The introduction of new CASs results in a multiplication of the number of different systems in the airspace. And this is not without consequences. During ACAS Xa integration tests, though the coordination was working perfectly, it was noted that the performances of TCAS were degraded. This highlighted the fact that CASs need not only to be able to coordinate but they also need to be interoperable.
Unlike other ACASs (ACAS II, ACAS X), ACAS Xu can provide both vertical and horizontal resolutions. The challenge is: how to choose between vertical and horizontal maneuvers? The choice is done by a module called Nucleus. However, little information on what Nucleus might be was publicly available until last September.