Tracing The Future

SITAONAIR’s François Bardin discusses the status and challenges of datalink in Europe
My involvement in European datalink services (DLS) began in 1997 with the Eurocontrol PETAL programme at Maastricht Upper Area Control Centre (MUAC). Its aim was evaluating the benefits on capacity and safety of implementing Controller-Pilot Data Link Communication (CPDLC) capabilities using the datalink technologies available at the time.
More than two decades later, a truly global and seamless DLS environment is still far from being a reality. Datalink fragmentation, as well as a number of technical and political hurdles, are still hampering the industry’s ability to unlock the greater capacity urgently needed for air / ground communication.
In the coming years, new generations of aircraft, generating unprecedented levels of Airline Operation Communication (AOC) data, will be flying in European skies. At the same moment, air navigation service providers (ANSPs) will need increased datalink capacity to enable enhanced CPDLC and 4D trajectory exchanges. But the current datalink system is not fit to cope with such an increase in demand.
A new DLS system, encompassing multi-link technologies to deliver the capacity and performance required to cope with these challenges, is urgently needed – delivered under ANSP governance and European Aviation Safety Agency (EASA) control.
The story so far
Of course, a great deal has been accomplished since those pioneering times I refer to above. After the PETAL project, Eurocontrol initiated the LINK 2000+ programme to co-ordinate the implementation of Aeronautical Telecommunication Network (ATN) B1 CPLDC by European ANSPs, paving the way for the future DLS Implementing Rule (IR).
However, a series of technical issues arose, preventing the operational use of datalink in day-to-day Air Traffic Control (ATC) operations. The tough realisation that the pre-existing VDL spectrum capacity could not support the complex ATN protocol; teething problems encountered with both onboard and ground components; stakeholders held waiting until the last minute of the DLS IR deadline to kick-off equipping, and, more generally, a lack of European leadership and co-ordination, led to what was referred to as the “Provider Abort crisis” (from the term “Provider Abort”, used in ATN terminology to identify a loss of communication).
In 2014, the European Commission tasked EASA to perform a root cause analysis of the issues, which led to the amendment of the Implementing Rule in 2015 to leave some time for the stakeholders to address and fix the problems identified.
The following year, the SESAR Joint Undertaking launched and co-ordinated the ELSA study, which arrived at a number of recommendations for addressing the issues highlighted with the VDL infrastructure.
A key conclusion of the ELSA study, in relation to observed Provider Aborts, was that the very limited spectrum available was contributing to VDL congestion. Only one single VDL frequency was deployed at the time (the VDL CSC – Common Signalling Channel, 136.975 MHz) and used by all airspace users regardless of their Communication Service Provider (CSP) contract.
Therefore, it was clear the most urgent work to undertake was to expedite the deployment of additional VDL frequencies (VDL Multi Frequency (MF)). In order to do so, the European Commission (EC) tasked the SESAR Deployment Manager (SDM) to produce a “DLS Recovery Plan”, and co-ordinate the deployment of VDL MF, funded by the Innovation and Networks Executive Agency (through the CEF 2016 Call For Tender process) to support stakeholders’ efforts.
In parallel, the EC also requested the Network Manager to implement DLS performance monitoring, and tasked EUROCAE with the update of the necessary industry standards, and EASA with the regulatory oversight.
The DLS Deployment project, led by the SDM, kicked off in March 2017, involving 16 ANSPs and SITAONAIR and Rockwell Collins as the two Communications Service Providers (CSP). Its double objective was to drive the deployment of VDL MF (Path I) and prepare for the future DLS Service Provision model (Path II). The latter included the definition of the future DLS Technical Architecture, its future governance model and business case assessment, as well as the transitional activities needed to make it happen.
The Good
Out in the field, the improvement with VDL / ATN performances have been spectacular. A number of additional VDL stations and radios have been deployed, completing the CSC initial infrastructure with additional coverage with the En Route frequency (ENR: 136.775 Mhz) and coverage with the Terminal frequency (TRM: 136.875) at main European airports. At the time of writing, SITAONAIR has deployed (either directly or via its VHF ANSP partners) a total of 218 VDL radios covering 141 airports in 28 countries. There are 149 radios on the CSC frequency, 35 on the Terminal and 27 on En Route, resulting in the comprehensive coverage illustrated below:
Figure 1: SITAONAIR CSC coverage (pink: online, blue: planned)

Figure 2: SITAONAIR ENR coverage (pink: on line, blue: planned)
This significant deployment effort, conducted by SITAONAIR among the CSPs and ANSPs, drastically decreased the rates of Provider Aborts. Of course, there is still room for improvement, but observed PA rates have gone down from 20-30 per cent at the peak of the crisis to two per cent or less, as recently reported by Maastricht UAC and DFS Deutsche Flugsicherung (DFS).
The Bad
These improvements are certainly encouraging. However, there still are a number of issues (technical and non-technical) preventing a smooth usage of CPDLC and requiring the DLS community’s attention.
At the ATN level, some technical problems remain, impacting communication with aircraft, especially when transitioning from one area to the other – a phenomenon known as ‘routing ambiguity’. Moreover, it has been observed that some avionics would rather remain connected to a VDL station with bad signal quality, rather than hopping to a better one, if this one advertises a different ATN identifier.
On the airborne side, some avionics do not seem to have transitioned to VDL multi-frequency protocols properly. In a nutshell, in order to fully benefit from a comprehensive VDL MF infrastructure and unleash the complete VDL MF capacity, it is essential for avionics to be properly tuned into the appropriate VDL frequency, either when advertised (Frequency Support List (FSL)) and especially when the aircraft is instructed to do so (Autotune).
Using FSL, the avionics would detect and select additional ENR or TRM frequencies, whereas with the Autotune mechanism, a CSP ground component (the Centralised VHF Management Unit or CVME) would track the aircraft and instruct him to switch onto ENR or TRM frequencies when appropriate. Although progressing, the current usage of VDL alternative frequencies remains relatively low for various reasons. As a case in point, SITAONAIR observed that only 60 per cent on average of autotune uplinks are acted upon, resulting in the aircraft switching frequency.
Lastly, the DLS community lacks a technical forum for addressing such issues, as well as other deployment challenges and technical topics in general. It is our understanding that the SDM should soon come forward with a proposal, but such a forum would have been very useful to have in place from the beginning of the VDL deployment programme.
The Ugly
At a time when the whole DLS community, ANSPs, industry and CSPs, should stick together and join forces in delivering today’s DLS system – enabling ANSPs to comply with the Implementing Rule and paving the way to full 4D trajectory management, while also working to shape the future DLS Service Provision model – a tremendous amount of time and energy is lost in useless dogmatic fights over what the VDL European architecture should be. To make a long story short, two VDL architecture models currently exist in Europe: the standard model, referred to as “Model B” (or dual RF) in the ELSA study, and the “Italian” model, referred to as “Model C” (or single RF) in the same study (or Model C/MF in its multi-frequency version).
The Model B describes the VDL infrastructure that has been deployed to date in most European countries, by both CSPs and partner ANSPs. It is made up of the two different VDL multi-frequency networks existing today, each of them advertising a specific CSP identifier. But in Italy, for example, the approach for deploying VDL has been different. The ANSP (i.e. ENAV) tasked a local industry manufacturer to develop a specific VDL station capable of advertising both CSPs’ identifiers. The deployed network would then capture the whole of the VDL traffic, route AOC data to CSPs and the ATN traffic to the ANSP.
The pros and cons of each approach have been debated at length by the stakeholders of the DLS Deployment project. To summarise respective positions, the promoters of Model C/MF claim it delivers higher VDL capacity, whereas the arguments of Model B’s supporters is that this stated increase remains to be seen in the field. In light of this, they believe the business case is not yet strong enough to justify replacing the comprehensive Model B VDL infrastructure already deployed over most European countries, especially with so many other non-technical aspects also having to be considered.
The two different models could co-exist and be operated in their respective service areas.
What happens next? Towards a new DLS landscape
In April, the SDM kicked off the second step of the Deployment Programme, the “DLS European Target Solution Assessment” – a two-year project gathering the same ANSPs / CSPs team from the previous phase, along with additional industry stakeholders. The objective of the project is to design the future Common European ATN Ground Network, and follow-up technical and non-technical activities from the previous project, including VDL capacity assessment.
Still, despite the divergences over the best-possible VDL MF architecture, there is a general consensus among the industry that a new datalink model is needed to tackle DLS fragmentation, prepare for initial and full 4D trajectory management, as well as for other SESAR concepts requiring an effective air / ground communication link. To deliver the required capacity, additional datalink media such as satellite, in multi-link with VDL, is a must, as is exploring next generation technologies such as LDACS, AEROMACS or LTE. And more so now than ever, the emergence of a new Service Provision model is necessary, driven by ANSP governance, with EASA oversight and delivered with industry and CSP support, for the benefit of each and every airspace user.
A lot has been achieved since European DLS deployment began and a lot remains to be done. Under the co-ordination of SDM, a new DLS landscape is now being shaped. It is time for ANSPs, industry and CSPs to step up, join forces and make it happen.

Francois Bardin is SITAONAIR director, Air Traffic Solutions