Nigel, you need to do some GA flying
I do plenty
but all in Permit aircraft (JPs, RVs and Yaks) so VMC only, and with, by defintion, "low tech" aids / instruments. Rest of flying is airliner, so 99.5% CAS, with the odd RAS outside briefly when circumstances demand.
Skywatch, in a similiar way to full TCAS, paints a bubble around the aircraft
TCAS doesn't really paint a "bubble"... it is time based, so someone can get quite close, but if assessed as not a collision risk, it says nothing (might show something but can get excluded from the display).... whereas if a potential collision is assessed, it goes TA, and RA when it gets drastic
In Skywatch's case if an aircraft enters the bubble you will get an audible warning and the display will indicate the height and position of the traffic relative to you, together with the positional trend of the traffic.
Answer depends on how accurately it plots the 3D relaitonship. We all understand the vertical (Mode C), but Range? Bearing? How do these things assess Range? I understood one system to rely on signal strength? But how does this work with different power transponders, and variations in power as, say, the Line-of-Sight might get interrupted by other antenna, gear legs, even wings in a turn etc.? Bearing I think we seem to agree is "indicative" only?
What do you do? Do you do anything until you have acquired the traffic visually?
In theory, it would seem "nothing". In practice, it would seem you need to ensure a separation is achieved in excess of the limitations / accuracy of the "system" (system comprising hardware units and user SOPs / ability) i.e. in a simplistic sense, if the "accuracy" of the hardware was, say 95% likely to 2NM, you would want to clear all targets by >2NM. But we'd quickly get into complex maths since I bet it is not a raw "x NM", but a combination of factors... And as stated above, you also need to factor in "what the other target" is? It might be using TCAS on your Mode C - and therefore unknown to you it might have an avoidance strategy completely incompatible to yours? And/or ATC (who at least will be trying for decent amounts of separation)... or someone just dozing along...
Whatever, you and I (and others) have been applying thought to the problem... and it might result in a solution sometime
NoD