thanks rudderrudderrat for your quick reply!
I also read "Then the TCAS computes the intruder trajectory, the Closest Point of Approach (CPA) and the estimated time (TAU) before reaching the CPA" in some online documents. It is the sentence which caused confusion,
1. the CPA concept is a objective one,
2. "computes the intruder trajectory" implies that the trajectory is in a 3D objective coordinate. If TCAS runs under the relative approach, then the "trajectory" is always pointing towards the TCAS plain itself, rather than pointing to the CPA.
You are very welcome to further discuss on this very curious topic!~
