Evo
especially as 'no mode Charlie' includes those with no transponder, not just those with only mode Alpha
Actually it doesn't. Transponding aircraft with no mode 'C' are a pain in the backside from both the 'TCAS' and similar equipped aircraft that are finding their way on the market, as well as from an ATC point of view. If you don't have a transponder well - such is life.
If you go back and read my post you'll understand - you can't avoid what you can't see, but you must avoid what you can't place 3 dimensionally.
The problem a lot of airfields have is they have their own primary radar head, but the SSR aerial can be up to 50 miles away, shared with other users and piped in down the phone line. Now - the average primary radar base of cover increases by 1000ft per 10nm away from the radar. This means that traffic 25 miles away below 2000ft is possibly going to be invisible.
Now - add an SSR aerial which is the other side of the target, on top of a hill, thereby increasing the range by an ability to 'look down' and you end up with SSR returns, but with no corresponding primary. Makes no difference - if you can see the Squawk you have to avoid it, even though primary radar performance may provide the assumption that it's low-level. The one thing you cannot do in that game is assume however.