Serious questions about ADS-B NPRM arise

 - October 29, 2007, 5:43 AM

While the FAA in early October released its Notice of Proposed Rule Making (NPRM) covering future mandatory carriage of ADS-B avionics, serious questions have already been raised about it. Generally, it was expected that the NPRM would cover the whole spectrum of ADS-B applications and would provide clear guidance to corporate operators anxious to benefit from the system’s full capabilities. Instead, the document focuses on the  system’s “lower end” application, where the aircraft would appear only on ATC radars and on the cockpit displays of other, better equipped, airplanes.

Concerns have also been expressed about equipage costs versus benefits; the length of the transition to more advanced applications; the true ownership, control and certification of the system; and its overall security once operational.

The NPRM covers the performance requirements for ADS-B out installations, with Jan. 1, 2020, to be the start date for mandatory carriage of that class of equipment. ADS-B out is similar to, but does not replace, today’s transponders, which will still be required. Instead of the current units’ responses to ATC secondary surveillance radar (SSR) interrogations, ADS-B signals are independently transmitted once per second omnidirectionally by the onboard avionics, normally via upper and lower antennas.

SSR and ADS-B signals are displayed in a similar fashion on air traffic controllers’ screens, but the ADS-B positions, derived from the required GPS/WAAS, are more accurate than those of SSR, particularly at increasing ranges from the radar. But like SSR, ADS-B out provides pilots with few, if any, direct benefits, other than in areas such as the Gulf of Mexico, which lacks significant SSR coverage.

The proposed rule would require ADS-B out equipage after 2020 in all aircraft operating in Class A, B and C airspace over the 48 contiguous states, plus Class E airspace above 10,000 feet msl. Carriage would also be mandatory up to 10,000 feet within 30 nm of FAA-specified busy airports. ADS-B out would also be required in Class E airspace out to 12 nm from the coastline over the Gulf of Mexico, at or above 3,000 feet.

Above FL240, the equipment must use the international 1090 MHz Extended Squitter (ES) datalink, while below FL240, operators may use 1090ES or the general aviation 978 MHz Universal Access Transceiver (UAT) link. ADS-B ground stations will translate signals received in either format and retransmit them in the other, to ensure commonality.

ADS-B is often portrayed as a system in which all aircraft would have a cockpit display of traffic information (CDTI) showing the identifiers, flight paths, altitudes and intent (climbing, descending or remaining level) of all traffic of interest plus, in some installations, weather and other flight data. ADS-B out does not provide these benefits.

Such information is available only with the more advanced–and more costly–ADS-B in variant, sometimes called “full ADS-B,” which the NPRM does not cover, other than to point out that some of its specific attributes are still being investigated. No mandatory ADS-B in date has been set, other than it will be beyond 2020. The NPRM provides no cost estimates for ADS-B in.

The FAA does, however, encourage operators to voluntarily equip with ADS-B in units. It points out that this would be beneficial in providing full traffic situational awareness–for example, in displaying the locations of ADS-B out aircraft–and it would be especially useful for UAT users, who could then receive the system’s weather and AIS broadcasts. (Weather and AIS are not available to 1090ES users because of the frequency’s bandwidth limitations.) The FAA also suggests that some operators could opt to install both 1090ES and UAT systems.

The NPRM includes a review of future benefits of ADS-B out and in and, unquestionably, the system potentially offers significant benefits to all users. Some might be less evident, such as the increased positional accuracy that could eventually lead to closer separations and increased airspace capacity, while others, such as enhanced situational awareness of proximate traffic and cockpit weather data, are obvious advantages.

Farther ahead are more advanced ADS-B in applications such as surface traffic awareness of taxiing aircraft and ADS-B-equipped vehicles, as well as enhanced visual approaches, where a CDTI would maintain “visual” contact with other traffic in low-visibility conditions. Eventually, the system is intended to support true airborne conflict resolution.

The FAA’s Aviation Rulemaking Committee (ARC), which assessed the FAA program shortly before the NPRM was issued, reviewed these and other applications extensively. (See story on page 78.) The ARC also discussed equipage incentives that it believes are essential to implementation.

Concerns about Implementation
Given the apparent limited benefits versus the costs of ADS-B out, many operators are understandably cautious about early equipment investments. Many pilots AIN interviewed said, “It’s obviously good for the controllers, but it does little for me.” Others felt that in the ensuing 13 years before mandatory compliance, technology advances would lower equipment prices and, possibly, could make units purchased before then obsolete.

Predictably, some pilots were skeptical about whether the FAA would follow through with its ADS-B plan. “I remember MLS,” said one. One industry observer cautioned that the avionics specifications in the NPRM could change with time, possibly requiring equipment upgrades. This might occur should there be a changeover from GPS/WAAS to GPS/Galileo, for example.

Conversely, one pilot posted this response on the Web site set up for the 90-day NPRM comment period, “The 2020 mandate is not aggressive enough and will not support the benefits provided by the NextGen ATC system in a timely manner. [A deadline of] 2015 would provide the National Airspace System with greater benefits in a far more timely fashion. What’s more, the proposal does not cover ADS-B in, which is vital to the successful implementation of NextGen and securing its full benefit. ADS-B in must be addressed, within the 2020 time frame.”

Following mid-October testimony to the House Aviation Committee by the FAA and other interested parties, several Congressmen questioned the ownership and control of the nationwide ADS-B infrastructure. This will be designed, built, installed and then owned, operated and maintained exclusively by ITT and its several private company team members, with the FAA paying an annual subscription fee for the service. The agency will be responsible only for monitoring its performance quality. While the FAA testified that the contract included a large number of checks and balances, informed sources report that the legislators might not have been entirely convinced.

Security Concerns Raised
Another concerned group was the Professional Airways Systems Specialists (Pass), composed of FAA ATC equipment engineers and technicians, which testified that changes in FAA regulations shortly before the award of the ITT contract actually prohibit them from inspecting and certifying the ADS-B ground stations, a move they described as unprecedented in the history of the NAS.

DOT Inspector General Calvin Scovel detailed several risk areas and stated that the program will require “an extraordinary level of oversight” to assess the potential security vulnerabilities of using ADS-B for managing air traffic. Noting that FAA documents identified “several specific concerns,” Scovel said, “We believe a full discussion of ADS-B security and potential vulnerabilities is inappropriate in an open forum.” He added, “The FAA needs to continue to work with the intelligence community and the Departments of Defense and Homeland Security” on these issues.

Honeywell raised this question in its presentation at an FAA ADS-B Industry Day in August 2006. An ICAO specialist panel echoed the sentiment more recently. However, AIN understands that before the IG’s Congressional testimony, the importance of system security might have been downplayed within the agency.
ADS-B is clearly a system with great potential, and will be the critical foundation for NextGen. But important issues must be resolved before it achieves full operational readiness.

The NPRM is available at

The DOT IG testimony is available at