Mode C
In the 80`s there were Military ATC units that didn`t have SSR or a link from other units.
At Brawdy, the old original AR1 was used until about 1989. It was fantastic at showing in great detail all the moisture, rain, birds and occasional shipping. On a good day, the aircraft picture was superb although any conflicting traffic had to be de-conflicted with either London sector 23 or avoid!
To us, SSR was for sometime in the future and therefore we didn`t have to worry about the legalities surrounding verification, ident, Mode C accuracy etc.