Salute!
What seems to be missing here is the advancement in clocks The folks working the "longitude problem" back then would have cut off part of their body to have the accuracy of a fairly modest wristwatch you can buy now at the drugstore.
Our clocks got so much better in later 60's and onward, that we could do things the old navigators could not even dream of. Further, the GPS sats give a time synch with the nav message, so your unit can back in the time the message was transmitted, and then do it for three or more sats. Easy peasy with three or more positions and nanosecond times, to plot the time of arrival arcs and such - think the MA370 problem, huh? For stationary receivers like surveyors use, you can track the carrier wave itself and resolve the phase of the wave to a few millimeters.
The advances in MEMS and super sftwe algorithms have made small GPS and GPS-aided inertial systems orders of magnitude better than what I flew back in the 70's and 80's.
Gums sends...