I know the navy is also working on a gps alternative for their nuclear submarines. They currently use inertial navigation, but they still have to surface period

I know the navy is also working on a gps alternative for their nuclear submarines. They currently use inertial navigation, but they still have to surface periodically to recalibrate it using radio signals. Ideally they would eliminate the need to surface for navigation, especially while operating underneath Arctic ice where surfacing is dangerous and not always possible.