Introduction to ACARS

ACARS (Aircraft Communications Addressing and Reporting System) is a digital communication system used to transmit and receive short messages between aircraft and ground-based systems. Developed in 1978 by ARINC, it was originally designed to automate routine communications between pilots and their airlines, replacing voice-based position reporting and reducing workload for both crews and dispatchers.

Plain Old ACARS (POA)

The original ACARS system — often referred to as Plain Old ACARS, or POA — transmits over VHF (Very High Frequency) radio, primarily on frequencies like 129.125 MHz and 131.550 MHz. POA uses a relatively simple modulation scheme (AM MSK) with low data rates, but its simplicity and reliability have kept it in widespread use for nearly five decades.

POA is line-of-sight, meaning it works when the aircraft is within range of a VHF ground station. This limits its coverage to continental areas and near-coastal regions. For communication beyond VHF range, ACARS data is carried over other mediums — see the Technology Overview for the full landscape.

What ACARS Carries

ACARS messages cover a wide range of aviation operations. Some of the most common message types include:

• OOOI Reports — Out, Off, On, In events that automatically log gate departure, takeoff, landing, and gate arrival times.
• Position Reports — Periodic latitude/longitude/altitude reports, especially important for oceanic flights.
• Weather Data — METARs, TAFs, NOTAMs, and PIREPs delivered to the cockpit.
• Pre-Departure Clearances (PDC) — Digital delivery of ATC clearances before pushback.
• Free-Text Messages — Airline operations, crew scheduling, gate changes, and other operational text.
• Maintenance & Health Reports — Engine parameters, fault codes, and airframe fatigue data transmitted in-flight.
• CPDLC and ADS-C — Controller-Pilot Data Link Communications and surveillance contracts used for air traffic management.

See the Introduction for the full list of ACARS use cases.

ACARS Frequencies

VHF ACARS operates on dedicated frequencies within the aviation VHF band. The primary frequencies vary by region:

Common Frequencies

Frequency	Region	Provider	Notes
129.125 MHz	Global	ARINC	Primary worldwide ACARS frequency
130.025 MHz	USA / Canada	ARINC	Secondary North American channel
130.425 MHz	USA	ARINC	Additional channel
130.450 MHz	USA / Canada	ARINC	Additional channel
131.125 MHz	USA	ARINC	Additional channel
131.475 MHz	Japan		Japan primary
131.525 MHz	Europe	SITA	European secondary
131.550 MHz	Global	SITA	Primary worldwide SITA channel
131.725 MHz	Europe	SITA	European channel

tip

You don't need to monitor all frequencies. Start with 129.125 and 131.550 — these two carry the majority of ACARS traffic worldwide. Add more channels as you learn which are active in your area. acarsdec can decode up to 8 channels simultaneously from a single SDR.

Receiving ACARS

VHF ACARS is the easiest entry point for hobbyists. A basic RTL-SDR dongle, a VHF antenna, and the acarsdec decoder are all you need to start receiving and decoding live ACARS messages in your area. Once decoded, you can feed the data to Airframes to contribute to the global network.

Quick Start

1. Get hardware — An RTL-SDR Blog V4 or Orange SDR and any VHF antenna (even a 53 cm wire works for testing).
2. Install acarsdec — Follow the installation guide.
3. Start decoding — Run acarsdec tuned to 129.125 and 131.550 MHz.
4. Feed Airframes — Configure the output to send data to the Airframes aggregator.

See the Getting Started guide for a full walkthrough.