The wheels move slowly in the air traffic control (ATC) world: the current radar-based surveillance infrastructure used in the National Airspace System (NAS) has been in place for some 50 years. That's why the FAA administrator's announcement in May of plans to replace up to one-third of the agency's secondary surveillance radars was so electric (see story, page 40). As expected, the agency's senior-level Joint Resources Council endorsed the approach, approving the first steps as this issue went to press.
The chosen technology is ADS-B, automatic dependent surveillance-broadcast. FAA intends to provide coverage in the continental United States and the Gulf of Mexico through a network of 400 ADS-B ground stations. Interest groups from the Aircraft Owners and Pilots Association (AOPA) to the Air Transport Association (ATA) have voiced support for the move. The International Civil Aviation Organization (ICAO) likewise has endorsed ADS-B as offering major safety and airspace capacity benefits worldwide, and programs are active or planned in Australia, Europe, Asia and Africa.
Onboard ADS-B equipment broadcasts an aircraft's identification, GPS position, altitude, heading, speed and intent once per second to other aircraft and ground stations that can forward the information to controllers' screens ("ADS-B Out"). Aircraft can receive traffic information from ground stations or other aircraft for display in the cockpit ("ADS-B In"). Overall the technology promises lower-cost, more precise ATC surveillance. Beyond that, there would be shared air-ground and air-air situational awareness and further delegation of responsibility for separation from the ground to the air. Shared separation management between aircraft will be key to expanding airspace capacity. And, as the system matures, the possibility exists to exchange much more information--including 4D flight paths--between aircraft, as well as with a ground-based ATC infrastructure, enabling air-to-air strategic conflict avoidance and ground-to-air profile management to optimize runway capacity.
An ADS-B Out notice of proposed rulemaking is expected for FY07 or 08, with a final rule in FY10. FAA hopes to have virtually all U.S. aircraft equipped by 2020. Employing a fairly limited set of data, ADS-B Out will support the same types of operations that are possible in today's radar environment. ADS-B In, with advanced data links, will support applications such as air-to-air separation and 4D trajectory management.
While the vision is powerful, more needs to be done. ADS-B will require a backup because it depends on GPS. There's also the question of Mode S frequency congestion, as progressively greater numbers of aircraft pump out Mode S-coupled ADS-B signals in high-traffic areas.
Another issue is how to navigate safely from today's radar environment--through a long period in a mixed radar/ADS-B environment--to an ADS-B-intensive environment. This issue resurfaced recently in Alaska (after years of success) and has been given a temporary fix. But more must be done at the technical level before the system goes live nationwide.
While FAA will work out the details of how to get the ground network built, thought needs to be given about how to stimulate early adoption of onboard equipment, especially ADS-B In.
To fully benefit from the efficiencies of the technology, operators need to adopt ADS-B In. But they won't do so until they see a significant operational benefit, such as reducing separation requirements to take advantage of ADS-B's increased temporal and spatial precision. The FAA's decision to go ahead with ADS-B is an historic opportunity for the agency to chart a long-term course from a human-centric airspace management infrastructure to a more automated and efficient infrastructure. It will be interesting to see how far down that path the agency goes.