Originally Posted by
draglift
The MRX does not know if the other aircraft is behind you or in front of you. It just looks at altitude, distance and closure rate and generates a warning accordingly.
So to answer your question. If an aircraft is going to pass very close behind you it will give you a warning until the distance starts to increase. If it is going to pass a significant distance behind you it will not give a warning in the first place.
PROVIDING the other aircraft also has TCAS/PCAS/Transponder. If it doesn't then your PCAS unit isn't going to pick it up.