All critical systems have used inertial navigation for decades. The systems may accept GPS corrections within the error margins of inertial navigation but nothing more.
Inertial navigation used to have larger margins of error than GPS. Since GPS was not trustworthy and inertial is, they accept GPS fine-tuning to the trusted inertial system. If the inertial system and GPS system disagree, GPS is ignored.
State-of-the-art inertial systems are now more precise than GPS.