Tutorial VDL2

Tutorial: VDL Mode 2 (VHF data link)

What it is

VDL Mode 2 is the modern VHF data link between aircraft and ground. It carries the same ACARS messages as the older ACARS channels, but at 31.5 kbit/s using AVLC frames, and also carries ATN (X.25 and CLNP), ADS-C and controller-pilot data link messages. OnAir decodes all channels in your capture at once and shows messages, aircraft, ground stations and a map.

What you need

• A radio that tunes about 136 MHz. The mode asks for 2 Msps, lowest working rate 250 ksps; the channel width in the mode table is 400 kHz. An RTL-SDR works.
• A VHF antenna with a clear view; height matters most.
• Channels from the project's database: Europe 136.725, 136.775, 136.825, 136.875, 136.900 and the common signalling channel 136.975 (worldwide). North America 136.650, 136.700, 136.800 and 136.975. Some channels near 131.5 MHz (131.525, 131.725, 131.825) also carry VDL2 in Europe. The mode starts at 136.85 MHz.

Before you start

One capture of 2 Msps can hold all European channels at once if you tune in the middle (about 136.85 MHz). Tune between channels, not on one. The bandwidth you capture limits how many fit: channels outside the sample band are marked "outside the sample band" in the Channels view.

Step by step

1. Choose the group Aviation, entry Data link, 136.9 MHz (mode VDL2).
2. In the tuner, Channels MHz takes a list separated by spaces. Empty means the European six (136.725 136.775 136.825 136.875 136.900 136.975). For North America enter 136.650 136.700 136.800 136.975. The simulated signal offers the sets "Europe (6)" and "136.975 only" for trying the screen without a radio.
3. Press Start. The status shows "Listening" then "Decoding".

What you should see

Frames from many aircraft in a minute on a busy day. The common signalling channel 136.975 carries logon and handover traffic and is the busiest for simple frames; the others carry data.

Reading the screen

• Messages: time (UTC), channel, SNR, from (aircraft or ground), frame kind, content. Kinds include link management (XID, S, U frames), ACARS, X.25 and CLNP. Click a message to see every layer of it. A filter takes an aircraft address (for example 4CA2B1), registration or flight.
• Map: aircraft with positions (from ADS-C or ACARS reports) and ground stations; positions are on the map after the second report when ADS-B is linked.
• Aircraft: ICAO, registration or flight, position, altitude, frames, SNR, last heard. The ICAO address is linked to ADS-B data only if the ADS-B mode has seen the aircraft in the same session.
• Ground stations: the stations heard, their location when known (from airports in the database), frames and last heard.
• Channels: activity per channel, active channels, frames and bad frames, repaired frames, tuning error.

Options and settings

Prediction settings on the map work like in ADS-B: Predict positions and uncertainty. On VDL2 the prediction is on by default because reports come every few minutes.

Troubleshooting

• No frames: you may be tuned outside the band; check the Channels MHz list matches your region.
• Channels marked "outside the sample band": tune to the middle of the channels or raise the sample rate.
• Only very weak frames: improve the antenna. Many bad frames mean low SNR.
• Frames in the wrong place: the antenna position only matters for distances; set it in the tuner.
• Positions missing: a position needs an ADS-C report or an ACARS position message; many frames carry none.

Tips

Run VDL2 and ADS-B together in two receiver slots; the VDL2 aircraft then join the ADS-B record by address. See Two receivers and the multi-decoder.

Legal and privacy notes

VDL2 is unencrypted. Rules for listening differ per country. Message text can contain personal details. Sharing to the public map is opt-in and removes phone numbers and e-mail addresses from ACARS text. See Legal and Privacy.

Under the hood

D8PSK at 10.5 ksymbol/s, 31.5 kbit/s, 25 kHz channels; AVLC link layer (HDLC-like, ISO 4335, with a CRC-16), carrying ACARS (ARINC 618), ATN network and transport layers (ICAO Doc 9705 / 9896) and ADS-C. The physical layer is specified in ICAO Annex 10 vol. III and ETSI EN 301 841.

Related

VDL2 mode page, Tutorial: ACARS, Tutorial: VDL4, Tutorial: ADS-B, Live map.