So, if an non-squawking, or just Mode A, aircraft inadvertently climbed into CAS from underneath, the controller would assume it was still underneath and would not instruct the IFR traffic to take avoiding action if horizontal seperation is lost?
In which case, would the IFR aircraft's TCAS (assuming a large transport) be able to calculate straight line proximity purely based on a Mode A return..and therefore provide a last line of defence?
I find it difficult to believe the system relies entirely on such an unsafe assumption that height unknown traffic are at not infringing.
If it does, I can certainly see the case for compulsory Mode C transponding/ Mode S.