ATM Modernization, Business & GA, Commercial

Link 2000+: CPDLC System Steams Ahead

By George Marsh | July 1, 2004
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While the U.S. Federal Aviation Administration’s (FAA’s) controller pilot data link communication (CPDLC) program in Miami cools on the proverbial back burner, Eurocontrol’s CPDLC program steams ahead with Link 2000+. Some 25 aircraft (mostly in the United States) are equipped for CPDLC, and funds are earmarked to equip another 124 aircraft. Since January 2003 the Maastricht upper airspace control center (ACC) in the Netherlands has been ready for CPDLC service, and the first European aircraft was certified for CPDLC last December, marking the beginning of the “rolling,” and at this time voluntary, Link 2000+ program.

Maastricht is the first center to offer a fully operational CPDLC service. However, other ACCs—Karlsruhe, Germany; Reims, France; and Madrid—are due to follow during 2005 and 2006.

As a result of a decision expected to be formalized this summer, Eurocontrol, the force behind Europe’s air traffic management (ATM) modernization, will provide funding assistance and test support to pioneer ACCs and aircraft operators. Typically, control centers need to modify their flight data processing systems by adding a data link server and update their controller positions by adding a human-machine interface (HMI) for selecting/composing and reading messages.

Link 2000+ will provide pilots and controllers with standard messages into which they will, where necessary, key in missing details. The program offers a repertoire of 110 basic messages, within four service groups:

  • The first is for basic messages, such as “hello,” “goodbye” and “accept,” and for handing over communications to the next controller.

  • The second is for clearances: turn left/right, hold heading, climb/descend to a new flight level, and so on.

  • The third is to alert aircrews to instances where a voice channel has been blocked due to a stuck microphone switch.

  • The fourth, which is virtually transparent to users, is for context management, for example, logging on and making contact and then reversing those steps on termination. Pilots will select standard responses such as “wilco” (will comply), “standby,” “unable” (e.g., due to weather or aircraft performance), “affirm,” “negative,” and “acknowledge.”

An aircrew will have to log on only once for a flight across Europe; the data link parameters will be passed on to successive control centers as the flight proceeds. Controllers will be able to see which aircraft are data link-equipped and communicate with them accordingly. Non-equipped aircraft will be contacted in the traditional way, by VHF voice. Where appropriate, controllers also will be able to uplink several clearances at a time and note the data-linked acknowledgments.

Other information, such as optimum area navigation (RNAV) routes, can be uplinked selectively, while advisories, air traffic information and similar communications can effectively be broadcast by asking the system to send the message to all affected aircraft. The system also will avoid much time-consuming voice communication between sectors. Instead Link 2000+ will be used to transmit data via the on-line data interchange (OLDI), ground-to-ground forwarding mechanism.

The Benefits

In the cockpit, data link communication will be more visual and tactile than traditional audio-centric voice communication. The HMI probably will comprise a data readout—utilizing either a special-purpose or existing display—and keyboard. The latter might be part of an existing unit with “soft” keys whose functions are configured through menu selections.

The system will be made as intuitive as possible so as not to divert pilot attention from basic heads-up monitoring. Software safeguards will be included to detect erroneous entries and correct them automatically or prompt the user to do so. Link 2000+ is well encrypted and inherently resistant to jamming and spoofing.

Eurocontrol estimates that CPDLC, when fully implemented, could potentially cut air traffic control (ATC) -induced delays by 14 percent. Simulations suggest that sector capacity, currently constrained by the time controllers spend on voice radio telephony, could increase by 11 percent if 75 percent of aircraft become data link-equipped.

These figures make CPDLC a technological time saver and capacity enhancer, alongside such measures as reduced vertical separation minimum (RVSM), basic area navigation (BRNAV) and 8.33-KHz VHF channel spacing. (The 8.33-KHz channel spacing is the most recent “fix” for limited channel capacity.) They are the reasons Europe wants to implement ground-air data link within its busiest air traffic regions, starting with upper airspace.

If an ATC system that broadcasts messages to everyone tuned in or within VHF radio range is wasteful and distracting, then adopting a system that digitally addresses messages to specific recipients should quiet the airways and free up capacity. So goes the case for digital pilot-controller communications.

Air traffic controllers, the argument continues, would spend less time on voice communication and, by selecting preformatted messages into which specific details can be entered, may be able to handle more aircraft. Because data-linked messages would be displayed clearly on a screen or as hard copy, there should be fewer call sign confusions, missed clearances, misunderstandings due to regional variations in the use and pronunciation of “ATC English,” and clarification repeats. Data link also would make it possible to automate the promulgation of air traffic services (ATS) information.

Sold Since 2000

Eurocontrol convinced itself almost before the millennium that data link would truly add value to the ATC system. The Preliminary Eurocontrol Trial on Air/Ground Data Link (PETAL), a three-year CPDLC program conducted in an operational ATC environment, began in 1998. During this trial—which attracted interest from many countries, including the United States—controllers in the Maastricht center communicated with aircrews using three data link technologies. These included:

  • The experimental North European automatic dependent surveillance-broadcast (ADS-B) Network’s (NEAN’s) VHF data link Mode 4 (VDL-4) infrastructure;

  • The future air navigation system (FANS) 1/A protocol, used mainly by long-haul operators on oceanic routes; and

  • The intended global system offering e-mail-style messaging, known as the aeronautical telecommunications network (ATN).

The first flight of an ATN-equipped aircraft took place in 2000, when a UK Defence Research and Evaluation Agency (DERA, now Qinetiq) BAC 1-11 and a Dutch National Aerospace Laboratory Cessna Citation sent data messages over the network in flight.

A European Pre-Operational Data Link Applications (EOLA) trial validated the operational use of air-ground data link and showed that it would be feasible to modify ATC data processing and airborne systems to accommodate it. A Prototype ATN (PROATN) trial and another evaluation for ATN Trials Infrastructure (ATIF) showed the feasibility of using an ATN-based e-mail-like service to meet air-ground communication needs. Recent U.S. CPDLC trials carried out in an operational setting confirmed the European findings and left FAA wishing to implement the system when budgets permit (see May 2004, page 6).

Program Evolves

Encouraged by the accumulated experience, Eurocontrol decided to build on it and implement an ATN-compliant data link, dubbed Link 2000+. (The name relates the project to Europe’s ATM 2000+ strategy for air traffic control, first drawn up during the 1990s.) In 2001 air navigation service providers (ANSPs) delivered the initial letters of commitment. Link 2000+’s geographical scope was defined, with service planned initially for the upper airspace of much of western and central Europe, including France, Germany, Austria, Switzerland, Italy, Spain, Portugal, Belgium, the Netherlands and Luxembourg. The UK, Ireland and the Scandinavian countries, though currently excluded, are keeping a watchful eye on developments and may be included later.

In 2002 a program steering group determined the system’s technology basis. Like FAA, Eurocontrol already had recognized the need for a faster, more reliable means of data exchange than is provided by the airborne communications addressing and reporting system (ACARS), the data link currently used for airline operational communications and limited ATC messaging. (In core continental airspace, ACARS is approaching the saturation point.) The agency believed a more powerful data medium was needed to supplement standard voice communications, in particular to support routine, low-urgency communications. The new link had to be able to convey both human and system-generated data messages.

After consultation with airlines, air traffic service providers, Europe’s Joint Aviation Authority (JAA), airframers, and communication equipment and service providers, a system based on VHF data link Mode 2 was decided upon as the initial baseline. VDL-2, also employed for FAA’s CPDLC program, was selected because airlines were fitting VDL-2 equipment as a more capable successor to ACARS. The technology, therefore, was available, posed minimal technical risk, and fitted well with stakeholders’ existing plans. VDL-2 meets the required communications performance (RCP) for Link 2000+ services and, using a carrier sense multiple access (CSMA) protocol, offers a data rate 10 times that of ACARS.

Airborne Equipment

On the airborne side, the prime requirement is a VDL-2-capable VHF data radio. This requires a transmit/receive antenna separate from the VHF voice radio antenna. Suitable equipment is available commercially and several operators already have upgraded to VDL-2. The HMI can be achieved by modifying a multifunction control display unit or by providing a dedicated keyboard and printer. Some data link clearance services—for instance, those including waypoints—may require inputs from the flight management system. The existing flight deck audible and visual alerting system should be modified to be able to alert the aircrew to the arrival of messages.

Finally, software to support Link 2000+/ATN services must be installed, normally as an upgrade to the communications management unit (CMU) or air traffic services unit. This software contains CPDLC applications to encode/ decode the messages, as well as the ATN end system and router protocols.

Alex Wandels, Eurocontrol’s Link 2000+ program manager, explains that a three-step approach is being adopted to equipping aircraft. Basically, these are to:

  • Support a number of airlines in pioneering the system,

  • Use incentives to widen the installation base, and

  • Mandate equipment fit.

The Link 2000+ program is providing financial help and integration support for the first 100 aircraft that add ATN/ CPDLC capability to their VDL-2 installations. Current “pioneer” operators include Lufthansa German Airlines, Scandinavian Airlines System (SAS), Airbus Transport International (ATI), Air Europa, Federal Express and Hapag Lloyd. SAS gained early certification of a VDL-2-equipped Boeing 737 in December 2003, while ATI is expected to follow this summer, with Lufthansa certification likely late this year.

Incentives to the Operators

After the pioneer stage, program managers plan to maintain momentum by offering route charge reductions to follow-on operators. This incentive is likely to be offered until a quarter of all flights taking place in the eligible airspace are using data link. Eurocontrol hopes that other operators will by then be keen to secure the evident benefits of CPDLC and will carry out retrofits, typically during engineering checks.

Ultimately, since full data link benefits would not be obtained until all aircraft in a given airspace are equipped, it may prove necessary to make the system mandatory. The International Air Transport Association (IATA) is, subject to conditions, prepared to accept this. Realistically, the Link 2000+ program office aims to ensure that three quarters of all flights and all centers in the eligible airspace are using the data link by about 2009 to 2010.

Wandels expects that airlines will increasingly perceive that CPDLC makes good business sense. He believes that as controllers and pilots become familiar with the new way of communicating, their confidence will grow, transactions will be faster, and sector capacity will consequently increase. This will result in reduced delays and more direct routings. There will be fewer errors and less controller and pilot fatigue, he says.

Voice communication will be reserved for dynamic situations where non-routine dialogue may be expected. The presence of the second communication channel will further enhance safety through redundancy.

An overall cost-benefit analysis suggested that Link 2000+ will have paid for itself four times over by 2020. This may be hard to conceive during the first few years when Eurocontrol, the ANSPs and aircraft operators are all investing in the system. But after that benefits should predominate. This analysis was part of a business case put together by Eurocontrol, FAA, aeronautical communication providers, air operators, aircraft manufacturers, a communication, navigation and surveillance/ air traffic management (CNS/ATM) focus team and a Boeing-led airline grouping.

Program Costs

Eurocontrol expects to fund the program from its European Air Traffic Management (EATM) budget, to the tune of 16 million euros ($19 million), excluding the cost of ACC and aircraft implementations. The cost per ACC is estimated at between 5 million and 10 million euros ($6 million and $12 million). Upgrading an aircraft is a substantial task, requiring anything from 50,000 to 100,000 euros ($60,000 to $120,000) in total, depending on aircraft type and system complexity.

However, if VDL-2 VHF is already fitted, the cost of the necessary software upgrade and other required changes could start at some 20,000 euros ($24,000). Rockwell Collins, Honeywell and Thales Avionics will offer Link 2000+ avionics, and Teledyne Controls reportedly plans to offer similar equipment. The European and U.S. data link systems have been shown, unofficially, to be interoperable. ARINC and SITA clients will be able to connect to it.

Are There Drawbacks?

Any system has drawbacks. The main issue highlighted with CPDLC is that, because pilots are no longer hearing radio traffic between the control center and other aircraft, they lose some situational awareness. Eurocontrol argues that this becomes significant only in the departure and arrival phases of flight when other traffic may be close. Agency officials add that the voice option is still available should the various parties prefer to use it. Wandels says that, so far, this issue has not been seen as a showstopper and that experience will yield procedures for dealing with it.

Overall, Wandels says, the system’s ability to deliver correct, readable and unambiguous messages to the appropriate aircraft without distracting other aircrews will quickly win acceptance with users and, through improved service, air transport operators.

Since budgetary constraints have forced FAA to put CPDLC implementation on hold in the U.S. at present, Europe is ahead of the game in delivering a viable ATC data link. Other regions will watch with interest to see whether Link 2000+ delivers on its considerable promise.

Link 2000+ Data Link Options
Other data link technologies, in addition to VHF data link Mode 2 (VDL-2) with carrier sense multiple access (CSMA) protocol, also were considered for Eurocontrol’s LINK 2000+ program. And they are not ruled out for the future, given that VDL-2 may become capacity-constrained in Europe as early as 2010.

VDL-3 uses the time division multiple access (TDMA) protocol and operates at the same channel data rate as Mode 2 but with improved bandwidth utilization. It would integrate voice and data communications on the same channel. Although the Federal Aviation Administration prefers VDL-3, the technology is less attractive in Europe, which relies on analog 8.33-KHz VHF for voice communications.

VDL-4 uses the efficient self-organized time division multiple access (STDMA) protocol, is faster than Mode 3, self-synchronizing and operable air-to-air without the need for ground stations. Moreover, it could support surveillance as well as communications systems. It is one of the data links favored for use with automatic dependent surveillance-broadcast (ADS-B). (The other main ADS-B contender is an enhanced [“extended squitter,” or message length] version of Mode S, a system associated with secondary surveillance radar.) Because VDL-4 is not yet commercially available and has spectrum availability issues affecting channel capacity, it is considered more of a future prospect than a realistic near-term option.

The universal access transceiver (UAT) data link has been advocated for ADS-B. Under development in the U.S. initially to support general aviation, UAT operates in a band reserved for navigation aids and has not yet secured a frequency assignment in Europe. It is still in the earliest developmental stages. But if a future upgrade had capability approaching that of VDL-4, it could ultimately be considered for European CPDLC.

All these systems are somewhat compatible with the aeronautical telecommunications network (ATN). While being primarily ATN-compliant, VDL-2 also can be made operable with the FANS 1/A protocol, although plans for doing this are limited at present. It was felt that VDL-2 was better suited to the CPDLC task than the surveillance-optimized Mode S, even in its extended-message variant.

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