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Figure 1: TCAS resolution advisory events in Boston area.

Traffic Alert/Collision Avoidance System (TCAS)

The Traffic Alert and Collision Avoidance System (TCAS) has had extraordinary success in reducing the risk of mid-air collisions by alertingpilots directly in the cockpit that a collision with another aircraft is imminent. TCAS also provides guidance to climb or descend to avoid a collision. Now mandated on all large transport aircraft, TCAS has been in operation for over a decade and has been credited with preventing several catastrophic accidents.

Lincoln Laboratory’s involvement in TCAS dates back to 1974, when the FAA tasked Lincoln to develop the surveillance subsystem and the MITRE Corporation to develop the collision avoidance algorithms, also known as the “threat logic.” In addition to surveillance development, in the mid-1970s Lincoln began TCAS-related monitoring of aircraft in the Boston airspace, using first a Lincoln-developed prototype Mode S sensor and later, FAA production Mode S sensors.

The Lincoln Laboratory Mode S Experimental Facility monitors TCAS performance.

Early monitoring focused on identifying errors in transmitted data that would impact the performance of a collision avoidance system, such as garbled aircraft-reported altitude. Later monitoring focused on assessing the appropriateness of collision-avoidance advisories and the impact of these advisories on airspace operation. In the mid-1990s, Lincoln undertook a third area of activity: assessing the threat logic (the algorithms for threat detection and resolution). Because of the growing complexity of the threat logic, Lincoln Laboratory and the FAA William J. Hughes Technical Center began developing simulation and analysis tools to perform specific types of threat logic assessment.

Since 2004, Lincoln has been assessing the frequency of an identified vulnerability in the TCAS threat logic and evaluating several proposed changes to the algorithms. Lincoln is also exploring the future of TCAS and how new technologies such as Automatic Dependent Surveillance – Broadcast (ADS-B) can be leveraged to improve the safety and efficiency of air transportation.

Major accomplishments

• Lincoln developed metrics and analysis techniques which enabled FAA upper management to understand the safety impact of the logic problem and to quantify the cost/benefit of proposed solutions.
• Lincoln monitored and analyzed TCAS performance in the Boston airspace (Figure 1), confirming the existence of the problem in the U.S. Lincoln is now leading the expansion of this monitoring effort to sites across the U.S.
• Lincoln developed a state-of-the-art TCAS logic simulation, making use of the Lincoln Laboratory parallel computing facility, LLGrid, to assess the current and proposed logic in a statistically robust manner.
• Lincoln provided the U.S. expertise on TCAS airspace modeling issues and currently leads a multi-organization team in development of a new U.S. airspace model. Lincoln also continues to lead a multi-organization team in understanding and documenting both the current and proposed logic.