addressing)
    • HFDL (LPDU, Ground stations, …)
    • Inmarsat (…)
    • Iridium (…)
  • Further reading: Standards
    • ARINC xxx
    • DO xxx
    • ICAO xxx