Helo Inertial
Some ancient history: The S-67 upgrade in 1972 had a version of the Litton ASN-86 ( LN-39 I believe ) from the Army OV-1B Mohawk. It was used for nav, running a CDC ( Canada ) moving map display from the A-7E, for the 20mm aim point calculations which fed to the F-111 head up display ( we borrowed from everybody! ), and the helmet tracking system that ran the 3 barreled turret. For 1972 it was pretty good.
Anyhow, that inertial spec was 0.5 nm/hr as I recall, but we got numbers in the 0.3 nm/hr on occasion. The environment was INU friendly: fairly low vibration levels and air conditioning.
Are you thinking of a stand alone INU nav system?