A short (unofficial) history of air/ground digital link – 2

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 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.
One of the unique features of the PETAL extension was the PETAL Integration Team or PIT. It was a multidiscipline airline/ANSP/industry design team construct run by Rob Mead about whom you can read in our Interesting People series. Rob ran a very tight ship and the PIT was super efficient and effective. In part this was the reason why they could get anyone they wanted on board… it was always clear that time spent on the PIT was valuable whichever way you looked at it. The PIT had the balls to allow on board only those who could contribute and no obfuscation was allowed. And people played along, in the meetings as well as at home.
The FAA was also an actively participating member of the PIT and I think in the end even AA acknowledged that the US project was getting back on track.

One more go at the business case

I have already mentioned the original difficulties the projects faced in creating a credible business case for CPDLC. About halfway through the project the “Data Link Business Case for the US” was published coming from the hand of the C/AFT. The CNS/ATM Focused Team was an industry group managed by Boeing and one of the things they did was develop cost/benefit analyses for various developments, digital link among them. The beauty of a C/AFT product was that it came from a group that also included the airlines and hence the results were rarely contested by the flying community afterwards.
If only we could have something like that… The C/AFT was financed for US work only but when we asked United Airlines whether they would agree

to have the C/AFT make a go at the European digital link cost benefit analysis, they sad yes. The rest of the members, including Boeing, also agreed and the activity resulted in a few absolutely wonderful days in Seattle plus a digital link CBA tailored to the European environment. EUROCONTROL was also involved in this, so much so that a little later they acquired the license to the software tool at the core of the CBA activity and thereafter put together the Emosia method which was for many years the standard cost/benefit methodology applied to all their projects.
As expected, the European C/AFT CBA was subject to a good deal less fighting than any CBA before (or possibly hence…).

FANS 1/A and ATN, a hard nut to crack

By the time trials with continental digital link took off, FANS over the oceans was also coming to maturity and the old discussion came back. What should happen with FANS equipped aircraft when they flew in airspace using VDL Mode2/ATN CPDLC? While FANS was still not up to the demands of continental airspace flying, airlines felt that they should not be required to fit two types of equipment. For the planners, the question was: do we teach the ground to speak FANS also or do we teach FANS equipped aircraft to speak VDL Mode 2/ATN also? From an operational perspective, FANS had its limitations but it was only a question of time before that got solved also.

The answer of the airlines was clear: no double stack in the air. No multi-lingual aircraft… It was then that under the aegis of PETAL a breakthrough ground based system was developed which allows aircraft with different and non-interoperable communications applications to operate simultaneously with the same air navigation service provider. In PETAL it was the Maastricht UAC which remains the only operational unit with this capability.
In spite of this elegant solution being available, in the end the upcoming European digital link mandate takes a step back and results, in effect, in double stack being required on board in all cases where the mandate applies to a FANS equipped aircraft.

LINK2000+ the PETAL follow up

Just like Microsoft who had this passing fad of naming their products by the year in which they were published, EUROCONTROL also had a period when everything seemed to take its name from the new European ATM Strategy for 2000+ and hence the PETAL follow up project got the name LINK2000+.
LINK had the no mean task of conducting the operational introduction of air/ground digital link. Of course when we talk about the link, we mean the physical link as well as the applications using it. The link on its own is of no use whatsoever. In other words, the airborne and the ground systems must also be modified to enable them to use the link.

LINK2000+ started its life in a varied environment. Those who were part of PETAL continued to support the activity and Maastricht UAC remained the flagship of implementation. Boeing was firmly on board while Airbus said they would look into this when their customers ask for it… some airlines were supportive, others non-committal and at least one big airline was positively not interested. Perhaps not coincidentally, their home State had no plans for digital link on the ground… Clearly, LINK2000+ had its task cut out for it. It did not help that the FAA’s digital link program was sputtering and American Airlines in turn had to scale back their plans also.

Pioneers to the rescue

To be continued…