A Short (Unofficial) History of Air/Ground Digital Link – The Clash of Technologies

Following years of testing and discussions on countless forums, VDL Mode 2 was emerging as the solution that, combined with the ATN protocol, could support the initial implementation of Controller Pilot Digital Link Communications. There was nothing else it could do but it had a huge advantage over everything else. There was agreement that it would do the trick! Some people tended to consider this virtue as being of little value but in fact it was as important as the link’s ability to perform. Achieving consensus on the scale needed to decide which link to use is an epic hurdle and when agreement is there, it should not be put in danger.


But that is exactly what was being done by the promoters of another technology that goes under the name VDL Mode 4. VDL Mode 4 can do everything, they claimed… It does voice, text messages and also ADS-B! Most of the claims were of course true and the initial hiccups with the system were no reason to discard it. Yet it never made it into the mainstream and the hard push did only one thing: delayed the inevitable, the final agreement on Mode 2. VDL Mode 4 lacked the most important element: industry agreement for implementation.

Mode 4 does service in a few places and in a way it represents a lost opportunity. If instead of taking the easy (=cheap) way out, the EUR RAN had demanded a new system instead of 8.33 kHz, VDL Mode 4 or an enhanced version of it, might very well have made it to the top. As it was, it became a nuisance for some, an obstacle for others, but it most certainly did not become the major new element in ATM technology it would have deserved.

When the USA also agreed that they would use VDL Mode 2 and ATN for CPDLC, the conga line was ready to move into the big league.