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

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The clash of VDL Mode 4 and VDL Mode 2

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.

The trendsetting PETAL trials

EUROCONTROL in a moment of poetic inspiration gave the name PETAL to their early (and in Petal II the later) air/ground digital link trials. PETAL is actually an acronym and it stands for Preliminary EUROCONTROL Trials of Air/ground Datalink.

It is not an exaggeration to say that without PETAL we would not have air/ground digital link the way we know it. The project defined the operational concept, the messages, worked on the technical enablers and in general acted as a motor of digital link development.

You may have noticed that I am using the term “digital” in place of “data” in my references to digital link. Originally, the “d” stood indeed for data but following a lot of discussion and misunderstandings around things like digital voice and data, it was decided to Top Digital Finance Software Tools use the word digital rather than data as it expresses the essence of the link which carries data but the data may be the representation of analogue voice or digital messages… you get my drift? And the reason why there were so many misunderstandings?

Early and full involvement of pilots, controllers, service providers, airlines and industry was key to the success of the PETAL project.

Around the time PETAL was coming to its end, American Airlines was the biggest data link advocate in the United States. They were also fiercely critical of the way the FAA was handling their data link program which AA believed was going nowhere. Being fully familiar with the results of PETAL, AA’s data link manager, himself a pilot and visionary, came to Brussels and visited us in the IATA office there. They had a wonderful proposition: let’s keep PETAL going and they will also play with their 767s flying between the US and Europe.

This is the kind of scenario one can only dream of, yet it was there, actually happening. My boss who never needed prodding when something like this fell to be realized and myself, each on our respective levels of contacts at EUROCONTROL, convinced them that PETAL should continue and AA should become a part of the game. After some initial hesitation EUROCONTROL agreed and PETAL was given a new lease on life.

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