Tutorial ACARS

Tutorial: ACARS (VHF aircraft messages)

What it is

ACARS is the short-message link between aircraft and airline or air traffic ground systems on VHF. It carries position and weather reports, flight-plan and gate information, maintenance data and sometimes free text from the crew. Each message is a block with a label (a two-character code that says what it is) and text. OnAir decodes every ACARS carrier in the band you capture and shows messages, a map and per-channel statistics.

What you need

• Any radio that tunes 118 to 137 MHz. The mode asks for 2 Msps and works from 1 Msps; the channel width shown in the mode table is 2 MHz, so one capture can hold several 25 kHz channels. An RTL-SDR is enough. See Supported radios.
• A VHF airband antenna or a simple vertical, outdoors if possible. Aircraft are line of sight, so height matters more than gain.
• Frequencies from the project's database: 131.550 MHz (ACARS primary, worldwide), 131.525 (secondary), 131.725 and 131.825 (Europe), 130.025 and 131.475 (US), 131.450 (Japan), 136.900 and 136.925 (global). The mode starts at 131.5 MHz.

Before you start

• A frequency near 131.5 MHz with 2 Msps covers 130.5 to 132.5 MHz, which holds the European and North American main channels at once. Stay away from the very edge of the band, where the filter rolls off and a DC spike sits at the centre.
• Keep gain moderate: strong FM broadcast stations just below 108 MHz can overload a radio with a wide front end.

Step by step

1. Choose the group Aviation, entry Aircraft messages, VHF (mode ACARS).
2. In the tuner, Channels MHz is a list of channels separated by spaces, for example 131.525 131.725 131.825. Leave it empty and every carrier found on the 25 kHz grid in the captured band is decoded. Europe uses 131.525, 131.725 and 131.825; North America 131.550 and 130.025.
3. Set THRESHOLD, the carrier-over-noise needed to start decoding a channel. Lower finds weaker aircraft but also decodes noise.
4. Press Start. The status shows "Searching" until a block decodes.

What you should see

Messages arrive in bursts: a handful per minute on a quiet channel, many near a hub. Good blocks have a valid parity and CRC. In the Channels view the BLOCKS OK % shows how many blocks pass.

Reading the screen

• Messages: time (UTC), channel, SNR, registration and flight, the block Label, Meaning (a plain name for the label) and Text. Click a message to read all of it. Latest messages keeps the newest first. A filter box shows only blocks whose label starts with what you type (for example H1 or 5Z), and a switch leaves out uplinks (ground to aircraft).
• Aircraft map: aircraft with positions, blue for ADS-C reports and amber for text reports, grey if older than 30 minutes. Click an aircraft on the map or in the list.
• Aircraft: one row per aircraft with ICAO address, registration, flight, last label, position, altitude and last heard.
• Channels: per channel level, carrier offset (CFO), SNR, good and bad blocks and state.

What the messages contain

• OOOI events: Out (left the gate), Off (took off), On (landed), In (at the gate), with times.
• Position reports, sometimes with altitude, fuel and wind; ADS-C reports when decoded.
• Free text between crew and ground, weather requests, ATIS and clearances, maintenance and engine reports. Label codes (for example H1 for a message sent through the flight management computer, 5Z for airline operational messages) are shown with their meaning where known.

Options and settings

Predict positions and uncertainty (on the map) dead-reckon an aircraft whose last report is a few minutes old and draw a 95 percent ellipse. Position prediction is an estimate, never a position report.

Troubleshooting

• Nothing decodes: confirm that your region uses the channels you listed; a US radio listening to European channels hears nothing. Leave the list empty to search the whole capture.
• Only noise decoded: raise the threshold.
• Few messages: ACARS traffic has fallen as airlines moved to VDL Mode 2; try VDL2 too.
• Strong breakthrough or ghost carriers: reduce gain.
• Weak channel beside strong ones: lower the gain a little. The simulated signal has a "weak ch." control to rehearse this case.

Tips

ACARS aircraft seldom send a position that is near-continuous; use ADS-B together with it and the card of the aircraft shows its ACARS messages. See Tutorial: ADS-B.

Legal and privacy notes

ACARS is a plain, unencrypted broadcast, but in some countries listening to or using what you hear is restricted. Messages can contain names, phone numbers and operational details; do not publish them. When you share to the public map, ACARS text is removed of phone numbers and e-mail addresses and only goes out if you opt in. See Legal and Privacy.

Under the hood

AM carrier with MSK at 2400 baud (minimum shift keying, 1200 and 2400 Hz tones), 25 kHz channel spacing. Blocks carry mode, aircraft registration, label, block identifier, text up to 220 characters, a CRC-16 and odd parity per character. Defined in ARINC 618 (air-ground protocol) and ARINC 620 (message formats).

Related

ACARS mode page, Tutorial: VDL2, Tutorial: HFDL, Tutorial: ADS-B, Live map, Troubleshooting.