I heard one guy today being warned off the LHR zone. His answer? "We're right on the line according to my GPS"
Maybe that is not the very best example, because prob99.99 he was in the right.
At say 20-30nm from the radar head, a GPS will be about 100x more accurate.
OCAS is OCAS.
I think the willy waving reference was re airports that want Class D to cover the one 737 per day.
Actually I have no problem with CAS - provided one gets a transit when one asks for it. Which is not the case, anywhere in Europe.