Mode VDL4

VDL Mode 4: synchronization and DLS bursts, CPR positions when decoded in the clear.

Group	Aviation
Default frequency	136.85 MHz
Default sample rate	2 Msps
In the mode list	STDMA link, 136.9 MHz

Step-by-step: Tutorial-VDL4

What VDL4 is

VDL Mode 4 is a VHF aviation data link that uses self-organising TDMA (STDMA) with GFSK on channels between 118 and 137 MHz, around 136.9 MHz. OnAir shows the synchronisation and data bursts and the positions that are sent in the clear.

Frequencies and settings

Setting	Value
Start frequency	136.85 MHz
Tuning range of the mode table	118 to 140 MHz
Sample rate asked	2 Msps
Lowest working rate	250 ksps
Channel width	400 kHz

OnAir takes any frequency you type; the numbers above are the mode's starting point and the range its table describes. In the app the shortcut label is STDMA link, 136.9 MHz.

What OnAir shows

VDL Mode 4: STDMA GFSK data link on 118 to 137 MHz (synthetic + spec; receive only).

Tips and caveats

• Checked with generated signals and impairments; recordings of real signals welcome.
• Which radio: Usually yes for narrowband signals: an RTL-SDR tunes roughly 24 MHz to 1.7 GHz at up to about 2.4 Msps. Wide signals such as TV channels need a faster radio, and OnAir says next to the source what the selected radio can do.
• Try it without a radio: most modes include a built-in synthetic test signal, so you can open the screen and try the controls first.
• Receiving and the law: Receiving aircraft broadcasts is legal in many countries, but some restrict listening to or using what you hear. OnAir is a receiver. Check the rules where you live. See Legal.

Related pages

• All modes, Aviation
• Supported radios, Troubleshooting, FAQ
• More in Aviation: ADS-B, Mode S uplink, ACARS, VDL2, HFDL, Airband, VOR, ILS, NDB, Drone Remote ID, UAT, FLARM, DJI drones