The Federal Aviation Administration (FAA) program for a nationwide network of automatic dependent surveillance-broadcast (ADS-B) ground stations was formally launched by Administrator Marion Blakey in early May. Since that time, agency officials have been providing prospective suppliers and the user community with more details about the program at public industry days and other forums. A schedule of activities has been established, with a screening information request planned for November 2006, followed by a request for offer to acceptable suppliers in March 2007, and a contract award planned for July 2007.
Between 400 and 500 ground stations are expected to be needed for coverage in the continental United States. The first stations would be commissioned in late 2009 and the nationwide network would be complete by 2012. At an industry day briefing, officials spelled out numerous intermediate steps, a process not typically undertaken in earlier major FAA procurements. And ADS-B is certainly a major project. A cornerstone of the FAA’s Next Generation Air Transportation System (NGATS), ADS-B is unofficially estimated to cost close to $2 billion, including 15 years of operational and maintenance support.
Yet several concerns have emerged, and not all of them have been entirely allayed.
Spoofing
The first of these concerns is the risk of "spoofing" by mischievous hackers-or others with more serious intent-which could potentially produce as many as 50 false ADS-B targets on an air traffic controller’s radar screen. First word of this risk came in April from Australia, which had launched its own nationwide ADS-B project in 2005. The alarm was raised by Dick Smith, a millionaire businessman who became famous for setting several dramatic solo helicopter records, including an around-the-world flight where he made mid-ocean refueling stops aboard freighters. An experienced pilot, he was formerly the head of Australia’s Civil Aviation Authority and, subsequently served as chair of the country’s aviation safety board. Smith stated that ADS-B spoofing could be readily achieved with a laptop computer, a general aviation ADS-B transceiver, and a $5 antenna.
ADS-B experts in the United States cautiously confirmed that such was, in fact, the case, and that Smith’s spoofing configuration had been tested and did indeed have that effect, although there was skepticism about the numbers of false targets which could be created. The individuals interviewed, however, were reluctant to discuss the issue in any depth, or any likely countermeasures to overcome it, probably because in next year’s contract negotiations, bidders for the ground stations will have to demonstrate to FAA their systems’ resistance to spoofing and all other types of interference. As FAA informed Avionics, "[It] is aware of potential vulnerabilities in building any new system, including ADS-B. That is why we conduct a preliminary hazard analysis prior to the commencement of designing and building a new system. As part of that process we also develop mitigation strategies in the design phase. There will be a requirement to address these issues in any request for proposal that goes out on ADS-B."
During Avionics’ visit earlier this year to ADS-B ground station manufacturer Sensis Corp., Syracuse, N.Y., one anti-spoofing countermeasures technique was demonstrated, using a setup similar to that described by Smith. Here, an unknown, but potentially false, target was moving across a simulated controller’s screen. In fact, it was a real target with signals transmitted from a general aviation transponder that had been modified with encrypted software. But it remained a potential false target until a company engineer inserted a personal password into the transponder control unit, effectively completing the security loop, at which point the target’s shape and color changed to show it was legitimate. But we were not necessarily seeing the technology which the company would demonstrate to FAA as its solution. A Sensis spokesman noted that "it was just one of a number of possible spoofing countermeasures."
Ins and Outs
Every second, an ADS-B transceiver sends out its identification, position, altitude and other data. This information, called "ADS-B Out," is received by all other aircraft within reception range and also by the local ground station. The ground station retransmits the information to the nearest air route traffic control center (ARTCC), where the aircraft then appears on controllers’ radar displays.
The ARTCC also can send weather maps and forecast data back to the ground station for uplinking to local aircraft plus, when necessary, the positions of non-ADS-B traffic in the vicinity, which an ADS-B-equipped aircraft would not otherwise see. Urgent NOTAMs and other data also can be transmitted by the ground stations. This uplinked information is called "ADS-B In." In official language, the uplinked weather and other data are known as flight information service-broadcasts (FIS-B), while the traffic advisories are traffic information service-broadcasts (TIS-B).
When FAA’s nationwide implementation plan began to emerge in 2005, it also started to appear that agency planners regarded the ADS-B Out function as the system’s most important benefit, and that uplinking of weather data, proximate traffic information and other data was a secondary feature, to be incorporated at some later date. From an air traffic control (ATC) point of view, this may have seemed a reasonable assessment, since ATC’s prime customers were airliners, most of which already received weather and other flight data over company data links and which also carried traffic avoidance systems to alert pilots of potential conflicts.
But this viewpoint overlooked general and corporate aviation, smaller regional air carriers and others, most of whom did not enjoy those benefits. Aircraft Owners and Pilots Association (AOPA) Vice President Andy Cebula told FAA officials at the industry day for users that to general aviation, "ADS-B is FIS-B and TIS-B," since otherwise it offers nothing more than is available today. AOPA members, he said, are not interested in simply buying a more advanced transponder in disguise. NBAA Director of Air Traffic Services Bob Lamond stated that "Most of the ADS-B Out benefits go to the FAA. But most of the operator benefits come with ADS-B In, and those must be achievable concurrent with equipage."
The overall consensus from the non-airline community has therefore been that without the coincident availability of FIS-B and TIS-B services from the nationwide ground station network, there will be little reason to invest in ADS-B avionics. This now appears to be well understood by FAA officials, whose current implementation schedule shows FIS-B and TIS-B marching in step with the ground stations. But after the briefings, one general aviation representative stated that from his past experience of major FAA initiatives, "It sounds good, but I’m from Missouri [the Show-Me state]."
Mandate Question
There is nevertheless general agreement within the user and regulatory communities that when fully implemented nationwide, ADS-B should be carried by all aircraft, both commercial and private, and in both instrument flight rules and visual flight rules conditions, if its optimum safety and traffic management benefits are to be realized. Inevitably, this will call for equipment mandates. Equally inevitably, users are wary of what FAA may have in mind. The agency plans to issue a draft notice of proposed rule making (NPRM) in September 2007 for industry comment, and expects to publish its final rule in November 2009.
But some user organizations already have staked out their territory. While NBAA is noncommittal on the subject, other than linking it with the full availability of benefits, AOPA has stated that to relieve the economic burden on its members by allowing attrition of current equipment, and recognizing the gradual transition of the general aviation fleet to any new system, the installation mandate should not be imposed until eight to 10 years after the nationwide ground station network is declared fully operational, including TIS-B and FIS-B.
Regional Airlines Association (RAA) Vice President Scott Foose is more concerned about the current lack of clear benefits to his members vs. the required investment. Regional operators, he says, must be able to present a convincing business case to company management before major expenditures could be authorized. While FAA is very aware of these concerns, ADS-B Program Manager Vincent Capezzuto cautions that "if NAS users demonstrate active opposition to avionics related-mandates, there may be delays in required rulemaking activities and/or the program may experience a reduction in benefits."
Affordable Avionics
The cost of ADS-B equipment raises the classic issue of supply and demand. While avionics manufacturers such as Garmin, Honeywell and Rockwell Collins have successfully built and certified system transceivers, these have not reached the production levels necessary to see quantity price reductions, as was the case with, say, GPS. Conversely, the potentially very large user community is holding off buying equipment until prices fall. AOPA’s Cebula has stated that the equipment is simply "too expensive," while NBAA’s Lamond has called for units that are "affordable and made sense to OEMs [original equipment manufacturers] and operators."
This is a difficult situation to resolve, and some observers feel there may not be much movement until the implementation of new ground stations gets under way in 2009. Yet it could take much longer. While specific numbers are not available, one source says that there is only "a modest degree" of independent operator usage of the over 25 ADS-B stations previously installed along the U.S. east coast. The exceptions appear to be the flight training schools operated by Embry Riddle at Daytona Beach, Fla. and Prescott, Ariz., and the University of North Dakota, Grand Forks, N.D., where a high proportion of aircraft are ADS-B equipped.
Capuzetto told attendees at one industry day that "if avionics suppliers are unwilling to accept investment risks, and/or unable to ramp up ADS-B equipment production, certification and installation, then the avionics ‘industrial base’ will lag user demand for ADS-B capabilities and make it difficult to meet the requirements of airspace mandates, and there will be a delay in benefits accrued to the program."
The airlines have relatively low-cost access to ADS-B since almost all current production Mode S transponders include the system’s extended squitter, or ES, signals in their response to secondary radar interrogations, and some earlier models can be readily upgraded to include them.
What If GPS Fails?
In the future ADS-B environment, FAA will, for the most part, place sole reliance on GPS for both the navigation of each aircraft and for ATC surveillance. But agency officials recognize that GPS is vulnerable to interference, as well as to satellite outages and, of course, onboard equipment failures. An independent backup system will therefore be essential.
FAA is reviewing candidate systems, although its first classification of these has raised eyebrows. The agency’s top three choices are, respectively, secondary radar, primary radar and multilateration (MLAT). Unfortunately, while all three would provide an ATC controller with an aircraft’s location, none could show the pilot where he is. Some feel that this is, once more, a reflection of the agency’s stress on the lessened importance of the aircraft operator’s needs. FAA’s second tier of GPS backup systems includes IRS/DME/DME or DME/DME, neither of which is common to smaller aircraft, as well as Enhanced Loran (e-Loran) and, curiously, "satellite navigation." Some of the most knowledgeable in the GPS community consider emerging e-Loran capability to be the logical GPS backup-with its potential to support position, navigation and timing, as well as RNAV needs. But the still to be matured e-Loran infrastructure lacks government support (outside of Congress), as well as avionics standards. It is not in FAA’s navigation evolution roadmap, says Daniel Salvano, FAA’s director of navigation services, who terms it a "tertiary system that does not have a business case from a navigation perspective."
A third tier listing unacceptable backups includes inertial reference systems only-presumably because of its uncorrected drift characteristics-VOR/DME, and two oddities not normally considered navigation systems: ILS localizer/DME and even more curious, microwave landing system/area navigation (MLS/RNAV).
ADS-B and Multilateration
Multilateration is sometimes used as a supplement to ADS-B, but the two systems are entirely different. MLAT is more comparable to secondary radar. The radar antenna’s beam continuously sweeps 360 degrees through the surrounding airspace, interrogating each transponder and determining the relative bearing and distance of each aircraft. With MLAT, on the other hand, four or more small, widely spaced ground stations simply listen for transponder responses within the same area and triangulate aircraft positions. An MLAT network is much cheaper in both acquisition and installation than a secondary radar, and reportedly significantly cheaper to maintain. It is more accurate, as well.
The linkage of multilateration with ADS-B frequently occurs in areas outside secondary radar coverage, but where ADS-B and non-ADS-B aircraft operate, thereby ensuring that all aircraft in the area can be accurately tracked. In such locations, the MLAT units can be combined with the ADS-B ground stations in a single, unmanned enclosure. In this configuration, one or more of the MLAT units would send out secondary radar-like interrogations, for triangulation of the conventional transponder returns.
Both Sensis and Rannoch Corp., Alexandria, Va., a separate ADS-B ground station supplier, provide combined ADS-B/MLAT systems. Mongolia and Taiwan installed such combinations built by Rannoch, while Sensis is currently installing a combined system in Tasmania. MLAT is also quite widely used as a standalone airport surveillance system, such as in FAA’s ADSE-X program, and in wider area applications, such as at Innsbruck, Austria, where a Sensis system provides more effective surveillance coverage than would a secondary radar. MLAT also could be a bridge to the ultimate ADS-B infrastructure, as multilateration tracks any airplane sending out a transponder return. It does not require ADS-B or even GPS equipage.
As the ADS-B program moves on, it seems certain that most of the user concerns described here will be accommodated. It behooves FAA to ensure that the system fully meets the operational requirements of the entire user community.