Two aircraft approaching each other head on with their widgets singing and dancing - how does the conflict get resolved? In other words do they climb and descend or do both aircraft turn right and will the widget indicate what to do?
WIth "full blown" TCAS - they systems "talk" and give mutually compatible avoidance actions in the vertixal plane
only. With a TCAS v Mode C Xpdr just the TCAS equipped aircraft avoids, and hopes/assumes the Mode C traffic keeps his eyes (and ears) shut and maintains everything.
AFAIK, all other systems rely on "pilot" interpretation / decisions. Even the PCAS manufacturer states bearing accuracy as +-22degrees i.e. if it says it's directly ahead of you, it could be 22degrees left or right. When you avoid laterally by visual means, your eyes are very accurate in direction, and also in "trend" which is important in collision avoidance/ assessment. I cannot see how a "reliable" turn could be made on such information, so the vertical plane would seem, again "best".
NoD