What is ACARS?

The Aircraft Communications Addressing and Reporting System (ACARS) is a digital datalink system used by aircraft to exchange short messages with ground stations and airline operations. Introduced in the late 1970s, ACARS was originally designed to automate routine tasks—such as logging departure times—to reduce crew workload and improve operational efficiency.

At its core, ACARS is a text-based messaging system for aviation. Unlike consumer messaging platforms, however, it is tightly integrated into aircraft avionics and airline infrastructure. Messages can be automatically generated by onboard systems or manually entered by the flight crew, covering everything from engine performance data to gate arrival notifications.

Today, ACARS operates across multiple communication mediums:

VHF (Very High Frequency) — Primary for short-range, line-of-sight communication
HF (High Frequency) — Used for long-distance oceanic communication
SATCOM (Satellite Communication) — Provides global coverage via systems such as Inmarsat

The Evolution of ACARS

1970s: The Birth of "OOOI"

ACARS was developed in 1978 by ARINC (now part of Collins Aerospace), and its original purpose was surprisingly simple: time tracking. Airlines needed a reliable, automated way to record four key flight events, known as OOOI:

Out
Aircraft leaves the gate (brakes released)

Off
Aircraft takes off (weight-off-wheels)

On
Aircraft lands (weight-on-wheels)

In
Aircraft arrives at the gate (brakes set)

This automation eliminated manual logging by pilots, provided accurate flight duration data, and improved payroll and operational efficiency.

1980s: From Timekeeping to Technical Monitoring

As avionics advanced, airlines realized ACARS could carry far more than simple timestamps. The system evolved into a teletype-style datalink, enabling:

Engine Health Monitoring — Aircraft systems could automatically report faults (e.g., engine temperature exceedances)
Weather Updates — Digital weather briefings sent directly to the cockpit
Pre-Departure Clearances (PDC) — Reduced long and error-prone voice exchanges with ATC

This marked the transition from a logging system to a true operational communication network.

1990s: Going Global (Satellite Integration)

Early ACARS relied heavily on VHF, which works well over land but fails over oceans. The introduction of SATCOM in the 1990s transformed ACARS into a global system:

Satellite ACARS via Inmarsat — Enabled communication far beyond VHF range
FANS (Future Air Navigation System) — Aircraft could automatically report position via satellite, allowing ATC to track aircraft outside radar coverage
Global Standardization — Two major providers—ARINC and SITA—established a worldwide ACARS network

Aircraft were now effectively "always connected," even mid-ocean.

2000s to Present: The Digital Shift

The original ACARS system uses MSK modulation at 2400 bps—reliable, but limited. Modern aviation has gradually evolved beyond this constraint:

VDL Mode 2
Higher data rates, more efficient spectrum usage

CPDLC
Controller–Pilot Data Link Communications

CPDLC represents a major advancement: ATC instructions sent as digital messages (e.g., "Climb and maintain FL350" delivered as text). Pilots can accept or reject messages and load instructions directly into flight systems. In essence, this is "texting with ATC," reducing voice congestion and miscommunication.

Types of ACARS Communication

VHF ACARS
118 – 137 MHz, MSK modulation, 2400 bps, line-of-sight (~200-300 km range)

HF ACARS
2 – 30 MHz, skywave propagation, oceanic/remote coverage, variable reliability

SATCOM ACARS
L-band (~1.5 GHz), near-global coverage, higher latency than VHF

VDL Mode 2
118 – 137 MHz, D8PSK modulation, up to 31.5 kbps, packet-based

Multi-Link Operation: Modern aircraft dynamically switch between available links based on signal strength, availability, cost (SATCOM is expensive), and message priority. On ground → VHF. Climbing → VHF. Over ocean → SATCOM or HF. Near destination → back to VHF.

Software Setup – Decoding with acarsdec

For decoding VHF ACARS, one of the most reliable and lightweight tools available is acarsdec. It is a fast, command-line decoder capable of handling multiple channels simultaneously while maintaining excellent performance—even in busy RF environments like Dubai International Airport.

Hardware
RTL-SDR Blog V3 dongle with telescopic antenna kit configured as a dipole

Filtering
RTL-SDR Blog FM band-stop filter to suppress strong local broadcast interference

Recommended Command

acarsdec -e -t 1800 --output full:file:path=/home/dragonos/Desktop/decoded-acars.log --output monitor:file: --rtlsdr 0 -g 40.2 -c 131.500 131.175 131.475 131.725 131.825

Option Breakdown

-e — Stop outputing empty messages
-t 1800 — Set forget time in seconds on live monitor mode
--output full:file:path=... — Save full decoded messages to log file
--output monitor:file: — Display real-time output to terminal
--rtlsdr 0 — Use RTL-SDR device index 0
-g 40.2 — Set tuner gain (adjust for your environment)
-c 131.500 — Set center frequency as 131.500Mhz

The command above monitors four ACARS frequencies: 131.175 MHz, 131.475 MHz, 131.725 MHz, and 131.825 MHz. Adjust frequencies based on your local airband activity.

ACARS Message Types & Labels

ACARS carries a wide range of operational, technical, and control data between aircraft and ground systems. Here's what you can expect to receive:

OOOI (Flight Progress)
Gate departure, takeoff, landing, and arrival timestamps

Operational / Dispatch
Load sheets, fuel data, delays

Maintenance Messages
Faults, warnings, and system performance data

Telemetry Reports
Engine status, APU usage, system data

Free Text Messages
Pilot–dispatch communication

ATIS / Weather
Airport weather and operational info, METARs

Pre-Departure Clearance (PDC)
Digital ATC clearances

CPDLC Messages
Digital ATC instructions

Position Reports
Location, altitude, speed

ACK/NAK
Acknowledgments and retransmission requests

Understanding Message Labels

Each ACARS message includes a two-character label, which indicates its purpose:

Label	Description
H1	Flight progress (OOOI and related reports)
A9	ATIS / weather information
Q0	Position reports
B9	ATC-related messages
5U	Weather requests
_d	Acknowledgment / control messages

Labels are not strictly standardized—meanings can vary slightly by airline and region.

ACARS Mode Types

In addition to labels, each message includes a mode, indicating its general format:

Mode	Description
Mode 2	Standard air-to-ground messages (most common)
Mode A	Ground-to-air broadcasts
Mode B	Broadcast/network messaging
Mode C	Control/system-level communication
Mode D	Air-to-ground with extended routing

Common Combinations: Mode 2 + H1 = Flight progress | Mode 2 + A9 = Weather/ATIS | Mode 2 + Q0 = Position reports | Mode 2 + B9 = ATC communication

Final Thoughts

ACARS isn't particularly fast, modern, or even efficient by today's standards—but that's not why it's interesting. What makes it compelling is that it's still everywhere. Every decode is a small piece of a much larger system: aircraft reporting their state, airlines coordinating operations, and ground networks quietly routing messages in the background.

You're not just receiving data—you're observing a system that has been evolving for decades, still doing its job. In practice, it's also a reminder that real-world RF rarely behaves perfectly. Some messages decode cleanly, others arrive incomplete, and many never make it through at all. Between overlapping transmissions, weak signals, and busy channels, what you see on your screen is only a fraction of what's actually on the air.

ACARS sits in an interesting place: old, but still relevant; simple, but deeply integrated. Whether you're using it as a first step into SDR or as a way to explore aviation systems more deeply, it offers a direct look at how aircraft communicate beyond voice—imperfect, continuous, and very much alive.

Exploring VHF ACARS: Decoding Aircraft Communications