Tutorial VDL4

Tutorial: VDL Mode 4 (STDMA link)

What it is

VDL Mode 4 is a VHF aviation data link in which stations share the channel by self-organising time slots (STDMA) and send short GFSK bursts: synchronisation bursts and data bursts that can carry position reports. It is far less used than VDL Mode 2. Expect few or no signals in most places, and be aware that OnAir's decoder here has been checked against the specification and a synthetic signal, not live traffic.

What you need

• An RTL-SDR or any radio that tunes about 136.9 MHz. The mode asks for 2 Msps; the lowest working rate is 250 ksps (the mode table shows a 400 kHz channel width).
• A VHF antenna with a clear view of the sky.
• Frequencies from the project's database: 136.925 MHz ("Band 136.9-136.975 reserved for VDL4 in the ICAO plan; little traffic"). The mode starts at 136.85 MHz.

Before you start

Use a moderate gain. Tune so that 136.925 lies inside the capture and away from the centre spike. Because VDL4 is rare, first check that the screen works with the built-in test signal.

Step by step

1. Choose the group Aviation, entry STDMA link, 136.9 MHz (mode VDL4).
2. Tune about 136.9 MHz and press Start.
3. Watch the Messages view and the Map. The side panel is headed AIRCRAFT, with a FRAMES summary of what was heard.

What you should see

Most of the time, nothing: the status will show that the receiver is running without frames. When a station is in range you will see synchronisation and data frames and, for those sent unencrypted, positions.

Reading the screen

• Messages: Time, MHz, From (the sender's address, six hex digits), Type and Summary. Click a row to see the detail of that message below the table.
• Map: aircraft whose messages carried a position.

Troubleshooting

• Nothing heard: normal in most regions; VDL4 has little traffic. Confirm with the test signal and with VDL2 that the radio and antenna are fine.
• Noise only: reduce gain and check that no strong carrier sits near 136.9 MHz.
• A position is shown that looks wrong: positions are decoded from compact position reports; treat them as indicative.

Tips

If you want something to watch in this band, VDL2 on 136.975 MHz is the busiest.

Legal and privacy notes

Receive only. Receiving aircraft broadcasts is lawful in many countries but not all; see Legal.

Under the hood

GFSK (modulation index 0.25) at 19.2 kbit/s on 25 kHz channels with a self-organising TDMA frame, defined in ICAO Annex 10 vol. III and ETSI EN 302 842. Check the panel for the exact message types shown.

Related

VDL4 mode page, Tutorial: VDL2, Tutorial: ACARS.