How does inertial navigation function?

Inertial navigation tracks the position of an object by measuring and comparing the distance it travels with the accelerations in various directions it experiences while moving. For clarification, gyroscopes are used that, once spun up, tend to remain in one position (like the gyroscopes we played with as kids). When the gyroscope position is moved, sensors detect the direction, speed, and duration (vector) of the movement which is then calculated to determine a resultant change in position of the gyroscope. Inertial Navigation Systems (INS) have a built in error due to the precessing (caused by the earth's movement) that all gyroscopes experience. In aircraft, this error can be as much as a half a mile per hour. Over a 5 hour flight, for instance, an INS could be 2.5 miles off and still be considered good. By comparison, a GPS over the same period of time will usually have an accuracy of 30 yards or less. For this reason, and the fact that inertial navigation systems are quite expensive, INS is no longer considered state of the art technology.