Allbore
I think you have a slight misunderstanding here. GPS is modern but is not a replacement for inertial navigation systems like Concorde has. GPS is helpful - in fact it is wonderful in 99% of situations - but it can not be a primary aid whereas INS can. It works anytime anywhere.
Just to emphasize a point already made, Concorde's INSs are corrected by VOR/DMEs. I think it's actually the DME bit.
McD I don't know who you are but interfering with other people's postings is extremely rude and if justified ever is certainly only justified in extremis.