Inertial navigation can be paired with GPS (and friends), terrain recognition, etc. to keep it accurate, and it can be used to detect jamming attacks on GPS. You can even use fairly low accuracy IMUs to detect jamming attacks on GPS, so it's certainly worth using.
With enough accuracy the frequency at which an IMU needs to be recalibrated with GPS or other systems can be low enough that a ship need only do it when at port, or a plane need only do it when parked or at an airport gate. I.e., at places where differential GPS can be available and/or where location can be known statically since the location only changes due to continental drift (i.e., very slowly).
With enough accuracy the frequency at which an IMU needs to be recalibrated with GPS or other systems can be low enough that a ship need only do it when at port, or a plane need only do it when parked or at an airport gate. I.e., at places where differential GPS can be available and/or where location can be known statically since the location only changes due to continental drift (i.e., very slowly).