A Short (Unofficial) History of the Air-Ground Digital Link – The Beginning


Talking to a group of young controllers the other day I suddenly realized that Controller Pilot Digital Link Communications (CPDLC) and its enabler, air/ground digital link were a kind of given for them… Their centre has either already implemented it or had plans for it and while their opinion diverged on the usefulness of the thing, they certainly did not consider it as anything exciting. In a way this is good. The more everyday air/ground digital link becomes, the more we can consider having cleared a major hurdle in implementing an important capacity enabler.

But not being familiar with the history of a particular development reduces our ability to understand its shortcomings and its future potential.

With this article I would like to put on the table a few, sometimes amusing, sometimes incredible, details from the last 15 years of so about air/ground digital link development in the hope that it will be provide some insight into what is after all a very exciting development in air traffic management.

The story will not be comprehensive; it is only a summary and is based mainly on my recollections. I was pretty close to the fire but possibly for that very reason I may have seen things in a light that was colored differently from the actual reality. If you have better information, do comment on my version of the tale.

The original double drivers

Work on digital link use in continental airspace was triggered by two concerns, both having to do with radio frequency congestion. On the one hand, experts calculated that with the forecast traffic growth, sector frequencies would reach saturation, preventing controllers from communicating with the aircraft under their control in a timely fashion. Making sectors smaller was not an option as their minimum size would have been reached earlier as dictated by traffic growth. Only an alternative communication means could offer a solution.

At about the same time, concerns arose about the possible forthcoming congestion on the ACARS frequencies, basically promising to paralyze this vital tool of airline operations. There again an alternative solution was apparently needed.

In the 1980s and early 1990s it was not easy to argue for starting serious work on air/ground digital link implementation. Part of the problem arose from what should have been a big help… but it was not. Seeing all the air traffic management problems in the world, Boeing pushed ahead with the airborne element of FANS, the Future Air Navigation System, which looked extremely promising except for one thing. Nobody on the ground took any notice with the result that hundreds of aircraft were flying over the world’s oceans with CNS capabilities that made the existing oceanic separation minima obsolete but still had to continue with them because the ATM providers on the ground balked at introducing the changes that would have made them able to use the new capabilities.

FANS was a bit like the FMS… the aircraft capability being light years ahead of the ground systems with the ground systems hardly moving at all.

In continental airspace, the situation was different. FANS was normally not installed on short and medium range aircraft while they faced the specter of ATC and ACARS frequency congestion, something that was not an issue (in this form) over the oceans.

ACARS has always been an air/ground digital link system, albeit of modest bandwidth and reserved for airline communications. Also the alternative means of communication for ATC has been identified as air/ground digital link with a set of new messages that would enable the controller to “talk” to aircraft even in the most congested situations. The previously strictly sequential method of voice communications could be replaced by the basically parallel method of digital link with the added bonus of enabling several members of the sector team to talk to aircraft without jinxing the system.

The idea was, and still is, to remove the capacity constraint represented by the limited ability of the controller to talk to the many aircraft otherwise able to operate in his sector.

Business case hurdles

It is well known that, safety issues apart, airlines will only bolt anything on their aircraft if there is a clear business case for doing so or if it is mandated. The real chance of ATC frequencies becoming totally jumbled was still far enough in the future to be considered a real driver and hence it was proving very difficult to make a business case for ATC digital link on its own… In the end, the ACARS problem first brought to the table by Lufthansa saved the day.

Combining the ACARS “replacement” with the elements needed for ATC digital link was able to deliver a compelling business case on the industry level. Of course, general digital link usage was not something smaller airlines took for granted… those without ACARS saw only one side of the equation, namely the costs and they were not particularly thrilled by the prospect.

But on the industry level the business case was water tight and it was explained to the nay-sayers that they too would benefit if capacity was not restricted because of communications issues.

Of course reservations remained. These were tied more than anything to the bitter experience with FANS where the reluctance of the ground to equip negated the benefits… Airlines insisted that any continental data link solution must make sure that the ground leads, period.

The war of technologies

While experts were already working on the kind of messages that would be needed for CPDLC, on the technology side the mother of all wars broke lose.

There was of course FANS 1 from Boeing (and its Airbus equivalent FANS A) and some thought this should be made prevalent everywhere… Unfortunately there were a number of shortcomings both from a technical and price perspective that made FANS less than ideal as a solution for continental airspace. Even more unfortunately, repairing FANS to eliminate the shortcomings was not an easy task and was therefore not even considered for a time. But it did not prevent Boeing from pushing FANS at every opportunity until they realized that the whole effort was becoming counter productive…

The war of technologies was being fought on two levels: the Aeronautical Telecommunications Network (ATN) versus Internet Protocol (IP) on the one hand and between the VHF Digital Link (VDL) modes on the other.

The war between ATN and IP was the least bloody. ATN is an ICAO standard and it took slightly more than forever to get agreed and standardized with the blessing of ICAO. When work on ATN started, IP was not even a gleam in engineers’ eyes and in the end ATN won not because it is so superior but because it was there in a standardized form and not many felt the strength back then to start the same effort on IP.

The VDL modes were another story. There, for a time, it looked like the US and Europe might end up with different and mutually incompatible systems that would have been a costly disaster for airliners operating on both continents.

The clash of 8.33 kHz channel spacing and VDL Mode 3

Although always claimed by the airspace users and never officially admitted by European States, the archaic and inefficient management of frequencies was leading to a situation where new sectors being opened to increase capacity would no longer be able to have their own frequency because there simply would be none left in the aviation spectrum with its 25 kHz channel spacing.

For some in the industry it was becoming clear that the whole VHF radio system used by aviation was outdated, inefficient and a blocking factor to the introduction of new capacity enablers of which the absolute shortage of frequencies was just one element. It was high time to devise something new. Something future proof, something that would serve aviation for many years to come… Of course this would have cost money but then there is no such thing as a free lunch…

Instead of taking the rational decision to force the development of a new communications system, the ICAO Special European Regional Air Navigation meeting (EUR RAN) in September 1994 decided to split the channel spacing in Europe to 8.33 kHz, thereby increasing the number of available channels. Replacing radios appeared to be the cheaper option in the short term…

In the United States where there has never been a shortage of frequencies (although the demand for frequencies is not smaller than in Europe) and 8.33 kHz was never considered seriously initially. Nevertheless, they too realized that a new communications system would be needed and they promoted VDL Mode 3. This system would be able to handle both digital voice and digital messages. In other words, VDL Mode 3 would offer a seamless service where a controller could talk or send text messages, depending on the prevailing operational needs.

For a time the map of the world seemed to have been fatally split. What has never happened in the past, no ILS or VOR being different on different continents was becoming the nightmarish reality on the threshold of a major ATM “improvement”. Aircraft would be required to carry voice radios with 8.33 kHz capability as well as VDL Mode 3 radios plus whatever Europe would come up with as the technology for air/ground digital link… Pilots would also need to figure out when to use what…

In the end, the implementation of VDL Mode 3 fell behind schedule and the danger receded… As we will see in the next section, at the end of one more major battle a single solution remained and that became the de facto world-wide standard.