IIRC.... NAVWASS on UK Jaguars and others used a system of the following.....
Program into the nav comp your known position using Eastings and Northings.
Aircraft gyros and pitot tubes giving references to turns, speeds etc will update the nav comp in real time using a nice mix of both analogue and digital electronic gubbins.
No GPS required and certainly no GPS was used in my time in avionics.
Navigational error? Erm, none that I can ever remember.