VDL Mode 2 (aircraft data link)
A faster, digital successor to ACARS for airliners.
What does it sound like?
A short, dense burst of digital noise.
Who uses it?
Airlines and aircraft.
Where and when can I hear it?
VHF airband, for example 136.975 MHz.
The science
VDL Mode 2 uses differential eight-phase-shift keying (D8PSK): each symbol is one of eight phase changes, carrying three bits, sent at 10,500 symbols per second for a raw rate of 31,500 bits per second inside a standard 25 kHz airband channel between 118 and 136.975 MHz. Data is protected by Reed-Solomon forward error correction and interleaving, and radios share a channel by carrier sense multiple access (CSMA), meaning each one listens and transmits only when the channel is clear. Frames use a link protocol called AVLC with 24-bit ICAO aircraft addresses, and they can carry ordinary ACARS messages as their payload.
Why use it?
At 31.5 kilobits per second it is more than ten times faster than 2400 bit-per-second ACARS on the same kind of VHF channel, so each channel can carry far more messages. It also carries controller-pilot data link communications (CPDLC), letting routine, non-urgent air traffic control messages arrive as text, which reduces misunderstandings on poor voice links and frees voice channels for more urgent calls.
Lore
Under Commission Regulation (EC) No 29/2009, the Data Link Services rule, aircraft operators flying above flight level 285 in European airspace must be able to use data link services over VDL Mode 2, and the rule has been in full force since February 2020.
Easily confused with
- ACARS (aircraft messages): ACARS is slower two-tone data on nearby channels