FAA Air Traffic Organization (ATO) officials early last month briefed government and industry representatives on the cost-benefit analysis of nationwide ADS-B implementation, as requested by the agency’s top-level Joint Resources Council (JRC). The ATO is scheduled to submit the analysis to the JRC in early June. The briefing covered most of the program management issues involved, but the agency’s perception of user priorities created concern among attendees.

ADS-B will be a major FAA program, affecting future communications, navigation and surveillance over the next 20 or more years, with an estimated implementation cost of well over $1 billion, plus continuing operations and maintenance expense.

Unlike, say, radar or VOR–which offer the same service to all users–ADS-B can be thought of as offering a menu of services to different members of the aviation community–members who all have their own ideas about which services should receive implementation priority. Since the system’s introduction can’t be a “big bang” event, that menu complicates the question of who gets first dibs on the services. It’s here that community opinions have arisen, since the FAA’s priorities appeared to put the airlines–which, other than freight carrier UPS, have not been pushing for ADS-B–ahead of groups such as AOPA and the Gulf helicopter operators, which want ADS-B.

In ADS-B, each aircraft broadcasts its ident, altitude, speed and intent on a common frequency every second, and this is received by all other aircraft within reception range and by ground stations that pass the information to local ARTCCs.

But the system uses two quite different, and incompatible, datalinks to handle these messages. Airliners and other large aircraft use their mode-S transponders, most of which can send out “extended squitter” ADS-B signals–sometimes called 1090ES, after their 1090 MHz mode-S transmissions– while smaller, mainly general aviation aircraft use a much lower-cost FAA-developed concept called the universal access transponder (UAT). Currently, several hundred smaller airplanes use UAT as part of the FAA’s Alaska Capstone project.

Dual Priorities

But there are major differences between the 1090ES and the UAT. Aircraft UAT units have a two-way datalink, broadcasting signals and receiving incoming signals from other aircraft, which allows crews with UAT cockpit displays to see traffic around them.

The airline 1090ES transponders cannot do this. They can transmit only their own aircraft’s data–hence the term “ADS-B OUT”–and, since their transponders receive only 1030 MHz ATC radar interrogations, they are unable to receive position data from other aircraft. In addition, they cannot receive uplinked weather or ATC alerts of non-ADS-B traffic.

To obtain that information would require investing in UAT avionics, which isn’t an option for many cash-strapped airlines. Besides, one FAA insider told AIN, TCAS already gives crews knowledge of local traffic, and airline pilots have excellent weather uplinks from their dispatch centers. The FAA source suggested that the ATO’s apparent early prioritization of airline ADS-B OUT was primarily to provide additional information to ATC, rather than promptly moving to provide the service to the broader user community.

A small group of government and industry experts has developed recommendations under the auspices of RTCA, and a source told AIN that the FAA’s plan does accommodate the group’s recommendations. Bob Lamond, NBAA director of air traffic services and a member of the RTCA group, put things best into focus, expressing his organization’s support for ADS-B, while emphasizing the importance of continuing dialog on the topic.