An Inertial Navigation System (INS) uses motion and rotation sensors along with a computer to figure out the position, orientation, and speed of movement of a vehicle without using the stars, Sun, Moon, or other outside visual references. Traditionally, this was done with mechanical gyroscopes and accelerometers.
A key consideration in inertial navigation systems is that even the most accurate units "drift" over time, thus requiring additional periodic navigation fixes. Because of this, INS units are connected closely with other navigational systems, ranging from star trackers for celestial navigation (as was used on the Lockheed SR-71 Blackbird) to GPS, or even signals for cellphone networks and wireless access points.
