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Inertial navigation is a way of navigation or position calculation in which measurements provided by accelerometers and gyroscopes are used to calculate the position of a vehicle relative to a known departure point.  This is done without the need for external references like radio navigation beacons or global position satellites.  An inertial navigation system, or INS,  is used on board ships, aircraft, submarines, guided missiles and spacecraft.

Basic Inertial principals
One of the principal elements of an INS are 3 single-axis accelerometers.

Because an aircraft can move in any 3 orthogonal directions,

Forward-backward (x), left-right (y) and up-down (z), there is an

Accelerometer provided for each axis.

To measure the real aircraft accelerations in 3 directions,

the x and y, accelerometers must be maintained parallel

to the  earth surface so they do not measure gravitational

acceleration.

For this reason the accelerometers are mounted on a stabilized platform and mechanically isolated with gimbals from rotational motion (roll, pitch, yaw) of the aircraft. The gimbals are mounted inside one another. Gyroscopes mounted on the platform give an output proportional to the angle through which they have been rotated.

The output of each gyroscope is amplified and connected to a servo motor driving the appropriate gimbal,
thus keeping the platform parallel to the earth surface and in the same direction.

For each accelerometer the computer integrates its output over time to calculate the current velocity in that direction.  Then it integrates the velocity over time to compute the current distance from the point of departure. With vector addition of the x,y and z distances, the actual aircraft position is found.

LN-3 Inertial Navigator on display: http://en.wikipedia.org/wiki/LN-3_Inertial_Navigation_System