Receiving ACARS messages with an SDR: acarsdec and ACARS Hub

The aircraft taxiing past your spot isn’t just broadcasting its position via ADS-B. At the same time, it sends short text messages to the airline: when it was pushed back from the gate, how much fuel is on board, whether a component has reported a fault. This datalink is called ACARS. With an inexpensive SDR stick, the right antenna and two open-source tools – acarsdec and ACARS Hub – you can technically see what passes back and forth between cockpit and ground. This guide explains frequencies, hardware, setup and evaluation. And it tells you plainly where the legal limits lie in Germany.

Receiving ACARS: the short answer

You need an SDR stick, an antenna for the VHF airband, a small Linux computer and the acarsdec decoder. You set acarsdec to the European ACARS channels between roughly 131.5 and 131.9 MHz, and it prints every message it receives as text. If you want to search everything conveniently, send the output to ACARS Hub, a web interface that runs in a Docker container. Before you get started, though, you should know the legal situation. More on that further down.

What ACARS is in principle and what airlines use it for is covered in the basics article ACARS explained. This one is about the practice at the receiver.

What is inside ACARS messages

ACARS is not voice radio but a datalink for short telegrams. The system was originally developed to automatically report the four phases of a turnaround: Out (away from the gate), Off (airborne), On (landed) and In (at the gate). Sensors on the doors, parking brake and landing gear trigger these so-called OOOI reports without the crew having to do anything.

Today, much more runs over it:

• Operational data such as fuel quantities, departure and arrival times or reasons for delays
• Weather, for example requested METARs and TAFs for destination and alternate airports – how to read such weather reports is covered in How to read METAR and TAF
• Maintenance and engine data that goes automatically to the airline’s technical department
• Position reports, especially on routes without radar coverage
• Free text between the crew and the operations center

Every message carries a fixed header. Among other things, it contains the aircraft’s registration, a flight number, a two-character label that identifies the message type, and block and message numbers. Interesting for spotters: The registration appears in plain text, not as a hex code like with Mode S. Why flight number and callsign are not the same thing is explained in Callsign and flight number.

Much of the content, however, is machine-readable and cryptic to humans: strings of numbers, abbreviations, airline-specific formats. ACARS Hub and the libraries behind it translate some of it into readable fields, but far from all.

ACARS, VDL Mode 2, HFDL: the datalink family

Classic ACARS on VHF is only one way these messages travel. For planning your receiving setup, you should know the relatives:

Method	Frequency range	Data rate	Typical use	Decoder
ACARS (VHF, “Plain Old ACARS”)	around 131 MHz	2.4 kbit/s	older and many current aircraft, near the ground and cruise over land	acarsdec
VDL Mode 2	136.700–136.975 MHz	31.5 kbit/s	modern VHF datalink, also used for CPDLC in Europe	dumpvdl2
HFDL	shortwave	much lower	oceans and polar regions	dumphfdl
SATCOM	L-band via satellite	variable	worldwide, independent of ground stations	satdump, JAERO

Important for Europe: A lot of traffic that used to run over classic ACARS has moved to VDL Mode 2. The common signaling channel is on 136.975 MHz. If you are disappointed by how little arrives after setting up acarsdec, that is often not down to your setup. How to pick up VDL2 as well is described in Receiving VDL Mode 2, and for shortwave there is the guide to HFDL.

The frequencies in Europe

For ACARS in Europe, frequency lists mainly give these channels:

• 131.525 MHz
• 131.725 MHz
• 131.825 MHz or 131.850 MHz – sources differ here, so it is worth checking both when testing

Worldwide, 131.550 MHz is considered the main channel. In North America, further frequencies are added, such as 129.125, 130.025, 130.450 and 131.125 MHz. Frequency lists online sometimes differ in the details. So don’t rely blindly on one list. Instead, check in the waterfall display of an SDR program such as SDR++ or SDR# where things regularly “flash up” at your location. ACARS bursts last only fractions of a second to a few seconds and are easy to recognize as short, wide streaks.

The practical advantage: The European channels are only a few hundred kilohertz apart. A single RTL-SDR stick captures them all at once without trouble, and acarsdec decodes them all in parallel. The documentation of the Airframes fork gives as a rule that all frequencies of one stick must lie within 2 MHz.

Hardware: what you really need

The good news: If you have already built an ADS-B receiver yourself, you already know most of the parts. There are two differences, though.

A stick of its own. An RTL-SDR can only receive one frequency window at a time. If your stick runs on 1090 MHz for ADS-B, you need a second one for ACARS. Which sticks have proven themselves is compared in SDR sticks compared. For VHF, a model with a TCXO, meaning a temperature-stable oscillator, is nice because the frequency then barely drifts. Besides RTL-SDR, acarsdec also supports Airspy, SDRplay and devices via SoapySDR – details on Airspy are in Airspy for ADS-B.

An airband antenna. An antenna for 1090 MHz is far too short for 131 MHz. You need an antenna for the VHF airband: a ready-made airband ground plane, a discone or a homemade quarter-wave radiator. A quarter of the wavelength at around 131 MHz is about 57 centimeters. As with ADS-B reception, height and a clear view beat any amount of gain. The principles from Finding the best antenna location apply here as well.

A computer. A Raspberry Pi is enough for acarsdec and ACARS Hub. In its current version, ACARS Hub needs a 64-bit system (arm64 or amd64). If you don’t have a Pi, you will find alternatives in ADS-B reception without a Raspberry Pi.

Optionally, an FM notch filter. Strong FM broadcast transmitters between 88 and 108 MHz are not far from the airband and can overload an inexpensive stick. A band-stop filter for the broadcast range helps if your waterfall is full of interference. The basics of filters and preamplifiers are in Boosting ADS-B range.

acarsdec: the decoder

acarsdec originally comes from Thierry Leconte and is free software. There are now several further developments, among them a fork by Airframes, which the ACARS Hub project recommends, and an actively maintained variant whose newer versions have a revised command line with --output options. So the exact syntax depends on which version you install. When in doubt, read the README of your variant.

The basic principle is the same for all of them: You choose the stick, set the gain and frequency correction and specify the channels. With the current variant, a test run on the European frequencies looks something like this:

acarsdec --output monitor:file -g 34 --rtlsdr 0 131.525 131.725 131.825 131.850

The most important adjustments:

• Gain (-g): According to the docs, automatic gain is usually set to maximum and so is rarely optimal. Work your way up manually, typically in the middle to upper range, and watch how many messages arrive cleanly.
• Frequency correction (-p): Inexpensive sticks without a TCXO are off by a few ppm. You determine the value once and enter it permanently.
• Bias tee (-B): Only needed if you power a preamplifier at the antenna mast through the coax cable.
• Output: acarsdec outputs messages as one-liners, verbose, in monitor mode or as JSON – to a file, via UDP or via MQTT. For ACARS Hub, JSON over the network is the usual way.

If the test run works and the first messages with registrations show up after a few minutes, your chain is working. At large airports you get more, in rural areas with overflights at cruising altitude less.

ACARS Hub: collecting and searching messages

Raw output in the terminal is exciting for ten minutes. After that, you want to search, filter and see statistics. That is exactly what ACARS Hub does. It comes from the sdr-enthusiasts project, which also maintains the Docker images for ADS-B around Ultrafeeder.

ACARS Hub does not decode any radio signals itself. It takes the output of external decoders:

• acarsdec for classic ACARS
• dumpvdl2 for VDL Mode 2 (vdlm2dec works as well, but is no longer being developed)
• dumphfdl for HFDL
• satdump or JAERO for Inmarsat satellite data, plus tools for Iridium

The web interface runs on port 80 by default (configurable) and shows messages in real time. Everything ends up in a SQLite database. By default, ACARS Hub keeps normal messages for seven days and hits on alert terms for 120 days. On a Raspberry Pi, the project recommends putting the database in RAM (tmpfs) to spare the SD card. With very large databases, search can become slow on weak hardware.

Connecting decoders

The most convenient way is to run acarsdec as a container too. The docker-acarsdec image from sdr-enthusiasts mainly expects a unique FEED_ID, the list of FREQUENCIES and the stick’s serial number in RTL_SERIAL. Output goes by default to acars_router on port 5550. This router collects the messages of all decoders, removes duplicates and passes them on to ACARS Hub. The project recommends TCP or ZMQ instead of UDP because transmission is then more reliable.

If you already feed several ADS-B networks via Docker, all of this will feel familiar. The setup follows the same logic as in Feeding multiple networks with Ultrafeeder.

Alert terms

In the web interface, you can store terms that ACARS Hub reacts to – for example a specific registration or a flight number. When a message matches, it is highlighted and stored for longer. For spotters, this is the most interesting feature: You see whether an aircraft you are waiting for is active within your reception range.

Map with ADS-B data

If you run an ADS-B receiver in parallel, ACARS Hub can read the aircraft.json from tar1090 or Ultrafeeder. The aircraft within range then appear on a map, and a click shows the messages that belong to them. To do this, you set ENABLE_ADSB and enter the address of the JSON file. How tar1090 itself works is described in Flight tracking on your PC with tar1090.

The legal situation: this is where it gets serious

As simple as the technology is, the legal situation in Germany is just as clear. ACARS messages are intended for airlines, datalink service providers and air traffic control. They are aimed neither at the general public nor at an indefinite group of people. So, in the widely held view, the same applies to them as to voice radio: § 5 TDDDG prohibits listening in on messages not intended for you, and also passing on their content or even the fact that you received them. Violations are punishable under § 27 TDDDG.

What this means in concrete terms is covered in detail in Listening to airband radio: technology and legal situation. The difference from ADS-B, which counts as an open broadcast, is explained in Is receiving ADS-B data legal?. Austria, Switzerland and other countries have their own rules, which you should check before operating.

This assessment is not legal advice. If you want to run a receiving setup permanently, it is worth getting expert advice.

Troubleshooting: when nothing arrives

Symptom	Possible cause	What you can do
No messages at all	wrong frequencies, stick not found	check the waterfall, verify serial number and device index
Bursts in the waterfall, but no decoding	frequency offset of the stick	determine the ppm correction and enter it
Lots of noise, few messages	overload from FM broadcast	lower the gain, use an FM notch filter
Only messages from aircraft on the ground nearby	antenna too low or unsuitable	mount the airband antenna higher and clear
Much less traffic than expected	datalink runs over VDL Mode 2	add a second stick with dumpvdl2 on 136.975 MHz

A note on expectations: ACARS traffic comes in waves. Around turnaround times at a nearby airport, a lot can be going on, while at night it is often quiet for a long time. So judge your setup over several days, not over one hour.

ACARS, ADS-B and spotting

For most spotters, ACARS is a technical side project, not a tool for everyday life at the fence. The useful information – which registration is behind a flight number, when an aircraft lands, where it is right now – you get legally and more conveniently via ADS-B and flight tracking. How to find a particular aircraft by its registration is shown in Tracking an aircraft by registration. How ADS-B and transponders are related is explained in Mode S and transponders.

If you like experimenting with radio technology, you still learn a lot from the ACARS project: how an SDR processes several channels at once, how Docker containers work together and how communication between aircraft and airline is built technically. You will find related projects in Receiving airband radio with an SDR stick.

Conclusion

Receiving ACARS is technically a manageable project: a second SDR stick, an airband antenna, acarsdec on the European channels between roughly 131.5 and 131.9 MHz and, if you like, ACARS Hub as a web interface. If you want to see more of the datalink, add VDL Mode 2. The real hurdle is not the technology but the law. In Germany, ACARS messages are not intended for you, and in the widely held view even passing them on is prohibited. Sort that out before you bolt the antenna to the roof.