Inertial navigation and accumulated drift

Ring-laser gyroscopes and related systems infer movement from very small changes in orientation and acceleration. They are especially valuable for submarines, missiles, aircraft, and other platforms that cannot always obtain an outside position fix. Their accuracy nevertheless degrades with distance and time from a known starting point, producing a drift rate that must be monitored. A submarine can periodically raise a mast to obtain GPS or another external fix, compare it with the inertial estimate, and correct the accumulated error. Modern systems can use repeated comparisons to refine their own programmed model, while fiber-optic and quartz approaches offer related ways of measuring motion.