Thanks for the info.
If the SSR response is OK on the other a/c then surely my equipment would be able to detect the reply and show direction with a blue dimond with no level or trend to the right of me, there was nothing until the other a/c was ahead of us.
OK about the software, it was a Learjet type of a/c (I'm not much of an anorak so can't tell you the type even though it was close), could his transponder be intermitant or switched off at the time ?
Cheers
