Decode ACARS Aircraft Messages

~100 minIntermediate

Decode real aircraft datalink messages at 131.550 MHz using acarsdec — from first decode through to building a permanent 24/7 monitoring station.

Prerequisites
Gather these before starting

Hardware

  • •RTL-SDR dongle (any model)
  • •Antenna for 131 MHz (1/4-wave whip or discone)
  • •Computer with USB and ideally outdoor antenna placement
  • •Location within ~200 km of a major airport

Software / Knowledge

  • •acarsdec (built in Step 3)
  • •rtl-sdr tools installed
  • •Linux/macOS terminal comfort
  • •Basic understanding of VHF reception
Step 1 of 714% complete
Step 110 min
What is ACARS (131.550 MHz)

ACARS (Aircraft Communications Addressing and Reporting System) is a digital datalink protocol used by commercial aircraft to exchange short text messages with airline operations, air traffic control, and automated ground systems. First deployed in 1978, it remains active on hundreds of thousands of flights daily worldwide.

Protocol basics

ModulationAM-MSK (2400 baud)
Primary frequencies131.550, 131.725 MHz
Channel bandwidth~2.5 kHz
ModeAM (not FM!)
Protocol layerARINC 618 over VDL Mode 0/A
EncryptionNone (plaintext)

What ACARS messages contain

  • Departure/arrival reports (OUT/OFF/ON/IN — OOOI)
  • Engine and performance data (ACMS reports)
  • Weather requests and ATIS text
  • Crew pre-departure clearances (PDC)
  • Aircraft position and fuel state
  • Maintenance fault reports (CMC messages)
  • Passenger counts and baggage weights

Key ACARS frequencies by region

North America primary131.550 MHz
North America secondary130.025, 130.450 MHz
Europe primary131.725 MHz
Europe secondary131.525, 131.550 MHz
Asia-Pacific131.550, 131.450 MHz
South America131.550 MHz

131.550 MHz is the primary ACARS frequency in North America. Europe uses 131.725 MHz as primary. Many aircraft transmit on both — check both frequencies when starting out.

What is ACARS (131.550 MHz)