• SJU reference # PJ.11-A1 /Release 9
  • Stakeholders
  • Benefits Safety
  • Status Available for industrialisation/deployment


Airborne collision avoidance systems (ACAS) currently receive information only from Mode C/S interrogations, yet there are other surveillance sources available, such as the more accurate automatic dependent surveillance – broadcast (ADS-B), which could enhance this safety layer. In addition, the performance of collision avoidance can be improved by updating the mathematical processes and modelling used in today’s traffic alert and collision avoidance system (TCAS).

Both these improvements form part of the ACAS Xa being designed for commercial aircraft with the aim of delivering the next generation TCAS in the 2020-2023 timeframe. By introducing additional surveillance data and optimised resolution advisories, ACAS Xa is expected to improve on today’s system without changing the cockpit interface, i.e using the same alerts and presentation.

ACAS Xa implements the surveillance improvement through a surveillance and tracking module (STM) which processes raw surveillance data coming from the surveillance sensors. Meanwhile the resolution advisory improvement is dealt with by the threat resolution module (TRM), which uses the estimated intruder parameters provided by the STM to choose an appropriate avoidance manoeuvre, if necessary.

SESAR has validated the solution using multiple fast-time simulations combined with encounter models designed to replicate rare ACAS events, and one real-time simulation. ACAS Xa is a US Federal Aviation Administration development so SESAR tested the technology in a European environment. The results showed ACAS Xa improves safety in a theoretical environment by as much as 16% when all aircraft are equipped.

ACAS systems need to be interoperable worldwide which is primarily ensured through the standardisation process. The European requirements identified by SESAR are being fed into the global standardisation process.

The solution is available for industrialisation.


Enhanced safety

Fewer nuisance alerts