The Story of Mode S:
An Air Traffic Control Data-Link Technology

In the 1960's, the United States air traffic control (ATC) system was in disarray. Delays were increasing, efficiency was low, and air traffic was growing at an rate that the system was unprepared to handle. By the end of the decade, a number of converging factors, on several different fronts, came together to solve these problems. The solution was Mode S, an air traffic control data link technology designed by MIT's Lincoln Labs to alleviate the existing problems of the day as well as to meet the challenges of future growth. Today, Mode S is an integral component of the Traffic Collision Avoidance System (TCAS), which is an international standard for commercial aircraft.

The story behind the development of Mode S is interesting to study, not just for the technical aspects of air traffic control, but also in the larger context of the design of a real-world, complex system. No technology is designed in isolation, so in order to truly understand the technology, we must look at the social climate in which it was developed, and the impact that it had on the technical decisions that were made. For Mode S, the slow adoption rate of new ATC technologies, due to a variety of reasons, caused interoperability to emerge as the driving force behind the design and development of the Mode S technology. Not only did Mode S need to meet the needs of the coming decades, it also had to be perfectly interoperable with its predecessor, the Air Traffic Control Radar Beacon System (ATCRBS).

On this site, we have provided you with the pieces of the Mode S puzzle to give you a clearer picture of both the technology and the influences at work at the time. The background page sets up the context and provides valuable information to understand the road to the development of Mode S, who the players were and what their concerns were. The Mode S technology page delves into the actual system and shows the impact of the interoperability constraint on specific design decisions. Lastly, you can browse through descriptions of the interviews and reports we used, to get an idea of the research that was done or to learn more about particular points. And if you're completely lost with all the acronyms and terminology, we've provided you with a glossary of commonly used terms.