Litton 51's
In the days of the Litton 51's we always updated along the way over a known point. The INS would remember that posistion unless you updated again, or deleted it.
If you didn't update, you could be a bit off after a 10-12 hour flight.....
The last airplane I flew had triple laserefs with dme/gps updating.
The system always remembered the last update position.