Receiving VDL Mode 2: dumpvdl2, frequencies and what the data reveals

If you already have ADS-B on your screen, you know positions, altitudes and speeds. What the aircraft discuss with their airline and air traffic control, you don’t see there. A large part of exactly that runs over VDL Mode 2, a digital datalink in the VHF band. With an inexpensive SDR stick and the open-source decoder dumpvdl2, you can receive and decode VDL2 yourself. This guide shows you which frequencies you need, how to configure dumpvdl2 and what is really in the messages.

VDL Mode 2 in one paragraph

VDL Mode 2 (VHF Data Link Mode 2, often VDL2 or VDLM2 for short) is a digital radio datalink between aircraft and ground stations. It works in the upper part of the aviation band just below 137 MHz, uses D8PSK modulation in channels 25 kHz wide and reaches a raw data rate of 31.5 kbit/s. By comparison, classic VHF ACARS manages 2.4 kbit/s. Mainly ACARS messages between aircraft and airline run over VDL2, and in Europe CPDLC communication between controllers and the cockpit. The ground networks are operated by service providers such as ARINC and SITA.

If ACARS isn’t quite a familiar term yet, it is worth a look at ACARS explained first. To understand VDL2, though, a simple distinction is enough: ACARS is the messaging system, VDL2 one of its transmission paths.

The VDL family: Mode 1 to Mode 4

The name “Mode 2” suggests there are other variants – and there were indeed. In everyday use, though, only one prevailed.

Variant	Principle	Status
VDL Mode 1	data transmission compatible with analog ACARS radio	intended only for validation, removed from the ICAO standards
VDL Mode 2	D8PSK, 31.5 kbit/s, channel access via CSMA	widely used in scheduled operations
VDL Mode 3	data and digital voice in time slots (TDMA)	trialed in the USA, abandoned in 2004
VDL Mode 4	self-organizing time slots, also aircraft to aircraft	standardized by ICAO, but displaced as an ADS-B carrier by Mode S on 1090 MHz

For you as a listener, almost only Mode 2 is therefore relevant. If you are interested in the ADS-B part of the story, you can find the background on the 1090 MHz method in Transponder modes A, C and S explained.

The frequencies: where to find VDL2

VDL2 uses a narrow section at the upper end of the aviation band. The central channel is 136.975 MHz, the so-called Common Signalling Channel (CSC). Aircraft check in here, and in many regions most of the traffic runs here. If capacity isn’t enough, network operators distribute the traffic across further channels.

Frequency	Role
136.975 MHz	Common Signalling Channel – always keep it running
136.875 MHz	additional channel in busy regions
136.775 MHz	additional channel in busy regions
136.725 MHz	additional channel in busy regions

Which additional channels are active near you depends on region and network operator. The most practical way: run all four at once and look after a few hours at which channels messages actually arrive. Because the frequencies are only a few hundred kilohertz apart, a single SDR stick can handle it. Classic analog ACARS, by contrast, sits lower in the band, in Europe for example on 131.525, 131.725 and 131.825 MHz. How to receive that is shown in Receiving ACARS messages with SDR.

You can find more on the structure of the aviation band and the channel allocation in Understanding aviation radio frequencies.

The hardware you need

The good news: VDL2 is not an expensive niche hobby. The basic equipment overlaps heavily with an ADS-B setup.

• SDR receiver: An RTL-SDR stick is the standard. dumpvdl2 also supports Mirics sticks, SDRplay receivers and, via SoapySDR, other devices – a sample rate of 2.1 million samples per second is required there. Devices with fixed sample rates such as Airspy receivers are not supported via SoapySDR, according to the documentation. Which stick suits what is compared in SDR sticks for ADS-B compared.
• Antenna: You need an antenna for the VHF aviation band. A 1090 MHz antenna from an ADS-B receiver is unsuitable for this. A simple quarter-wave ground plane for 137 MHz has a radiator of a good 50 centimeters in length and is easy to build yourself. A broadband scanner antenna for the aviation band works just as well – you can read more on antennas and reception in the VHF band in Receiving aviation radio with an SDR stick.
• Computer: dumpvdl2 runs on Linux, for example on a Raspberry Pi or an old PC. If dumpvdl2 reports lost samples on the Pi, the documentation advises, among other things, decoding fewer channels at once.
• Optionally a filter: Near strong FM broadcast transmitters, a filter against the broadcast band can reduce overload of the stick.

For the location, the same basic rules apply as for ADS-B: as high as possible, as clear a view as possible, short cables. VHF is somewhat more forgiving than 1090 MHz, but here too line of sight determines range. Tips are in Finding the best antenna location.

Installing and starting dumpvdl2

dumpvdl2 is an open-source decoder and protocol analyzer for VDL Mode 2, developed by Tomasz Lemiech and released under the GPL-3.0. It decodes several channels at once and breaks the messages down into the individual protocol layers.

A typical start with an RTL-SDR stick looks like this – here with all four common channels:

dumpvdl2 --rtlsdr 0 --gain 40 --correction 42 136725000 136775000 136875000 136975000

The 42 here stands for your stick’s ppm value and is only an example. In the header of every decoded message, dumpvdl2 shows the measured frequency offset. If it is persistently far from zero, you adjust the correction.

If you prefer to work with containers: the sdr-enthusiasts project offers a Docker image for dumpvdl2 in which you set frequencies, gain and a feeder ID via environment variables. How this ecosystem works in general is shown in Feeding several networks at once with Docker/Ultrafeeder.

Setting gain and signal quality correctly

With VDL2, less is often more. The dumpvdl2 documentation gives about 40 dB as a starting value for RTL-SDR sticks, about 75 dB for Mirics sticks, and expressly advises against turning the gain fully up. The goal is a signal about 15 dB above the noise. Too much gain lifts the noise along with it and makes the stick overload on strong transmitters.

A pragmatic approach:

• Start with 40 dB and let dumpvdl2 run for a while.
• Watch the signal and noise levels in the message headers.
• Change the gain in small steps and compare over at least half an hour how many messages per channel come in.
• SDRplay receivers usually cope well with automatic gain control, according to the documentation.

If you want to optimize seriously, send dumpvdl2’s statistics via StatsD to an external collector and analyze them graphically – similar to ADS-B with the tools from Measuring reception performance.

Output: text, JSON and network

dumpvdl2 can do much more than write text to the terminal. That matters as soon as you want to process the data further.

• Formats: readable text, JSON for your own analysis, a single-line ACARS format for Planeplotter and a binary format for raw frames.
• Destinations: files with hourly or daily rotation, UDP and ZeroMQ. For reliable transmission over the network, the documentation recommends ZeroMQ because UDP doesn’t guarantee delivery.
• Enrichment: With a ground station list, you map station addresses to an airport and location. From a Basestation SQLite database, dumpvdl2 adds aircraft data such as registration and type.
• Filters: With a message filter, you hide uplinks from the ground or empty ACARS messages, for example.
• Identifier: With a station ID, you tag the JSON output when you merge data from several receivers.

It gets convenient with ACARS Hub from the sdr-enthusiasts project. The web interface collects the output of external decoders, stores it in a database, offers search and alert terms and can overlay ADS-B targets on a map. The project page names dumpvdl2 as the preferred VDL2 decoder there. Besides VDL2, ACARS Hub also accepts classic ACARS, HFDL from shortwave and satellite datalinks – an introduction to shortwave is offered by Receiving HFDL.

What the data reveals

Now it gets exciting: what is actually in the messages? dumpvdl2 breaks every frame down into its layers. Roughly, three levels can be distinguished.

The link layer (AVLC)

Every VDL2 frame carries a source and a destination address. For aircraft, that is the ICAO 24-bit address – the same hex code you know from ADS-B. That lets you assign every message unambiguously to an aircraft, even if there is no registration in the content. What is behind this address is explained in ICAO 24-bit address (hex code). Ground stations have addresses of their own, which you resolve with a station list. In addition come management frames (XID), with which aircraft set up connections to ground stations and switch from one station to the next.

ACARS over VDL2

Most of the traffic consists of ACARS messages transported over VDL2. They usually contain the registration and often the flight number, plus a label that identifies the message type. Typical content:

• OOOI reports: Out (left the parking position), Off (took off), On (landed), In (at the parking position) – the timestamps from which airlines derive block and flight times.
• Weather and operations: weather requests, loading information, fuel and performance data.
• Technical messages: maintenance and engine data, often cryptically encoded.
• Free text: messages between crew and operations center, for example to flight dispatchers.

Via libacars, dumpvdl2 additionally decodes embedded applications: FANS-1/A CPDLC and ADS-C, MIAM for larger data transfers and media advisory messages, which reveal which datalinks (VDL2, HF, satcom, VHF ACARS) are currently available.

ATN and CPDLC

The third level is the Aeronautical Telecommunication Network (ATN). In Europe, the air traffic control CPDLC services run over VDL2: controllers send clearances, frequency changes or direct instructions as text messages to the cockpit, and the crew confirms at the push of a button. Maastricht Upper Area Control has operated ATN CPDLC since 2003, and the Eurocontrol Link 2000+ program brought the service to many more flight information regions. dumpvdl2 decodes the protocols required for this, from X.25 via CLNP to ATN B1 CPDLC. What CPDLC means in operation is shown in CPDLC explained; articles of their own cover the role of Eurocontrol and of DFS.

What spotters get out of it

For plane spotters, VDL2 is above all a supplement to ADS-B. A few use cases from practice:

• Confirming registrations: ACARS messages carry the registration in plain text. If your photo is blurry or the registration was hidden, a match via time and hex code helps – more in Identifying an unreadable registration.
• Completing sightings: OOOI times and flight numbers help you fill in your notes afterward, similar to matching with ADS-B logs.
• Aircraft without position data: Not every aircraft shows up completely on public maps. A datalink contact at least shows you that a particular registration was active within range of your receiver.
• Understanding operations: If you read along for a while, you get a feel for how much communication is behind each rotation – from the weather request to the arrival report.

For finding a specific aircraft, ADS-B remains the tool of choice; Tracking an aircraft by registration or hex code shows how.

Legal: reading along is not passing on

Datalink messages are addressed to airlines and air traffic control, not the general public. Many countries, including Germany, have rules for radio content not intended for you – especially regarding passing on and publishing. So don’t share any content, screenshots or logs before you have checked the legal situation for your country. Listening to aviation radio: the legal situation and Is receiving and sharing ADS-B data allowed? offer a starting point. This article is not legal advice.

What to keep in mind

• Always include 136.975 MHz. Without the Common Signalling Channel, you miss a large part of the traffic.
• Check the frequency correction. A wrong ppm value quickly costs many messages with digital modes.
• Don’t maximize gain. 40 dB on the RTL-SDR is a good start, not the maximum.
• Choose the right antenna. A 1090 MHz antenna receives poorly in the VHF band. A simple ground plane for 137 MHz brings considerably more.
• Tell uplinks and downlinks apart. Ground stations often transmit more strongly than aircraft at a great distance. If you mostly see uplinks, your location may be unfavorable for air traffic.
• Read first, then share. Don’t publish anything without knowing the legal situation.

Conclusion

VDL Mode 2 opens up the part of aviation that ADS-B doesn’t show: the digital communication between aircraft, airline and air traffic control. With a second SDR stick, a simple VHF antenna and dumpvdl2, you receive the common channels around 136.975 MHz simultaneously. The messages are often cryptic but deliver hex codes, registrations, flight numbers and operating times – valuable building blocks for confirming sightings and understanding flight operations better. If you want to take the next step, add classic ACARS and HFDL to the setup.