ATN: Harmonizing Aero Communications

There was a time when life in aeronautical communications was simple. There was analog voice radio or nothing. But how times have changed. Today we face competing technologies, divergent regional solutions, a profusion of data and data links, and incompatibilities between legacy and successor systems.

No magic bullet will make life simple again. However, the emerging aeronautical telecommunications network (ATN)–espoused by the International Civil Aviation Organization (ICAO), if not yet by most aircraft operators–could certainly help. When ICAO sired the future air navigation system (FANS) back in the late 1980s, it realized that multiple communications solutions would present themselves. It therefore proposed an overarching network that could "talk" to them all. The idea was to have this aeronautical Internet insulate users from concerns about which subsystems and protocols are in use by handling all data and automatically routing messages through the best communication data links and subnetworks available. ATN, despite slow uptake by users so far, still can deliver on that vision.

But consider the challenge faced by ATN developers. Although digital voice shows signs of inevitably replacing analog voice, the older communication mode still must be accommodated. A further complication is the fact that Europe, threatened with saturation of aeronautical VHF channel capacity, has adopted 8.33-KHz interchannel spacing, whereas the less pressed United States still maintains the 25-KHz standard. And allowance must be made for the different data links, which include the following:

The aircraft communications addressing and reporting system (ACARS) carried on analog VHF or via satellite,
The aeronautical mobile satellite service (AMSS) operated by Inmarsat to support future FANS functionality in oceanic regions, and particularly,
HFDL, the HF radio-based system used over extended ranges.

These, in turn, will become legacy systems once they are superseded by higher-capacity, new-generation VHF and satcom links. The successors will be some combination of VHF digital link (VDL) Modes 2, 3 or 4 and satellite data links such as Inmarsat’s Swift 64. There will be others–for instance, various ground wireless links (gatelinks) and a dedicated, combined air-ground/air-air data link for meteorological data. Ideally, an evolving, overarching network can integrate all of these.

The need for data links to support surveillance as well as communications adds to the complexity. Expanding ATN beyond its present communications focus will mean interfacing with links used for radar, such as Mode S.

These surveillance links support communications, as well. But, given expected traffic growth, they may start to reach their capacity limits within a decade. Their replacements also will need to be compatible with automatic dependent surveillance-broadcast (ADS-B), the emerging technology that can augment radar or, in certain circumstances, act as a stand-alone system.

Everybody’s Talking

Suddenly everyone–including airborne and ground-based systems–wants to communicate. The flight crew needs a dedicated link for air traffic services (ATS), backing up analog voice-based air traffic control (ATC) communication and information services. ATC authorities might want a direct link between their computers and the onboard flight management system (FMS). Another link is needed for airline operational communications (AOC) between the flightdeck and airline operations centers. A similar link, between the cabin and ground, may be needed for airline administrative communications. And passengers need broad data "pipes" to enjoy telephone, e-mail, Internet, intranet and entertainment services. These in-flight entertainment (IFE) data links will connect passengers to telecommunications systems and service providers on the ground.

Air-ground and air-air data links are likely to multiply to satisfy exploding communications demand. Clearly, considerable technical and regional diversity will have to be accommodated. Among the VHF data links, for example, VDL-2 is a carrier sense multiple access (CSMA) link favored by Europe as the basis of its Link 2000+ program because it is compatible with analog VHF voice, which Europeans expect to maintain for some time. The time division multiple access (TDMA), multiplexed, combined digital voice/data link, VDL-3, once championed by FAA, is on hold at this time.

It even is possible that these different VHF solutions might be operated concurrently. And, in any case, both VDL-2 and -3 eventually could give way to VDL-4, a potentially superior, more flexible, self-organized TDMA (STDMA) link that could serve the needs of multiple diverse users. VDL-4 has good range and capacity, and is designed to be independent of ground infrastructure. But ATN does not care; it can operate with all of them.

Still More Diversity

An alternative strategy of achieving greater data link commonality across a range of aircraft communication applications would be difficult, not least because of the life-cycle issue. As Tony Whyman, a director and cofounder of UK consultancy, Helios Information Services, points out, flightdeck technologies are generally expected to serve for many years, whereas IFE equipment in the passenger cabin typically is used only a few years before it is replaced with a new generation, which may rely on a different air-ground data link technology. An all-embracing "supernetwork" is a more sensible approach.

Another source of disparity is the links used for surveillance purposes. For instance, while existing radar-associated links–Mode S/1090ES–use the 1090-MHz channel, UAT is a 978-MHz random squitter system. The Mode S data link differs in its infrastructure, protocols and capability from ADS-B.

Add to all this diversity the emergence of dissimilar equipment from diverse vendors, plus a range of different services from ARINC, SITA and others, and it looks as though ATN will have to be ready for virtually anything. In addition, it is intended to be future proof, accommodating new kinds of subnetworks that do not yet exist.

ATN’s proponents hope this internetwork can rise above the confusion of data link and network technologies, standards and protocols by providing a common network layer with which they are all compatible. It should, they argue, be scalable and support the introduction of new data links for fresh applications as they arise. It should meet safety and security criteria, for example, prioritizing certain types of messages and resisting unauthorized interference. A necessary condition would be that individual data links be ATN-compliant. Operators then will have to equip their aircraft with the appropriate, compliant VHF data radios and/or satcom systems.

ATN in Practice

The ATN supernetwork utilizes various telecommunications protocols to access and interconnect the diverse subnetworks–such as VHF, satellite and Mode S–by which content is relayed to where it is needed. ATN, as specified in ICAO standards and recommended practices (SARPs), uses a layered protocol architecture based on the open system interconnection (OSI) standard championed by the International Standards Organization. The primary airborne element is a router, a software subsystem within the aircraft’s communications management unit (CMU) or, on Airbus aircraft, the air traffic services unit (ATSU).

The onboard router, matched by routers on the ground, chooses between the most appropriate HF, VHF or satellite data links in a transparent manner. Like the Internet, it attaches a destination address to a message and then dispatches it to that address by the most appropriate communications channel available. The software comprises a mix of routing protocols that can be configured according to specific routing policies.

Three companies delivering this technology are Ireland-based Airtel ATN with its TAR router, France-based Sofreavia with a product called ProATN, and Thales with its RRI product. Sofreavia laid much of the foundation for all three routers. The company developed trial software for Eurocontrol’s prototype ATN (ProATN) initiative and subsequent ATN trials in the late 1990s. The French company was part of a team headed by ATN infrastructure specialist Aeronautical Communications International LLC, which received the prime Eurocontrol contract to develop a "reference router." Airtel and Thales forerunner, Airsys ATM, also contributed to that team effort. The routers from all three companies comply with ICAO’s communication, navigation, surveillance/air traffic management (CNS/ATM) Package 1 SARPs, the technical specification pertaining to ATN.

Rockwell Collins was the first avionics vendor to bring an ATN-compliant CMU to market. It incorporates Airtel router software in its CMU-900. Honeywell is following with its Mark II/III CMU upgrades. Both vendors can adapt their ATN technology to ATSUs for Airbus aircraft. (Indeed, Airbus reportedly has opted for Collins’ technology rather than the more homegrown Thales router.) Teledyne Controls, which supported Delta Air Lines in U.S. trials of ATN/CPDLC (controller pilot data link communications), also has incorporated ATN software into its CMU.

Progress towards full ATN realization has been slower than first hoped because, though there is little opposition to the concept, airlines are obliged to invest only where it clearly benefits the bottom line. Estimates suggest that retrofitting ATN into a Boeing or Airbus all-digital aircraft could cost up to $400,000. This includes the cost of ATN-compliant data links, along with multiple interfaces to onboard computers and other equipment. Fleet fittings and factory installations would, naturally, reduce the per-aircraft figure. The cost of achieving worldwide air and ground ATN implementation is estimated at several billion dollars. Moreover, data link service providers such as SITA and ARINC will have to reflect in their charges the costs of incorporating ATN functionality into their networks.

Clear business cases can persuade airlines to invest, however. Thus, the promise of CPDLC for improving flightdeck ATC communication is attracting takers, particularly in Europe, where efficiency savings can be fed back to operators via reduced service charges. Airlines have started to sign up for Europe’s Link 2000+ program because they can see measurable benefits from CPDLC and other applications, in particular context management (CM), ADS-B and AOC. By the end of this year, more than 100 aircraft should be equipped for CPDLC delivered via ATN-compliant, VDL-2 radio. Altogether, some 150 aircraft will be equipped as part of Eurocontrol’s "pioneer phase," intended as a shakedown and type certification period.

European Rule

An implementation rule expected this year will result, by the end of this decade, in a requirement that virtually all new aircraft destined to operate over core Europe at flight level 285 and higher be equipped with ATN/CPDLC avionics. Five more years will be allowed for retrofit programs. Historical precedents suggest that high-end corporate flyers will wish to equip, too.

Between May 2003 and May 2004 air traffic controllers at Europe’s upper air center at Maastricht handled more than 4,000 flights using CPDLC to ease voice channel congestion. That figure is expected to rise to 100,000 flights a year by 2006, by which time at least another 15 ATC centers are scheduled to be equipped for ATN/VDL-2 operation. While the existing ACARS system will be maintained, transporting legacy ACARS application data to the new system may be possible. And, at that time, the new system might also be conveying aeronautical operational communications.

Last September, communications provider SITA launched an ATN service, following earlier flight trials that had demonstrated interoperability with Eurocontrol’s Link 2000+ and its own VDL AIRCOM service. SITA’s ATN service connects onboard routers to SITA VDL stations covering the core area of Europe. Because it is part of the AIRCOM environment, the ATN service is fully supported by 24/7 monitoring and help desk services. Lufthansa and Federal Express have selected SITA to provide ATN/VDL-2 services across Western Europe.

ARINC, too, has rolled out an ATN/VDL-2 network in the context of Eurocontrol’s Link 2000+ program. In December 2003 this network, comprised of 12 ground stations (including the Maastricht UAC), supported a successful supplemental type certificate (STC) flight by a Scandinavian Airlines B737. Last November Boeing announced that it had type-certified its B737NG aircraft with Collins technology, the first time an aircraft manufacturer had a forward-fit aircraft approved with CPDLC for ATN. ARINC says it can build ATN functionality onto its GLOBALink platform.

Both SITA and ARINC have deployed ATN technology in the U.S., including in support of CPDLC implementation at the Miami air route traffic control center (ARTCC). The FAA decision two years ago to defer an extension of CPDLC coverage beyond Miami for financial reasons was a short-term setback for ATN. U.S. authorities showed signs of wanting to advance an alternative ATN-compliant solution–VDL-3 supported by NEXCOM (next generation communications) data radios–but budgetary constraints again intervened.

Despite the measured, to say the least, pace of ATN uptake, operators equipping or preparing to equip with ATN avionics are reported to include Air Europa, Air Berlin, Hapag Lloyd and Airbus Transport International in Europe and American Airlines and Continental in the United States, as well as Lufthansa and FedEx. More ground stations in Europe are becoming ATN-equipped: Germany is expected to enhance its coverage this year, and Portugal appears to be about to commit. The Cape Verde Islands already have equipped for ATN in a recent, radical ATM upgrade. Many U.S. ground stations, in addition to those serving the Miami ARTCC, have equipped for ATN VDL-2, while Japan and Brazil are implementing the infrastructure.

In the surveillance category, avionics equipment vendors have begun to produce transponders capable of data link communications. In the future, transponders configured for ATN compatibility are likely to become the rule rather than the exception.

A Look Ahead

Beyond the near-term delivery of data link-enabled CM, CPDLC, AOC and ADS functions, ATN should have a broader future, since other aircraft subnetworks are likely to come into its portfolio. Whyman believes that eventually–at least a decade from now–an improved, second-generation ATN may be needed. One radical improvement, for example, might be to transition from OSI to an Internet protocol (IP)-based architecture.

A reason for preferring OSI originally was that the IP standard at the time, IPv4, was considered inadequate for ATN. However, the more recent, enhanced IPv6 standard overcomes many earlier objections, making it a potential candidate for any future revision of the ATN spec. Since IP is an open, freely available, industry-driven standard, it could reduce costs by enabling the selection of commercial off-the-shelf (COTS) equipment.

Whyman suggests, however, that the cost benefits may be marginal. Even if most air transport aircraft become ATN-equipped, he says, the economies of scale would hardly compare with the numbers that are everyday fare in the personal computer industry, which has clearly profited from open standards. ATC centers might gain little, since the cost of ATN equipment content would be a mere fraction of total cost.

Rather, argues Whyman, the benefit may be felt at the application level via improved communications and faster response rates. An enhanced ATN could be a natural complement to an improved VHF infrastructure such as VDL-4.

Still further ahead, broadband, satellite-based systems such as Connexion by Boeing could change the landscape, imposing their own style of unification on data com diversity. Meantime, ATN remains the best hope for avoiding communications anarchy.

At A Glance

Expectations are high for the aeronautical telecommunications network (ATN) as an overarching framework. ATN avionics systems exist, and Europe is moving ahead with its ATN-compliant Link 2000+ program. But can ATN harmonize all of the following systems?