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dumpvdl2 is a lightweight, standalone VDL Mode 2 message decoder and protocol analyzer.

Current stable version: 1.5.0 (released Nov 17, 2018)


  • Runs under Linux (tested on: x86, x86-64, Raspberry Pi)
  • Supports following SDR hardware:
  • Decodes up to 8 VDL2 channels simultaneously
  • Outputs messages to standard output or to a file (with optional daily or hourly file rotation)
  • Outputs ACARS messages to PlanePlotter over UDP/IP socket
  • Supports message filtering by type or direction (uplink, downlink)
  • Outputs decoding statistics using Etsy StatsD protocol


dumpvdl2 screenshot

Protocol support status

  • AVLC - supported
  • ACARS over AVLC - supported
  • ISO 8208 (X.25) control packets - supported
  • ISO 8473 (CLNP) - partially supported (CLNP header is skipped over without decoding)
  • ISO 9542 (ES-IS) - supported
  • ISO 10747 (IDRP) - partially supported (decoding of some less important attributes is TODO)
  • ISO 8073 (COTP) - supported
  • ICAO ATN-B1 CM (Context Management) - supported
  • ICAO ATN-B1 CPDLC (Controller-Pilot Data Link Communications) - supported
  • ICAO ATN-B1 ADS-C (Automatic Dependent Surveillance - Contract) - not supported
  • FANS 1/A CPDLC - supported
  • FANS 1/A ADS-C - supported


Install necessary dependencies (unless you have them already). Example for Debian / Raspbian:

    sudo apt-get install build-essential libtool libglib2.0-dev pkg-config
RTLSDR support (optional)

Install librtlsdr library (unless you have it already). Under Raspbian you can install packaged version:

    apt-get install librtlsdr-dev

or get the source from Git and compile by hand:

    apt-get install git autoconf libusb-1.0-0-dev
    git clone git://
    cd rtl-sdr/
    autoreconf -i
    sudo make install
    sudo ldconfig
    sudo cp $HOME/rtl-sdr/rtl-sdr.rules /etc/udev/rules.d/rtl-sdr.rules
Mirics support (optional)

libmirisdr-4 is an open-source alternative to SDRPlay binary driver (Mirics is the chipset brand which SDRPlay RSPs are based on). However, as of December 2017, it works properly with RSP1 only. For other RSP types (RSP2, RSP/1A) gain control does not work too well, so the native closed source driver is a better option (see next section). libmirisdr-4 is a good choice for RSP1 and various Mirics-based DVB-T dongles which are detected as RSP1 device. An advantage over RSP binary API is lower CPU utilization in dumpvdl2 thanks to a lower sampling rate.

Install libmirisdr-4 library:

    apt-get install git cmake libusb-1.0-0-dev
    git clone
    cd libmirisdr-4
    cd build
    sudo make install
    sudo ldconfig
    sudo cp $HOME/libmirisdr-4/mirisdr.rules /etc/udev/rules.d/mirisdr.rules
SDRPLAY RSP support (optional)

Download and install API/hardware driver package from Make sure you have selected the right hardware platform before downloading, otherwise the installer will fail.

Compiling dumpvdl2

Download a stable release package from here or clone the repository:

    git clone
    cd dumpvdl2

If you only need RTLSDR support, it is enabled by default, so just type:


Mirics support has to be explicitly enabled, like this:

    make WITH_MIRISDR=1

If you only need Mirics or SDRPlay, you may disable RTLSDR support:




Note: every time you decide to recompile with different WITH_* or USE_* options, clean the old build first using make clean.

To list available command line options, run:

    ./dumpvdl2 --help
Optional: add support for statistics

Install statsd-c-client library from

    git clone
    cd statsd-c-client
    sudo make install
    sudo ldconfig

Compile dumpvdl2 as above, but add USE_STATSD=1:

    make USE_STATSD=1

Basic usage


Simplest case on RTLSDR dongle - uses RTL device with index 0, sets the tuner gain to 40 dB and tuning correction to 42 ppm, listens to the default VDL2 frequency of 136.975 MHz, outputs to standard output:

    ./dumpvdl2 --rtlsdr 0 --gain 40 --correction 42

Device ID numbers are not persistent - they depend on the USB device order and the sequence which they were plugged in. You may specify the device by its serial number to get deterministic behavior:

    ./dumpvdl2 --rtlsdr 771111153 --gain 40 --correction 42

Use rtl_test utility to get serial numbers of your devices. dumpvdl2 will print them to the screen on startup as well.

If you want to decode another VDL2 channel, just add its frequency as a last parameter:

    ./dumpvdl2 --rtlsdr 0 --gain 40 --correction 42 136725000

dumpvdl2 can decode up to 8 VDL2 channels simultaneously. Just list their frequencies at the end of the command line:

    ./dumpvdl2 --rtlsdr 0 --gain 40 --correction 42 136725000 136975000 136875000

If your receiver has a large center spike, you can set the center frequency a bit to the side of the desired channel frequency, like this:

    ./dumpvdl2 --rtlsdr 0 --gain 40 --correction 42 --centerfreq 137100000 <channel freqs here...>

Mirics is similar, however libmirisdr-4 library currently lacks support for configuring correction in ppm. If your receiver needs a non-zero correction, you can pass the appropriate value in Hertz, instead of ppm. Note: this value will be subtracted from the center frequency, so if your receiver tunes a bit too low, the parameter value shall be negative:

    ./dumpvdl2 --mirisdr 0 --gain 100 --correction -2500

Device serial number can be given instead of ID, the same way, as for RTLSDR receivers.

libmirisdr-4 supports two types of hardware: generic Mirics (0 - the default) and SDRPlay (1). SDRPlay users should add --hw-type 1 option. It uses frequency plans optimized for SDRPlay and reportedly gives better results than the default mode.

If you get error messages about lost samples on Raspberry Pi, try adding --usb-mode 1. This switches USB transfer mode from isochronous to bulk, which is usually enough to rectify this problem. If it does not help, it might be that your Pi is overloaded or not beefy enough for the task. Try reducing the number of decoded VDL2 channels as a workaround. See also the FAQ section "CPU usage on Raspberry Pi is very high" below.

SDRPLAY RSP native driver

SDRPlay RSP native driver supports several advanced configuration options:

  • switching antenna ports (RSP2)
  • bias-T (RSP2, RSP1A)
  • notch filter for AM/FM broadcast bands (RSP2, RSP1A, RSPduo)
  • tuner selection (RSPduo)
  • Automatic Gain Control

Type ./dumpvdl2 --help to find out all the options and their default values.

SDRPlay driver has a concept of "gain reduction", which is an amount of gain (in decibels) which shall be deducted from the maximum gain. As a result, --gain option is not available with this driver - use --gr option to specify requested end-to-end gain reduction instead. The smallest possible value is 20. The highest value depends on receiver type, but it's not that important, because in dumpvdl2 you will hardly be using a GR larger than 59 dB.

Another way to go is to skip the --gr option altogether. This will enable Automatic Gain Control with a default set point of -35 dBFS, which shall converge to a reasonable gain reduction value in a couple of seconds after the program starts. AGC set point can be changed with --agc option, but treat this as an "expert mode" knob, which is hardly ever needed.

Example 1: use SDRplay device ID=0, with auto gain and three VDL2 channels:

    ./dumpvdl2 --sdrplay 0 136975000 136875000 136775000

Example 2: use SDRplay device with serial number 35830222, set gain reduction to 40 dB, use antenna A port, disable Bias-T, enable AM/FM notch filter, set frequency correction to -1ppm:

    ./dumpvdl2 --sdrplay 35830222 --gr 40 --correction -1 --antenna A --biast 0 --notch-filter 1 136975000

Output options

  • Decoded messages are printed to standard output by default. You can direct them to a disk file instead:

      ./dumpvdl2 --output-file vdl2.log [other_options]
  • If you want the file to be automatically rotated on top of every hour, add --hourly option. The file name will be appended with _YYYYMMDDHH suffix. If file extension is present, it will be placed after the suffix.

  • If you prefer daily rotation, --daily option does just that. The file name suffix will be _YYYYMMDD in this case. If file extension is present, it will be placed after the suffix.

  • Add --utc option if you prefer UTC timestamps rather than local timezone in output and filenames.

  • Add --raw-frames option to display payload of AVLC frames in raw hex for debugging purposes.

  • Add --dump-asn1 option to display full ASN.1 structure dumps of CPDLC and CM messages.

Integration with Planeplotter

dumpvdl2 can send ACARS messages to Planeplotter, which in turn can extract aircraft position information from them and display blips on the map. First, configure your Planeplotter as follows:

  • Stop data processing (press 'Stop' button on the toolbar)

  • Go to Options / I/O Settings...

  • Tick 'UDP/IP Data from net'

  • Set 'UDP/IP local port' to some value (default is 9742)

  • Close the settings window by clicking OK and restart data processing

Supply dumpvdl2 with the address (or host name) and port where the Planeplotter is listening:

    ./dumpvdl2 --output-acars-pp [other_options]

That's all. Switch to 'Message view' in Planeplotter and look for incoming messages.

Message filtering

By default dumpvdl2 logs all decoded messages. You can use --msg-filter option to ignore things you don't want to see. If you do not want messages sent by ground stations, run the program like this:

    ./dumpvdl2 --msg-filter all,-uplink [other_options]

Or if you want to filter out empty ACARS messages, because they are boring, use this:

    ./dumpvdl2 --msg-filter all,-acars_nodata [other_options]

For full list of supported filtering options, run:

    ./dumpvdl2 --msg-filter help

Refer to file for more examples and details.


The program does not calculate statistics by itself. Instead, it sends metric values (mostly counters) to the external collector using Etsy StatsD protocol. It's the collector's job to receive, aggregate, store and graph them. Some examples of software which can be used for this purpose:

  • Collectd is a statistics collection daemon which supports a lot of metric sources by using various plugins. It has a StatsD plugin which can receive statistics emitted by dumpvdl2, aggregate them and write to various time-series databases like RRD, Graphite, MongoDB or TSDB.

  • Graphite is a time-series database with powerful analytics and aggregation functions. Its graphing engine is quite basic, though.

  • Grafana is a sophisticated and elegant graphing solution supporting a variety of data sources.

Here is an example of some dumpvdl2 metrics being graphed by Grafana:


Metrics are quite handy when tuning the antenna installation or receiving parameters (like gain or correction). Full list of currently supported counters can be found in statsd.c source file. dumpvdl2 produces a separate set of counters for each configured VDL2 channel.

To enable statistics just give dumpvdl2 your StatsD collector's hostname (or IP address) and UDP port number, for example:

    ./dumpvdl2 --statsd [other_options]

Processing recorded IQ data from file

The syntax is:

dumpvdl2 --iq-file <file_name> [--sample-format <sample_format>] [--oversample <oversample_rate>]
  [--centerfreq <center_frequency>] [vdl_freq_1] [vdl_freq_2] [...]

The symbol rate for VDL2 is 10500 symbols/sec. dumpvdl2 internal processing rate is 10 samples per symbol. Therefore the file must be recorded with sampling rate set to an integer multiple of 105000. Specify the multiplier value with --oversample option. The default value is 10, which is valid for files sampled as 1050000 samples/sec. For example, if you have recorded your file at 2100000 samples/sec, then use --oversample 20 (because 105000 * 20 = 2100000).

The program accepts raw data files without any header. Files produced by rtl_sdr and miri_sdr programs are perfectly valid input files. Different radios produce samples in different formats, though. dumpvdl2 currently supports following sample formats:

  • U8 - unsigned 8-bit samples. This is the format produced by rtl_sdr utility.
  • S16_LE - 16-bit signed, little endian. Produced by miri_sdr utility (by default).

Use --sample-format option to set the format. The default format is U8.

The program assumes that the VDL2 channel is located at baseband (0 Hz), ie. the center frequency of your radio was set to the VDL2 channel frequency during recording. If this is not the case, you have to provide correct center frequency and channel frequency. For example, if your receiver was tuned to 136.955 MHz during recording and you want to decode the VDL2 channel located at 136.975 MHz, then use this:

    dumpvdl2 --iq-file <file_name> --centerfreq 136955000 136975000

Putting it all together:

    dumpvdl2 --iq-file iq.dat --sample-format S16_LE --oversample 13 --centerfreq 136955000 136975000 136725000

processes iq.dat file recorded at 1365000 samples/sec using 16-bit signed samples, with receiver center frequency set to 136.955 MHz. VDL2 channels located at 136.975 and 136.725 MHz will be decoded.

Launching dumpvdl2 in background on system boot

There is an example systemd unit file in etc subdirectory (which means you need a systemd-based distribution, like Debian/Raspbian Jessie or newer).

First, go to dumpvdl2 source directory and install the binary to /usr/local/bin:

    sudo make install

Copy the unit file to the systemd unit directory:

    sudo cp etc/dumpvdl2.service /etc/systemd/system/

Copy the example environment file to /etc/default directory:

    sudo cp etc/dumpvdl2 /etc/default/

Edit /etc/default/dumpvdl2 with a text editor (eg. nano). Uncomment the DUMPVDL2_OPTIONS= line and put your preferred dumpvdl2 option set there. Example:

    DUMPVDL2_OPTIONS="--rtlsdr 0 --gain 39 --correction 0 --output-file /home/pi/vdl2.log --daily 136975000 136875000 136775000"

Reload systemd configuration:

    sudo systemctl daemon-reload

Start the service:

    sudo systemctl start dumpvdl2

Verify if it's running:

    systemctl status dumpvdl2

It should show: Active: active (running) since <date>. If it failed, it might be due to an error in the DUMPVDL2_OPTIONS value. Read the log messages in the status output and fix the problem.

If everything works fine, enable the service, so that systemd starts it automatically at boot:

    systemctl enable dumpvdl2


dumpvdl2 can decode FANS-1/A CPDLC messages. They are embedded in ACARS as hex strings:

    [2017-03-04 20:47:12] [136.975] [-18.8/-47.0 dBFS] [28.1 dB]
    10A41A (Ground station, On ground) -> 78100B (Aircraft): Command
    AVLC type: I sseq: 6 rseq: 6 poll: 0
    Reg: .B-7898 Flight:
    Mode: 2 Label: AA Blk id: G Ack: ! Msg no.:

The second field in the message (.AT1) indicates that this is a CPDLC message (other message types which also contain CPDLC are: .CR1, .CC1, and .DR1). The above message is decoded as follows:

    FANS-1/A CPDLC Message:
    CPDLC Uplink Message:
      Msg ID: 2
      Timestamp: 07:47:10
     Message data:
      CLIMB TO AND MAINTAIN [altitude]
       Flight level: 340
      REPORT LEVEL [altitude]
       Flight level: 340

ACARS messages can be conveyed over various radio links - POA (Plain Old ACARS over VHF), AoA (ACARS over AVLC over VDL-2), HFDL (High Frequency Data Link) or SATCOM (Inmarsat, Iridium). dumpvdl2 only deals with VDL-2 radio link, however the FANS-1/A message format is common, regardless of physical link used for transmission. So if you monitor ACARS, HFDL or Inmarsat, you may wonder how to decode messages received on these links.

decpdlc is a small utility included in dumpvdl2 source tree which uses dumpvdl2's FANS-1/A decoder to decode CPDLC messages supplied manually from command line or from a file.


  • Clone the dumpvdl2 repository ( or download a stable release package from here and unpack it.

  • Go to the source directory and build the program:

      cd dumpvdl2
      make decpdlc
  • After a while you should have decpdlc binary in the current directory. Run:

      ./decpdlc -h

for further instructions.

Frequently Asked Questions

What is VDL Mode 2?

VDL (VHF Data Link) Mode 2 is a communication protocol between aircraft and a network of ground stations. It has a higher capacity than ACARS and a lot more applications. More information can be found on Wikipedia or SigIdWiki.

Who uses it?

Civil airlines - not all, but many. Military? Umm, no.

What frequencies it runs on?

The most ubiquitous is 136.975 MHz (so called Common Signalling Channel). In some areas where the capacity of a single channel is not enough, 136.725, 136.775 or 136.875 is used as well. Because they are closely spaced, dumpvdl2 can receive all of them simultaneously with a single receiver.

Is it used in my area?

Yup, it's quite probable. Launch your favorite SDR Console (like SDRSharp or GQRX), tune 136.975 MHz and place your antenna outside (or near the window, at least). If you see short bursts every now and then, it's there.

What antenna shall I use?

VDL2 runs on VHF airband, so if you already have a dedicated antenna for ACARS or airband voice, it will be perfect for VDL2. However VDL2 transmissions are not very powerful, so do not expect thousands of messages per hour, if your antenna is located indoors. If you have already played with ADS-B, you know, what to do - put the antenna outside and high with unobstructed sky view, use short and good quality feeder cable, shield your radio from external RF interference.

Two hours straight and zero messages received. What's wrong?

It basically comes down to three things:

The signal has to be strong enough (preferably 15 dB over noise floor, or better)
  • set your tuner gain quite high. I get good results with 40 dB for RTLSDR and 75 dB for Mirics dongles. Do not be tempted to crank the gain up to the max. Keep your noise floor low because higher noise yields higher bit error rate and may cause signal clipping when the transmission is strong (eg. the transmitting aircraft is just overflying your antenna). On SDRPlay it should be good enough to use auto gain control.

  • check SDR Console with the same gain setting - do you see data bursts clearly? (they are very short, like pops).

  • if your DC spike is very high, set the center frequency manually to dodge it (use --centerfreq option).

  • RTL dongles are cheap - some of them have higher noise figure than others. If you have several dongles at hand, just try another one.

Channel frequency must be correct
  • initially, just don't set it manually, use the default of 136.975 MHz. It is used everywhere where VDL2 is deployed.
PPM correction setting must be (more or less) accurate
  • oscillators in cheap receivers are not 100% accurate. It is usually necessary to introduce manual correction to get precise tuning. There is no one-size-fits-all correction value - it is receiver-specific. See next question.
How do I estimate PPM correction value for my dongle?

Method 1: use rtl_test utility which comes with librtlsdr library. Run it with -p option and observe the output:

    root@linux:~ # rtl_test -p
    Found 1 device(s):
      0:  Realtek, RTL2838UHIDIR, SN: 00000002

    Using device 0: Generic RTL2832U OEM
    Found Rafael Micro R820T tuner
    Supported gain values (29): 0.0 0.9 1.4 2.7 3.7 7.7 8.7 12.5 14.4 15.7 16.6 19.7 20.7 22.9 25.4 28.0 29.7 32.8 33.8 36.4 37.2 38.6 40.2 42.1 43.4 43.9 44.5 48.0 49.6
    [R82XX] PLL not locked!
    Sampling at 2048000 S/s.
    Reporting PPM error measurement every 10 seconds...
    Press ^C after a few minutes.
    Reading samples in async mode...
    real sample rate: 2048207 current PPM: 101 cumulative PPM: 101
    real sample rate: 2048159 current PPM: 78 cumulative PPM: 89
    real sample rate: 2048137 current PPM: 67 cumulative PPM: 81
    real sample rate: 2048184 current PPM: 90 cumulative PPM: 84
    real sample rate: 2048163 current PPM: 80 cumulative PPM: 83
    real sample rate: 2048165 current PPM: 81 cumulative PPM: 82
    real sample rate: 2048140 current PPM: 69 cumulative PPM: 81
    real sample rate: 2048178 current PPM: 87 cumulative PPM: 81
    real sample rate: 2048168 current PPM: 82 cumulative PPM: 81
    real sample rate: 2048117 current PPM: 57 cumulative PPM: 79
    real sample rate: 2048202 current PPM: 99 cumulative PPM: 81
    real sample rate: 2048173 current PPM: 85 cumulative PPM: 81
    real sample rate: 2048164 current PPM: 80 cumulative PPM: 81
    real sample rate: 2048135 current PPM: 66 cumulative PPM: 80
    real sample rate: 2048179 current PPM: 88 cumulative PPM: 80
    real sample rate: 2048170 current PPM: 83 cumulative PPM: 81
    real sample rate: 2048167 current PPM: 82 cumulative PPM: 81
    real sample rate: 2048155 current PPM: 76 cumulative PPM: 80
    real sample rate: 2048160 current PPM: 78 cumulative PPM: 80
    real sample rate: 2048159 current PPM: 78 cumulative PPM: 80
    real sample rate: 2048154 current PPM: 75 cumulative PPM: 80
    real sample rate: 2048155 current PPM: 76 cumulative PPM: 80
    real sample rate: 2048181 current PPM: 89 cumulative PPM: 80

After a couple of minutes the cumulative PPM value converges to a stable reading. This is an approximate correction value for your dongle. Run dumpvdl2 with --correction <value> option. dumpvdl2 can compensate correction errors up to a certain amount. Once you have received some messages, look for the frequency offset field which is printed in the header of each message (it's the value expressed in ppm). Your tuning is good, when this value is close to 0. If you see a systematic offset from 0, tweak your correction value to compensate it.

High CPU usage on Raspberry Pi

Default compiler flags should work fine on most platforms. However you may get a performance boost (read: lower CPU usage) by adjusting the flags to the CPU which you intend to run dumpvdl2 on.

dumpvdl2 has predefined knobs for three flavours of Raspberries:

    make PLATFORM=rpiv1 (or rpiv2 or rpiv3)

Using a recent version of GCC is important as well. Version 4.9 (supplied with Raspbian Jessie) is ancient and generates suboptimal code for ARM platforms. 6.3 (available in Raspbian Stretch) or 7.1 (available in Arch Linux) both perform significantly better.

What do these numbers in the message header mean?
    [2017-02-26 19:18:00 GMT] [136.975] [-18.9/-43.9 dBFS] [25.0 dB] [0.4 ppm]

From left to right:

  • date and time with timezone.

  • channel frequency on which the message has been received.

  • signal power level (averaged over all symbol sampling points in the burst). Full scale is 0 dB.

  • noise floor power level. Full scale is 0 dB.

  • signal to noise ratio (ie. signal power level minus noise floor power level).

  • frequency offset of the received burst from the channel center frequency, in parts per million.

There is an --extended-header command line option which enables additional fields:

    [2017-02-26 19:18:00 GMT] [136.975] [-18.9/-43.9 dBFS] [25.0 dB] [0.4 ppm] [S:0] [L:34] [F:0] [#0]
  • number of bit errors corrected in the VDL2 burst header (up to 2).

  • burst length in octets.

  • number of octets corrected by Reed-Solomon FEC.

  • number of frame in this particular transmission. Multiple AVLC frames (messages) may be concatenated and sent as a single transmission burst. When a multiframe burst is received, frames will be numbered incrementally.

Can you add support for [my favourite SDR receiver type]?

Maybe. However do not expect me to purchase all SDRs available on the market just to make dumpvdl2 work with them. If your life absolutely depends on it, consider donating, or at least lending me the hardware for some time for development and testing.

Alternatively, if you can write code, you may do the work by yourself and submit it as a pull request. Most of the program code is hardware-agnostic anyway. Adding new device type basically comes down to the following:

  • dumpvdl2.c, dumpvdl2.h - add new input type and necessary command line options.

  • rtl.c, rtl.h - this is the code specific to the RTLSDR hardware. Make a copy and modify it to use the API of your SDR device. Or you can start off from mirics.c and mirics.h, if you prefer.

  • demod.c - if your SDR device uses a sample format other than 8-bit unsigned and 16-bit signed, it is necessary to write a routine which handles this format and converts the samples to signed float in the <-1;1> range. Refer to process_buf_uchar() and process_buf_short() routines for details.

  • Makefile - add new WITH_DEVICE compile time option and your new source files, add necessary LDLIBS, etc.

Can you add support for Windows?

To be honest, I don't use Windows very often and I don't know the programming intricacies of this OS. However, if you feel like you could port the code and maintain the port later on, please do so. Pull requests welcome.

Credits and thanks

I hereby express my gratitude to everybody who helped with the development and testing of dumpvdl2. Special thanks go to:

  • Fabrice Crohas
  • Dick van Noort
  • acarslogger
  • Piotr Herko, SP5XSB
  • LamaBleu


Copyright (c) 2017-2018 Tomasz Lemiech

Contains code from the following software projects:

  • libfec, (c) 2006 by Phil Karn, KA9Q

  • acarsdec, (c) 2015 Thierry Leconte

  • librtlsdr-keenerd, (c) 2013-2014 by Kyle Keen

  • asn1c, (c) 2003-2017 by Lev Walkin and contributors

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see