The Nexcom Nexus

The US and Europe need to harmonize their separate visions for resolving increasing voice and frequency congestion

By John Croft
Air Transport World, March 2005, p.42     Buy this issue

Airlines soon may get an inkling as to which radio technology to buy-or not to buy-for their future aircraft.

The enlightenment will commence in a meeting room in Montreal next month when technical experts from FAA, NASA and Eurocontrol unveil to a 30-member ICAO aerocommunications panel their top six or so ideas for what type of communications system will make the most economical and functional sense for global interoperability through 2030.

The experts are tackling the subject as part of the three-year, ICAO-sponsored Future Communications Study. Their goal in the remaining two years of the project is to come up with a single cohesive US-European communications strategy, one that in theory could set in stone the voice and data link technology that carriers will be required to have in their aircraft by 2015. "The idea is to have one system everyone could migrate to," says James Eck,

FAA's chief systems engineer-communication and a member of the team. If successful, Futurecom will close the gap between the disparate solutions the US and Europeans have been designing or implementing to solve their common yet unique problems with rapidly increasing voice and frequency congestion, a more pressing problem in Eastern Europe than in the US. Based on the preliminary deliberations of the Futurecom group, the form and function of the 2015 system is anyone's guess. In a presentation Eck made to FAA management in December, the potential candidates included not only custom narrowband VHF solutions favored by US and European civil aviation authorities but also second- and third-generation cellular phone technologies, IEEE 802 derivatives, specialized mobile radios, satellite communications and others.

While the agreed-upon long-term plan since 1995 had been to adopt the FAA-proffered Nexcom all-digital communications system for voice and data air traffic services by 2000, delays at FAA and an immediate need for relief in Europe put the two aviation authorities on diverging paths. Nexcom promised a fourfold increase in frequency assignments by digitizing voice and data and sending both in the same channels separated only by time.

Though ICAO adopted the Nexcom plan in 1995, FAA did not begin developing the ground and air portions of the network until 1998, in part because its existing communications system had available capacity and also because of more pressing problems with air traffic control modernization. European authorities, meanwhile, were moving ahead with an ICAO-approved interim solution to subdivide the standard 25-kHz separation between VHF voice channels by three, down to 8.33 kHz, a decision that would require some carriers to buy new radios, while others were upgradable. By subdividing, Europe gained about a factor of two increase in the number of frequency assignments available.

Doubts about the grand plan increased as 2000 came and went without a completed Nexcom, and European 8.33-kHz radios became more and more prevalent in international aircraft. Airlines began to question whether there was truly a need to buy yet another new radio for the now-projected 2009 rollout of Nexcom (few converted radios support Nexcom, nor necessarily do new 8.33-kHz radios).

"Europe had backed itself into a corner," says Melvin Reese, director-communication and surveillance for Eurocontrol. "We had converted so many 25-kHz radios to 8.33 kHz that there was no way to back out to a 25-kHz system like Nexcom." Europe required the 8.33-kHz radios for flights above 24,500 ft. starting in 1999 and will continue the rollout next January with 8.33 kHz spacing for flights above 19,500 ft. Reese says about half of the US airline fleet already is equipped with radios that can handle both the 25-kHz and 8.33-kHz spacing.

Efficiency gain Support for Nexcom eroded further once the Europeans began experimenting with addressable controller-pilot data link communications in conjunction with 8.33-kHz radio communication. With CPDLC, pilots and controllers "speak" through canned data messages rather than voice, cutting language-based operational errors and radio usage by as much as 70% for routine transmissions. There also is an efficiency gain. Eurocontrol simulations show that if 75% of the aircraft in a sector are equipped with CPDLC, capacity can be increased by 11%.

Under Eurocontrol's Link 2000+ program, Maastricht upper airspace control began testing CPDLC with a handful of participating aircraft using a high-speed (31.5 kbps) digital technology called VHF Datalink Mode 2 in January 2003. Mode 2 originally was deployed as a high-speed follow-on to ACARS. Under the program, pilots have a set of 28 canned downlink commands and controllers have 66 canned uplinks, including commands like flight-level clearances, speed, heading and rate of climb. The addressable messages are passed back and forth from the aircraft to the Maastricht controllers through ground stations typically used for passing Aeronautical Operational Communications, the most common of which are ACARS messages. ARINC and SITA provide the digital ground infrastructure used for CPDLC as neither FAA nor Eurocontrol yet has a digital-capable ground network.

FAA halted a similar but less comprehensive CPDLC test in Miami with American Airlines in October after two years of testing with as many as 31 aircraft using 12 downlinks and 11 uplinks. Ron Hawkins, staff VP-aviation and air traffic services at ARINC, says the freeze was a combination of FAA budget problems and lack of participation by cash-strapped airlines. Eck says the agency learned what it needed from the trials and will use the knowledge in putting together a nationwide rollout of CPDLC beginning in 2010. Still, some airlines criticized FAA for delaying CPDLC and question the agency's rationale (ATW, 9/03, p. 44).

Favorable reactions from Miami and an earlier Maastricht test in 2001 helped convince IATA member airlines that a switch to Nexcom, though a superior technology, would not be cost-effective. "The airline choice is clear for Mode 2," the organization has stated. Critics of IATA's position argue that Mode 2 will eat further into the remaining spectrum because of the required guard frequencies on either side of the assigned frequency and that the technology will be prone to delayed transmissions when multiple users try to transmit or receive simultaneously.

IATA's position prevailed at an ICAO meeting in late 2003, says Reese, which led to a resolution to maintain the status quo in equipment while launching the Futurecom study. FAA afterward delayed its Nexcom ground station development but kept its ground and airborne radio programs alive.

Airwave congestion The basic problem facing both the US and Europe is the depletion of VHF aerial spectrum from 118 MHz to 137 MHz. In the US, 760 channels are available in the band using 25-kHz spacing; 524 are for ATS and the remainder for AOC, general aviation and special operations like flight testing. Since frequencies can be reused beyond certain distances (VHF signals have a range of about 240 nm at 30,000 ft.), the total number of channel assignments available nationwide is about 14,000 of which about 11,000 were being used at the end of 2003. FAA reports that the average number of new frequency assignments per year since 1974 was 255, which would mean there would be no assignments left by 2014, though officials point out that ATC system changes and the anticipated growth of air traffic will boost frequency assignments "significantly."

In order to make legacy 25-kHz radios last until Futurecom takes over, FAA is investigating every nook and cranny for the extra channels needed for growth, such as new runways, air traffic sectors and, in the future, CPDLC. "Unlike previous activities, this time significantly more aggressive actions needed to be considered," the agency stated in its Radio Spectrum Plan update for 2003 (the 2004 edition is not yet complete). Even the frequencies allotted to Nexcom are fair game: "Such luxuries are no longer possible, since every possible spectrum resource must be marked for near-term use," FAA continued. Currently there are 25 potential sources of additional frequency slots for ATS, including the buffers around the international mayday frequency of 121.5 MHz, two flight check frequencies, law enforcement channels, air show and firefighting frequency assignments and automated weather observation broadcast channels.

FAA estimates it could gain as many as 2,100 slots from the exercise, enough to support growth with today's radios until the replacement system comes online midway through the next decade, assuming that "air traffic system conditions and restrictions are implemented to limit the growth of new frequency assignment requirements." If that doesn't work, the backup plan very well may be to take the European approach, subdividing the 25-kHz slots down to 8.33 kHz and using Mode 2 for data link, a move that would require reequipping at least part of the fleet.

On the AOC side, ARINC in 2000 began its own independent frequency decongestion effort by starting to upgrade its 200 or so worldwide ground stations with Mode 2 capabilities, allowing for digital messaging and CPDLC at rates up to 31.5 kpbs versus the 2.4 kpbs attainable with the legacy analog ACARS system. At the time, the enhancement was crucial as ACARS, the predominant choice for AOC, fast was approaching saturation in 2004. That date now has slipped several years due to the 2001 slowdown. Hawkins says ARINC had upgraded 175 of its ground stations for Mode 2 at the end of 2004 and was seeing messaging traffic running at less than 10% capacity, with about 500 aircraft using the system on a weekly basis. Currently, only a small percentage of the fleet is equipped with Mode 2 for messaging and/or CPDLC.

Despite the Nexcom detour, avionics manufacturer Rockwell Collins is proceeding with its plan to certify a Nexcom radio for Boeing and Airbus aircraft early next year following a successful TSO effort last year. The company's VHF-2100 VDL Mode 3 system, built over the past 2.5 years as part of the Nexcom program and the baseline radio for the 787, is a multimode unit that can operate in the analog VHF spectrum as well as in the digital modes for both ATS and AOC. "It has a lot of room for future growth," says Rockwell Collins Nexcom Program Manager Nancy Guzak. The radio's architecture is flexible enough, she adds, that potential Futurecom permutations-like changing to 8.33-kHz frequency spacing for Mode 3 data link, a suggestion Rockwell Collins made to the Futurecom group-in theory could be accomplished with just a software upload.

That is, unless Futurecom's ultimate answer in 2007 is to move ATS out of the VHF band altogether, a possible outcome, according to Reese. "If we can't find something that fits in the current band, we would consider moving into the L-band (satcom)," he says.

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