| Introduction | | | | thrilled by the prospect. |
| Talking to a group of young controllers the other day | | | | But on the industry level the business case was |
| I suddenly realized that Controller Pilot Digital Link | | | | water tight and it was explained to the nay-sayers |
| Communications (CPDLC) and its enabler, air/ground | | | | that they too would benefit if capacity was not |
| digital link were a kind of given for them... Their | | | | restricted because of communications issues. |
| centre has either already implemented it or had plans | | | | Of course reservations remained. These were tied |
| for it and while their opinion diverged on the | | | | more than anything to the bitter experience with |
| usefulness of the thing, they certainly did not | | | | FANS where the reluctance of the ground to equip |
| consider it as anything exciting. In a way this is good. | | | | negated the benefits... Airlines insisted that any |
| The more everyday air/ground digital link becomes, | | | | continental data link solution must make sure that the |
| the more we can consider having cleared a major | | | | ground leads, period. |
| hurdle in implementing an important capacity enabler. | | | | The war of technologies |
| But not being familiar with the history of a particular | | | | While experts were already working on the kind of |
| development reduces our ability to understand its | | | | messages that would be needed for CPDLC, on the |
| shortcomings and its future potential. | | | | technology side the mother of all wars broke lose. |
| With this article I would like to put on the table a | | | | There was of course FANS 1 from Boeing (and its |
| few, sometimes amusing, sometimes incredible, | | | | Airbus equivalent FANS A) and some thought this |
| details from the last 15 years of so about air/ground | | | | should be made prevalent everywhere... Unfortunately |
| digital link development in the hope that it will be | | | | there were a number of shortcomings both from a |
| provide some insight into what is after all a very | | | | technical and price perspective that made FANS less |
| exciting development in air traffic management. | | | | than ideal as a solution for continental airspace. Even |
| The story will not be comprehensive; it is only a | | | | more unfortunately, repairing FANS to eliminate the |
| summary and is based mainly on my recollections. I | | | | shortcomings was not an easy task and was |
| was pretty close to the fire but possibly for that | | | | therefore not even considered for a time. But it did |
| very reason I may have seen things in a light that | | | | not prevent Boeing from pushing FANS at every |
| was colored differently from the actual reality. If you | | | | opportunity until they realized that the whole effort |
| have better information, do comment on my version | | | | was becoming counter productive... |
| of the tale. | | | | The war of technologies was being fought on two |
| The original double drivers | | | | levels: the Aeronautical Telecommunications Network |
| Work on digital link use in continental airspace was | | | | (ATN) versus Internet Protocol (IP) on the one hand |
| triggered by two concerns, both having to do with | | | | and between the VHF Digital Link (VDL) modes on |
| radio frequency congestion. On the one hand, | | | | the other. |
| experts calculated that with the forecast traffic | | | | The war between ATN and IP was the least bloody. |
| growth, sector frequencies would reach saturation, | | | | ATN is an ICAO standard and it took slightly more |
| preventing controllers from communicating with the | | | | than forever to get agreed and standardized with |
| aircraft under their control in a timely fashion. Making | | | | the blessing of ICAO. When work on ATN started, IP |
| sectors smaller was not an option as their minimum | | | | was not even a gleam in engineers' eyes and in the |
| size would have been reached earlier as dictated by | | | | end ATN won not because it is so superior but |
| traffic growth. Only an alternative communication | | | | because it was there in a standardized form and not |
| means could offer a solution. | | | | many felt the strength back then to start the same |
| At about the same time, concerns arose about the | | | | effort on IP. |
| possible forthcoming congestion on the ACARS | | | | The VDL modes were another story. There, for a |
| frequencies, basically promising to paralyze this vital | | | | time, it looked like the US and Europe might end up |
| tool of airline operations. There again an alternative | | | | with different and mutually incompatible systems that |
| solution was apparently needed. | | | | would have been a costly disaster for airliners |
| In the 1980s and early 1990s it was not easy to | | | | operating on both continents. |
| argue for starting serious work on air/ground digital | | | | The clash of 8.33 kHz channel spacing and VDL Mode |
| link implementation. Part of the problem arose from | | | | 3 |
| what should have been a big help... but it was not. | | | | Although always claimed by the airspace users and |
| Seeing all the air traffic management problems in the | | | | never officially admitted by European States, the |
| world, Boeing pushed ahead with the airborne | | | | archaic and inefficient management of frequencies |
| element of FANS, the Future Air Navigation System, | | | | was leading to a situation where new sectors being |
| which looked extremely promising except for one | | | | opened to increase capacity would no longer be able |
| thing. Nobody on the ground took any notice with | | | | to have their own frequency because there simply |
| the result that hundreds of aircraft were flying over | | | | would be none left in the aviation spectrum with its |
| the world's oceans with CNS capabilities that made | | | | 25 kHz channel spacing. |
| the existing oceanic separation minima obsolete but | | | | For some in the industry it was becoming clear that |
| still had to continue with them because the ATM | | | | the whole VHF radio system used by aviation was |
| providers on the ground balked at introducing the | | | | outdated, inefficient and a blocking factor to the |
| changes that would have made them able to use the | | | | introduction of new capacity enablers of which the |
| new capabilities. | | | | absolute shortage of frequencies was just one |
| FANS was a bit like the FMS... the aircraft capability | | | | element. It was high time to devise something new. |
| being light years ahead of the ground systems with | | | | Something future proof, something that would serve |
| the ground systems hardly moving at all. | | | | aviation for many years to come... Of course this |
| In continental airspace, the situation was different. | | | | would have cost money but then there is no such |
| FANS was normally not installed on short and medium | | | | thing as a free lunch... |
| range aircraft while they faced the specter of ATC | | | | Instead of taking the rational decision to force the |
| and ACARS frequency congestion, something that | | | | development of a new communications system, the |
| was not an issue (in this form) over the oceans. | | | | ICAO Special European Regional Air Navigation |
| ACARS has always been an air/ground digital link | | | | meeting (EUR RAN) in September 1994 decided to |
| system, albeit of modest bandwidth and reserved for | | | | split the channel spacing in Europe to 8.33 kHz, |
| airline communications. Also the alternative means of | | | | thereby increasing the number of available channels. |
| communication for ATC has been identified as air | | | | Replacing radios appeared to be the cheaper option in |
| ground digital link with a set of new messages that | | | | the short term... |
| would enable the controller to "talk" to aircraft even | | | | In the United States where there has never been a |
| in the most congested situations. The previously | | | | shortage of frequencies (although the demand for |
| strictly sequential method of voice communications | | | | frequencies is not smaller than in Europe) and 8.33 |
| could be replaced by the basically parallel method of | | | | kHz was never considered seriously initially. |
| digital link with the added bonus of enabling several | | | | Nevertheless, they too realized that a new |
| members of the sector team to talk to aircraft | | | | communications system would be needed and they |
| without jinxing the system. | | | | promoted VDL Mode 3. This system would be able |
| The idea was, and still is, to remove the capacity | | | | to handle both digital voice and digital messages. In |
| constraint represented by the limited ability of the | | | | other words, VDL Mode 3 would offer a seamless |
| controller to talk to the many aircraft otherwise able | | | | service where a controller could talk or send text |
| to operate in his sector. | | | | messages, depending on the prevailing operational |
| Business case hurdles | | | | needs. |
| It is well known that, safety issues apart, airlines will | | | | For a time the map of the world seemed to have |
| only bolt anything on their aircraft if there is a clear | | | | been fatally split. What has never happened in the |
| business case for doing so or if it is mandated. The | | | | past, no ILS or VOR being different on different |
| real chance of ATC frequencies becoming totally | | | | continents was becoming the nightmarish reality on |
| jumbled was still far enough in the future to be | | | | the threshold of a major ATM "improvement". |
| considered a real driver and hence it was proving | | | | Aircraft would be required to carry voice radios with |
| very difficult to make a business case for ATC digital | | | | 8.33 kHz capability as well as VDL Mode 3 radios plus |
| link on its own... In the end, the ACARS problem first | | | | whatever Europe would come up with as the |
| brought to the table by Lufthansa saved the day. | | | | technology for air/ground digital link... Pilots would also |
| Combining the ACARS "replacement" with the | | | | need to figure out when to use what... |
| elements needed for ATC digital link was able to | | | | In the end, the implementation of VDL Mode 3 fell |
| deliver a compelling business case on the industry | | | | behind schedule and the danger receded... As we will |
| level. Of course, general digital link usage was not | | | | see in the next section, at the end of one more |
| something smaller airlines took for granted... those | | | | major battle a single solution remained and that |
| without ACARS saw only one side of the equation, | | | | became the de facto world-wide standard. |
| namely the costs and they were not particularly | | | | |