ADS-B: Will It Be Filled In?
For all its potential as an air traffic management and pilot awareness tool, many questions still surround ADS-B.
Adrian Gerold
At A Glance: Automatic dependent surveillance-broadcast (ADS-B) is poised to expand in the National Airspace System and overseas. This article discusses: The status of the U.S. network, ADS-B in Alaska and the Lower 48, ADS-B's future--U.S. and worldwide.
Automatic dependent surveillance-broadcast (ADS-B) promises to bring new levels of safety and operational efficiency to aviation. Unlike some other efforts, it appears to be an FAA program that all users want. In September 2005, FAA's top-level Joint Resources Council (JRC) called for a study of the estimated cost of a nationwide, GPS-based, ADS-B network. JRC stated that, depending on the study's findings, a go/no-go decision on National Airspace System (NAS) implementation could be made this July. Industry observers suggest that, should ADS-B get a green light, installations could begin in late 2007 or early 2008 and continue over a four- to six-year period. Recently, the JRC announced that its decision would be brought forward to June 2006. The aviation community is holding its breath. The JRC initiative stemmed from a comprehensive agency review of three future air traffic control (ATC) surveillance options--secondary radar, multilateration and ADS-B. FAA officials considered radar to be the least preferred option: Not only are many of the agency's high-cost secondary surveillance radars due for replacement in the next few years, but continuing with them would merely extend the status quo and bring little technological advancement to the NAS. The multilateration technique, which uses ground-based monitoring receivers to tri-angulate aircraft ATC transponder, traffic alert collision avoidance system (TCAS), ADS-B and military identification-friend or foe (IFF) signals, is equivalent to radar in range but is much less costly, has a higher update rate and provides much better accuracy. But ADS-B's advantage is that it offers multilateration's performance while adding in-flight traffic awareness for pilots plus cockpit displays of key uplinked flight information, which neither of the alternatives provides. And for the chronically underfunded FAA, the transition to ADS-B, coupled with the eventual decommissioning of secondary radar and multilateration systems, offers long-term cost savings well in excess of $4 billion, according to an analysis for the agency's Air Traffic Organization (ATO). What is ADS-B? Called by some the next-generation ATC transponder, ADS-B units radiate their aircraft's type, identification, GPS position, altitude, heading, speed, intent (i.e., climbing, descending or level) and other data. Unlike conventional transponders, however, which transmit just identification and altitude--and only when interrogated by an ATC secondary radar or a nearby TCAS-equipped aircraft--ADS-B transceivers continuously transmit signal bursts once per second. These bursts are received by all other ADS-B aircraft within reception range and the data is presented on cockpit displays. But it is ADS-B's unmanned, automatic ground stations--the focus of the JRC initiative--that are the linchpins of the system. These ground-based transceivers (GBTs), spread out up to 100 miles (161 km) apart across an operational area, are data-linked with participating aircraft and, separately, with nearby air route traffic control centers (ARTCCs). The GBTs receive the aircraft transponder signals and instantly retransmit them to the nearest ARTCC for presentation on controller scopes. In the opposite direction, the ARTCC sends the positions of non-ADS-B traffic, plus weather, notices to airmen (NOTAMs) and other critical flight data, back to the GBTs for retransmission to aircraft in their areas. In ADS-B terminology, the uplinked traffic data is called the traffic information service-broadcast (TIS-B), while uplinked weather and other data is called the flight information service-broadcast (FIS-B). The traffic uplink is important, since without it, non-ADS-B aircraft would not appear on a pilot's cockpit displays--a critical deficiency until all aircraft are equipped. The system today can be seen in action at the Anchorage, Alaska, ARTCC, where controllers routinely use aircraft data retransmitted from GBTs hundreds of miles away--at FAA's Capstone ADS-B evaluation site in Bethel, Alaska--to monitor traffic well below the ARTCC's radar coverage. In turn, the ARTCC sends back key, but otherwise unobtainable, information to the GBTs to be uplinked to the pilots of those aircraft. It is this overall capability and its attendant safety benefits, combined with its lower cost, which makes ADS-B such a strong candidate for the future surveillance and navigation requirements of the NAS. The International Civil Aviation Organization (ICAO) has endorsed ADS-B as offering major safety and airspace capacity benefits worldwide, and several programs are active or planned in Australia, Europe, Asia and Africa. These followed FAA's pioneering Capstone initiative around Bethel and Juneau, Alaska, where, since 2000, more than 350 smaller commercial aircraft have been equipped. The success of the Capstone project has led to the proposal to extend the system throughout Alaska, which is known for its forbidding terrain, poor communications and sparse navigation facilities. The project has been so successful, in fact, that Alaskan aircraft equipped with ADS-B transceivers--which incorporate GPS wide area augmentation system (WAAS) technology--have special FAA approval to use GPS as the "sole means" of navigation within the coverage of the Bethel and Juneau GBTs. This allows the aircraft to fly below the regular minimum en-route altitudes laid down by FAA. It is an extremely important safety benefit in Alaska since it allows Capstone participants to fly below icing levels. "ADS-B and WAAS allow more people to fly more safely in IFR [instrument flight rules conditions]: it's that simple," says Sam Seery, marketing manager at Garmin AT in Salem, Ore. But ADS-B is not restricted to small aircraft. UPS flies 107 Boeing 757 and 767 freighters fitted with full ADS-B installations, according to Capt. Jim Walton, UPS supervisor of advanced flight operations. And 80 to 90 percent of the remainder of the company's 240-aircraft fleet carries partial installations, he says. Walton regards the JRC initiative as "very encouraging," not only for surveillance but also for its potential to support new applications such as continuous descent arrival procedures and, further into the future, to allow safe, "assisted visual separation" in marginally VFR (visual flight rules) approach conditions. UPS is considering an ADS-B option from Phoenix-based ACSS for monitoring airport surface movement. FAA has installed GBTs in the Ohio Valley (used by UPS), North Dakota (used by the University of North Dakota), and Arizona and Florida (both used by Embry-Riddle Aeronautical University). There is also a string of stations--some funded by individual states--along the East Coast, as well as the Safeguard ADS-B security cordon around Washington, D.C. FAA is studying how these stations would be integrated into the nationwide infrastructure of as many as 550 GBTs now under consideration. For flight school aircraft, ADS-B provides trainees with clear awareness of other traffic in busy exercise areas and allows ground staff to monitor all aircraft movements. All data is recorded, enabling instructors to conduct postflight reviews with solo students, a valuable but previously unachievable capability. And at Embry-Riddle's air traffic management research facility at Daytona Beach, Fla., research associate Ian Wilson points out that a nationwide ADS-B system would allow monitoring of all aircraft movements in the NAS--a clear safety and traffic management benefit. "In our laboratory today," Wilson says, "we can watch aircraft start up, taxi out, take off and fly within the present U.S. GBT coverage." Wilson also notes the value to future airspace managers of having downlinked trajectory, intent and other data for flight plan conformance monitoring. The Aircraft Owners and Pilots Association (AOPA) has strongly endorsed the move to ADS-B. Technical Vice President Andy Cebula raises the thought that the system could prevent inadvertent breaches of small areas of restricted airspace which briefly appear and disappear around the United States, as security demands. From discussions such as these, it would appear that the only thing which ADS-B can't do is replace the close-in collision avoidance commands of TCAS, although it does, to a degree, provide traffic advisories. The question sometimes arises about what happens when aircraft are out of range of the GBTs. They would, of course, lose contact with the stations and no longer receive uplinked TIS-B and FIS-B information. But since their once-per-second ADS-B transmissions will continue, they still will be safely seen by other ADS-B-equipped aircraft, and vice versa. However, tests flown by the Capstone group and the University of Alaska in 2004 demonstrated the potential of using a satellite link to maintain communications with the GBTs when beyond reception range, or when, as is more often the case in Alaska, the aircraft is deep in a valley and its signals are blocked by intervening mountains. The tests used a small, lightweight Iridium satellite communications transceiver. The evaluation was deemed very successful, but it is understood that a detailed report submitted to FAA in mid-2005 has yet to be acted on. To industry observers this is curious since, as ADS-B use expands internationally, a satcom link would appear to offer significant ATC benefits over remote and oceanic areas of the world where GBTs cannot be installed. Two interesting overseas programs also should be mentioned. Airservices Australia is nearing completion of its country-wide ADS-B network, intending eventually to replace its ATC secondary radars. But to ensure that all targets are positively tracked at all times by ATC and other aircraft, Airservices is reviewing bids for the supply of 1,500 small, low-cost units, which it may subsidize, or even give, to general aviation operators. In China, meanwhile, following successful early trials, the national Civil Aviation Flight University in Guanghan, Sichuan Province, plans to install six GBTs and 160 avionics packages during 2006. China's Civil Aviation Authority is reported to be evaluating the system for broader deployment. Frequency Issue Despite ADS-B's impressive advances, a number of issues remain unresolved. The domestic and international move towards ADS-B, for example, raises the issue of the international standardization of the data link radio frequencies used between GBTs and aircraft. Three separate but incompatible frequencies are in early GBT use. These are the Swedish VHF-based Mode 4 (known as VHF digital link, or VDL-4), the U.S.-developed, 978-MHz universal access transceiver (UAT) and the international, 1090-MHz, Mode S ATC transponder. Industry specialists say that although Sweden and Russia plan to adopt VDL-4, it probably will remain a regional system. The same is true for UAT, which was designed, at FAA's direction, as a small, low-cost avionics solution for Capstone and U.S. general aviation aircraft. Mode S, on the other hand, is used worldwide, and its current transponders already can transmit ADS-B's "extended squitter" data bursts or can be readily upgraded to do so. These transponders are described colloquially as 1090ES or, more simply, 1090. UPS, therefore, along with almost all other U.S. and international airlines, has adopted the 1090 option, which seems set to become the de facto world standard. Accordingly, future U.S. GBTs will be dual-frequency units, simultaneously uplinking both 1090 and UAT data. Interestingly, 1090 Mode S cannot accommodate FIS-B's data link demands, while the low-cost UAT units can. This is not a major drawback for 1090 users, however, since most are air carriers with strong dispatch support for weather and other information. Like UPS many operators have opted for partial installations, called "ADS-B Out," where their 1090ES transmissions simply allow enhanced ATC tracking. This avoids flightdeck retrofit costs in older aircraft, which may not be in service when the full ADS-B environment is in place, although some operators are considering electronic flight bag installations to display uplinked TIS-B data. Go-Ahead Decision While the JRC decision is not likely before June, most observers expect an affirmative outcome. Aside from the increasing international adoption of ADS-B, there appears to be wide support for it in the United States. A senior FAA official states: "The response we're seeing is unprecedented. I don't remember any previous system we've proposed getting this level of user agreement." Jeff Brabender, president of Coherent Solutions in Salt Lake City, echoes this view. "The aviation industry has had difficulties with new technologies because so few offered industry-wide value and thereby won industry-wide support," he says. "ADS-B will succeed because it offers benefits across the whole community. Other technologies, like WAAS, LAAS [local area augmentation system] and digital VHF data link, have struggled to take hold because they simply didn't provide benefits to as wide an audience." Brabender has been involved with ADS-B development and testing for 10 years. Nevertheless, some concern has been expressed in the user and manufacturer community about the recent repositioning of FAA's ADS-B program office into the ATO's en-route branch, where its terminal and airport applications could risk being sidelined and its emphasis on FIS-B and its weather uplink could be lessened. For most general aviation and Gulf of Mexico helicopter operators, FIS-B weather is a major incentive--and in the Gulf of Mexico, the main incentive--to equip with ADS-B. Indeed, some industry experts say that implementation planners who ignore those groups could alienate a significant number of users. The JRC decision will, of course, be based on detailed cost/benefit analyses. And those costs present a major problem to FAA. But a consortium of major industry suppliers, working together as "Project Mercury," is understood to have raised an intriguing possibility. Industry sources suggest that the Mercury team may have proposed to FAA that it could undertake the nationwide implementation of ADS-B and then offer it to the agency under a turnkey arrangement, thereby relieving FAA of a very large capital investment. While unconfirmed, the concept is thought to be part of a larger, closely guarded, Mercury proposal, which also would include provision of a nationwide controller pilot data link communications (CPDLC) network, together with transition to a digital communications environment. Both of these have been on FAA's wish list but were considered unaffordable. Future teaming of major industry players for specific, multibillion-dollar, government programs under the newly minted "co-opetition" philosophy was announced at the annual convention of the Air Traffic Control Association in September 2005. Industry insiders feel that ADS-B could provide an excellent launching pad for the concept. Please enable JavaScript to view the comments powered by Disqus.