ICBMs and SLBMs use primarily inertial navigation (with a starting position provided from the launch facility); the stellar navigation systems correct it by add

ICBMs and SLBMs use primarily inertial navigation (with a starting position provided from the launch facility); the stellar navigation systems correct it by adding a mid-course position fix. I do not know for sure, but I would be willing to bet, that the missiles are capable of reaching their target with reasonable accuracy in the absence of the mid-course stellar fix, since in a nuclear war with enough exo-atmospheric detonations, it might not be possible to visualize the fixed stars at the right time (and a requirement to do this would be an obvious vulnerability for a missile defense system, e.g. you could hit the missile with a laser from a space platform at the right moment and spoof the stellar nav system).