ATM Modernization, Business & GA, Commercial

Europe’s CASCADE: Blueprint for Data Link Services

By George Marsh | April 1, 2006
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Building on experience with en-route controller-pilot data communications in the Link 2000+ program, Europe is moving on to the next level of data link services. Its CASCADE program, launched early last year, will extend the repertoire of air-ground data communications that can be transmitted over the VHF Mode 2 digital link (VDL-2). This should further ease the workload of air traffic controllers in Europe's high-density, en-route airspace, while reducing congestion on VHF voice channels.

CASCADE, however, goes a big step beyond expanding the scope of controller-pilot data link communications (CPDLC) services. Significantly for the future of European air traffic services, the program will address the surveillance dimension of the communications, navigation and surveillance (CNS) triad. By encouraging the implementation of automatic dependent surveillance-broadcast (ADS-B), CASCADE will enhance pilot and controller awareness of the air traffic situation and enable the two sides to share the same derived overview. Although details of how the necessary equipage will be funded are not yet clear, CPDLC and ADS-B, between them, will lay the groundwork for more cooperative modes of air traffic control (ATC) in which pilots could have a greater say over the positioning of their aircraft.

CASCADE stands for Cooperative Air traffic services through Surveillance and Communications Applications Deployed in ECAC. ECAC, in turn, signifies the European Civil Aviation Conference group of countries. The CASCADE program aims to expand CPDLC and implement ADS-B, although both efforts--at least in the early going--depend on the willingness of the airlines and the air navigation service providers to invest in the avionics and ground-based infrastructure on which to build data link services. While much of the CPDLC infrastructure is already in place, thanks to pioneering efforts like Link 2000+, the ADS-B ground network remains to be built, and all parties persuaded that it will be worth the cost.

According to Alex Wandels, Eurocontrol's CASCADE program manager, the overall goal is to reduce delays, maintain safety and increase air traffic management efficiency in the upper airspace over "core Europe." CASCADE focuses on coordinating technology and service implementation, as well as ensuring that ECAC states do not adopt too many different solutions. It includes standardization activities and efforts to ensure that Europe moves forward on air traffic control data link in step with other regions. A more detailed plan for implementing specific services and technologies--based on safety and business cases now being developed--will be drawn up after trials help determine which options can best optimize airspace usage.

Two Phases

CASCADE will progress in two overlapping phases. Phase 1, which began last year, relies on ground and air infrastructure that should be widespread from 2008 onward and requires little investment over and above that needed for Europe's ongoing Link 2000+ (CPDLC) and Mode S (surveillance) programs. Eurocontrol's Maastricht Upper Air Center has been using VDL-2 for 20 months, and other centers are due to follow its example. Europe also is installing improved secondary surveillance radars (SSRs) to enhance radar surveillance. SSR has its own associated data link, Mode S, which, in its extended squitter (ES) form, has the capacity to carry ADS-B data. CASCADE's Phase 2, which overlaps with Phase 1 and proceeds largely in parallel, envisions the implementation of ADS-B-based surveillance in Europe.

As Wandels told delegates at the ATN 2005 conference held in London last year, "The challenge is not about equipage--it's how to effectively use the equipage that's already available." Airline representatives at the conference agreed. Their common plea was: "Give us the services so that we can use the avionics we already have." (ATN, the aeronautical telecommunications network, provides the overarching layer that will allow different air traffic control data links to interoperate.)

Over the next two years, Phase 1 will address services that can be provided at minimal extra cost to airspace users. Phase 2, however, which Eurocontrol hopes to see completed by 2011, will prove more expensive. Obtaining the full benefits of data link in both CPDLC and surveillance will require upgrades to onboard and ground-based systems. In the air that means the addition of data link receivers, traffic displays and interfaces that network the pilot, flight management system (FMS) and ground control with the communications infrastructure.

In the first phase CASCADE will increase the messaging capability available under Link 2000+, Eurocontrol's current push into CPDLC. Pilots soon will be able to use VDL-2/ATN to obtain clearance for all ground movements--pushback, taxi clearances, etc.--and to alert controllers and ground authorities about emergency situations. Further, it will become possible to access a range of reliable aeronautical information stored in digital form, such as digital flight information services (D-FIS) and terminal information services (TIS).

Digital OTIS

A 6-month Digital Operational Terminal Information Service (D-OTIS) trial scheduled to conclude in March suggests some of the benefits that can be gained with CPDLC in practice. Eurocontrol has collaborated with Belgium's Belgocontrol and SN Brussels Airlines in the project. Pilots flying the 38 aircraft SN Brussels Airlines has equipped for the trial can call up data from the European Aeronautical Information Services Database (EAD) made available by Eurocontrol. Weather and destination airport details are also displayed via digital VOLMET and digital automatic terminal information service (D-ATIS) provided by Belgocontrol. The carrier equipped 32 BAE Systems Avro RJ85/RJ100s, three Airbus A319s and three A330-300s.

Pilots can access the data, via VDL-2/ATN data link, whether they are in the air or on the ground. SITA's Information Networking Computing unit has developed and is hosting the service. According to George Paulson, air traffic management program director at Eurocontrol, the CASCADE program "is a fine example of how different players--air navigation providers, communications service providers, airlines and Eurocontrol--continue to join forces to enhance aviation safety."


CASCADE's second major aim is to enhance air traffic situational awareness, providing pilots and controllers a shared overview. The identity, position and velocity of each aircraft equipped with the appropriate transponder will be broadcast via data link to all other aircraft within range and to air traffic controllers on the ground. Once received, it is processed along with similar data from other aircraft to form a combined air traffic picture.

Supporting this automatic air reporting, ADS-B function, at least in the medium term, is the SSR-associated, 1090-MHz Mode S data link. Mode S has been mandated for aircraft using European airspace. Around 95 percent of these aircraft should be equipped by 2007 and the other 5 percent by 2009.

All will have extended squitter capability, which equates to more data and longer messages, and allows Mode S to meet ADS-B traffic surveillance requirements. The ability to exploit existing data link equipment reduces the total cost of implementing ADS-B.

ADS-B data enhances controller situational awareness by complementing radar and, in areas where radar coverage is not available, substituting for it. In the air pilots of equipped aircraft will have this surveillance picture on a cockpit display. If pilots can become as aware of traffic in their proximity as controllers are, the possibility emerges for the air side and the ground side to cooperate in managing air traffic spacing, arrival and departure sequencing and, more ambitiously, 4D trajectories.

Ultimately, ADS-B could revolutionize the way air traffic is managed. Enhanced pilot awareness has other benefits, such as being able to see and avoid other aircraft in poor visibility and in airspace where ATC cannot ensure separation.


Assessments of using 1090-MHz Mode S with ADS-B have been taking place as part of Eurocontrol's CoopeRative valIdation of Surveillance Techniques and AppLications (CRISTAL) activities, which are closely linked with CASCADE. Trials, simulations and studies involving 10 ECAC states are intended to evaluate the surveillance potential of data link-enabled ADS-B in Europe across a range of airspace types and scenarios.

Generally, the trials rely on the VDL-2/ATN link but, in certain areas where investment has been made in the VDL Mode 4 infrastructure, notably in Sweden, this has been used in order to expedite validation.

One study, undertaken by the UK's National Air Traffic Services Ltd. (NATS) and supported by Helios Technology Ltd. and Cranfield University, suggests that ADS-B could be useful even where radar coverage is extensive, as in southeast England, especially the London terminal area. The initial take on this "paper trail" was that ADS-B could add to the radar surveillance picture and possibly increase capacity in this congested area.

A trial in France tested the effectiveness of ADS-B at a traffic hub. Eurocontrol collaborated with the French Directorate of Air Navigation Services, Airbus, Thales ATM and Alticode on the project in Toulouse, using a 1090-MHz ES ground station that had been installed there. Equipped aircraft were reliably detected out to as far as 250 nautical miles (nm) on occasion, at high altitude.

A related program, CRISTAL MED, brings Eurocontrol together with air navigation services providers, ENAV (Italy), AENA (Spain) and HCAA (Greece), to assess the use of enhanced Mode S infrastructure for Mediterranean airspace. An important aim of this pre-operational trial is to establish the use of aircraft-derived data for surveillance in areas without radar coverage and under procedural control, as well as for radar areas.

A further trial in Iberia (Spain and Portugal) is assessing ADS-B's ability to fill gaps in existing radar coverage. Both CRISTAL MED and CRISTAL Iberia also aim to improve surface surveillance, using ADS-B to track both aircraft and vehicles on the ground at major airports.

Yet another CRISTAL trial in Sweden has demonstrated the usefulness of ADS-B in continental airspace, while a trial in Ireland, involving the Irish Aviation Authority and Raytheon, provided satisfactory results in long-range oceanic airspace. At Innsbruck, Austria, ADS-B has been tested in combination with multilateration, a high-integrity solution that is appealing in some applications. According to Wandels, Eurocontrol is prepared to mount further trials where ADS-B could address other difficulties.

FMS in the Loop

Implementing the data communications and surveillance infrastructure, linking it to FMS systems and providing interfaces to bring flight crews into the loop will pave the way for further air traffic control enhancements and automation. The necessary avionics and ground equipment changes could be in place by 2011, implemented in line with CASCADE Phase 2 objectives. It should then be possible to redistribute some tasks between controllers and flight crews in a more balanced way, delegating responsibility for certain tasks from controllers to pilots. This should boost traffic handling capacity in hard-pressed airspace sectors. The new data communications infrastructure also will permit a more balanced distribution of tasks between different ATC sectors, aided by increased automation.

Historic Flight

Cooperative activities that will become possible once FMS is in the loop include enhanced sequencing and merging--also known as arrival spacing. Since they will be aware of other aircraft over a substantial radius, pilots will be able to maintain their own spacings, as instructed by ATC. Autonomous separation, says Wandels, is likely to be the first example of a task that is delegated from the controller to the pilot. And, though such delegation may not be to every controller's taste initially, benefits could include increased sector efficiency.

The next step toward autonomy could be trajectory negotiation--made possible with 4D FMS and an additional onboard graphical interface. Indeed, this application is already here. In an historic flight on Jan. 19, 2006, the crew of a Scandinavian Airlines System (SAS) Boeing 737 in revenue service negotiated with air traffic control over the aircraft's intended trajectory in order to optimize the flight path from takeoff to landing. The flight culminated in a "green" (constant descent) approach and an arrival that was within seconds of the planned ETA. A key enabler was the ability of the Smiths Aerospace FMS to downlink the intended flight plan from the aircraft to the controller on the ground, and to follow up with subsequent change negotiations and revised ETAs--all via CPDLC data link.

An automation tool used at Stockholm's new ATC center played a major role in this seminal event. The Collaborative Information Exchange System (CIES) enables controllers to view arriving flights with their ETAs and agreed-upon approach patterns on a tabular display that is updated by the 4D trajectories received from aircraft flight management systems. SAS, seeking to exploit the full advantages of its onboard avionics, expects to experience less radar vectoring and fewer delays, resulting in significant fuel savings and reduced emissions. Obtaining clearances early via data link means less voice communication on the approach, freeing pilots to concentrate on their instruments, safety procedures and pre-landing checks. In April six SAS 737s are expected to take part in a trial to study how well the system works with multiple 4D trajectory-capable aircraft.

Clearly data link is a very live issue in Europe. As Wandels summarized at last year's ATN 2005 conference: "We continue to explore the potential of data link, as it is key to realizing future concepts. It's part of our wider initiative at Eurocontrol to improve sector productivity. Our task with CASCADE is to put in the further work needed to make it all happen."

Cascading Benefits

Already flight crews on the 260-plus aircraft fitted with VHF digital link, Mode 2 (VDL-2) avionics are seeing the benefits of routine messaging via data link. Alex Wandels, who managed Eurocontrol's Link 2000+ program and now heads the agency's CASCADE program, states:

"CPDLC [controller-pilot data link communications] offers much the same advantages that e-mail has over the telephone. It can take care of all routine, non-critical communications, easing the workload of pilots and controllers by providing silent, easily readable messages to the correct recipient."

Under Link 2000+, some 110 basic messages, in distinct service groups, can be passed as digital data without recourse to voice. The types of messages are, broadly, those associated with: "hello," "goodbye" and "accept" protocols; handoffs from one controller to the next; turn, heading and level clearances; stuck microphone switch alerts; and context management, e.g., making contact and logging on. Messages are unambiguous because they appear as text in a standard format and are free from the vagaries of voice, pronunciation, analog radio distortion and background noise. Pilots and controllers save time previously used in routine voice communications and there is less congestion of VHF channels.

Around the year 2000 Eurocontrol first became convinced that a new data link would add value to its air traffic control system. This was partly as a result of the three-year Preliminary Eurocontrol Trial on Air/ground dataLink (PETAL) that had begun in 1998. The existing airborne communications addressing and reporting system (ACARS) air-ground VHF data link had been able to support early air traffic services applications such as pre-departure clearance (PDC) and automated terminal information service (ATIS), but it was becoming saturated and would not long be able to support the required level of service in dense continental airspace. Radar-based data link (Mode S) lacked sufficient communications capacity, as well.

PETAL and subsequent trials indicated that VDL-2 offered a 10-fold capacity improvement over ACARS. The trials showed that this data link, within an aeronautical telecommunications network (ATN) environment, would provide a suitable and cost-effective platform for air traffic and other air-ground data services for at least a decade. Eurocontrol launched its Link 2000+ program to bring about VDL-2/ATN equipage and implement new data services, particularly CPDLC.

The recent addition of Finnair to the Link 2000+ program brings the total number of participating aircraft to more than 266, from 12 airlines. These include the 29 Airbus A320s that Finnair intends to equip with VDL-2 this year. The airline will install new Airbus air traffic services units (ATSUs) for CPDLC messaging. Communication service provider ARINC is supporting Airbus through a connection to ARINC's ATN/VDL-2 network, deployed since 2004 in Europe.

Eurocontrol's Maastricht Upper Air Center has been using ATN/VDL-2 for 20 months to handle a steadily increasing proportion of air traffic control communications in its congested airspace. The center also accommodates the FANS oceanic data link, boosting the number of aircraft with which Maastricht can conduct CPDLC operations. Recently the center introduced an automatic CPDLC service in which secondary surveillance radar codes are linked to aircraft as they proceed from sector to sector without human intervention. Data link parameters are another obvious candidate for such automation. On average, at least 1,000 data link messages per month pass between aircraft and the Maastricht Centre, and this figure is rising. Other control centers are expected to follow Maastricht's example, and eventually some 15 centers throughout Europe will be equipped to offer a similar level of CPDLC service.

CASCADE builds on this background, while also exploiting the secondary surveillance radar, Mode S-based data infrastructure for the surveillance element of the program.

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