The thing that I have been having difficulty with is contradictory information regarding TCAS logic on the Net. On the one hand I have read that TCAS logic doesn't use the bearing (because of errors due to antennas) and it only uses range and closure rate to compute the time-to-go to CPA, and in doing so it compares this value to TAU threshold (Is that right??).
To be honest, since I'm only trained to follow whatever the TCAS commands me to do, I have no idea what logic is being used.
Although, I have been told that the indication we have in the cockpit of the bearing of an other aircraft is not 100% accurate, while the range is being calculated very accurately. (Because of the time between interrogation and reply)
If I have to create a failsafe system, I would only take the accurate measurements, which are range, closure rate and altitude (measured by the other aircraft) and only use the bearing for situational awareness.
I'm afraid I can't help you any further.