It will definitely require corrections from GPS or other systems, but if made sufficiently accurate; it could be months or even years before the accumulated errors necessitate a correction.

What seems more concerning to me is a system like this would require 100% up time between outside corrections.

A gps receiver can acquire its position from a completely powered off state. Inertial guidence though, needs to be told its current position; then it can keep track of where it goes from there. If there's any hiccup with power, you've completely lost your location fix and can't reacquire it alone.

Put the two together though, and the inertial guidence can accurately fill in the gaps between gps service while also getting regular updates/corrections when you do have that signal available.