SSR

And it does make a positive contrubution to air safety, I used to watch the London air traffic centre screens a lot during my time as a mil assistant. The transponder secondary return is marked as a small cross which is superimposed on the primary return.Due to the masking effects of terrain we would loose primary returns when aircraft operated at lower levels but the SSR return could be seen down to 100 feet. Smaller light aircraft are harder to detect on primary radar especially when they are performing sudden changes of attitude eg aerobatics /thermalling. The primary radar return becomes intermittent but the SSR return is very clear.

So if you have SSR then keep it on !