Mode S Technology
Air Traffic Control Advisory Committee (ATCAC)In 1967, two fatal collisions of general aviation aircraft and air carrier aircraft occurred; one in Urbana, OH where 26 people were killed, and one in Hendersonville, NC, where 82 people died. These collisions once again raised a flag in the public's awareness that there were problems with the air traffic control system, and helped push the newly formed Department of Transportation (DOT) into forming a committee that would conduct an in-depth study of air traffic control.
OriginsThis committee, the Air Traffic Control Advisory Committee (ATCAC), was created in 1968 and put under the control of Larry Goldmuntz, who was in the office of the Assistant Secretary for research and technology in the DOT. Its purpose was to examine the state of air traffic control, project growth in air traffic, and recommend an ATC system for the 1980's and beyond. The recommendations were to be used as the basis for a new development program.
Larry Goldmuntz began recruiting people from industry, academia, and government agencies to join the committee. These people had strong backgrounds in a variety of areas, including air defense/surveillance, communications, and economics, to join the committee. Among these were Jack Ruina of MIT, who recommended Ben Alexander, ``an outstanding technical person'' who ``also happened to be a private pilot'' to act as chairman - he turned out to be ``a marvelous choice''. The technical staff of 150 were drawn from all segments of the aviation community, the FAA, NASA, and the Department of Defense. The committee also maintained liaisons with various aviation organizations, including the military, NASA, AOPA, and many others, thereby receiving advice on both the technical and policy levels from all affected parties.
ProceedingsIn order to come up with a viable solution, the committee developed a methodology of systems analysis and requirements development. Forecasted user needs defined the requirements, various system alternatives were developed to meet these requirements, and then a preferred system was selected on the basis of cost, technical risk, implementability, and performance.
User needs were determined by talking with people from different parts of the aviation community, ``not just to find out what they have to say but also so they know their thoughts are taken into account in the committee's decisions.'' Because the aviation community includes groups with widely divergent interests, such as technical advisors, pilots, controllers, small airplane owners, airline and airport owners, as well as equipment manufacturers, any proposed change involved a great deal of research. At the same time, the committee studied air traffic and its projected growth and predicted that aviation activity would double by 1980, and then double again by 1995. Correspondingly, demands for ATC service would rise even faster, tripling by 1980 and then again by 1995.
After examining the existing system, the committee determined that ATCRBS as it stood was too limited to handle the predicted increases in traffic. It lacked accuracy and reliability of position data and had limited capabilities. The frequencies they used were overloaded, which resulted in data loss and garbling. Garbling occurred when there were timing conflicts between two planes replying to different interrogators, when planes within a certain range replied to the same interrogator and their signals overlapped, or when multipath reflections occurred that distorted the signals. In order to solve these problems, ATCAC investigated a number of different system alternatives, from completely replacing ATCRBS to suggesting modifications/upgrades to the existing system. The final recommendation of ATCAC was to upgrade ATCRBS, allowing it to handle traffic into the 1990s.
RecommendationsThe ATCAC proceedings in the late 1960's resulted in a vision for future air traffic control systems and technologies extending well into the 1980's. In order to achieve this vision, ATCAC suggested upgrading ATCRBS, the existing surveillance technology, changing runway and air separation standards, and possibly developing collision avoidance systems. The significant features of the recommended ATCRBS upgrade were indicative of the main concerns that ATCAC considered: cost, implementability, and performance.
Essentially, ATCAC felt that better air traffic control depended on a two way point-to-point air-to-ground data link, with better data transfer rates than previous technology. By improving beacon technology, this data link could be incorporated into the current ATCRBS system for obtaining aircraft identification and altitude information.
The fundamental problem with existing beacon communication was an overload of the allocated frequencies, 1030 MHz for interrogations and 1090 MHz for replies. To increase the scalability of beacon communications, ATCAC advised the use of discrete addressing to reduce the amount of traffic on the assigned communications channels. Discrete addressing would mean that only the targeted aircraft would respond to any given interrogation, rather than all aircraft in the range of the beacon. Additionally, if beacon technology were improved, it would require fewer interrogations and replies for a given aircraft.
ATCAC was extremely sensitive to the needs of the user; it recognized that aircraft owners and airlines could not easily replace their old system with an entirely new one. Therefore, it unanimously agreed that ATCRBS should be upgraded, rather than replaced by an entirely separate system. Any new technology that was developed would have to be completely interoperable with ATCRBS to accommodate those who did not adopt the new system. This decision was made despite the fact that ATCAC had ``basically been charged with a fresh piece of paper to look at air traffic control.'' The motivation for this was primarily to minimize deployment costs in order to facilitate acceptance of the new technology within the conservative aviation community. Additionally, the distributed nature of the air traffic control system made it implausible to enforce an instantaneous conversion from one system to another. Based on recommendations from technical experts, ATCAC concluded that incremental upgrade was feasible, and that the benefits of reduced risk and cost outweighed the increased design difficulty of the new system.
Much of the above came from the Report of Department of Transportation Air Traffic Control Advisory Committee, Dec. 1969