Inertial navigation is a technique of navigation that does not depend on external references such as compasses, stars, gravity or GPS, but computes the present position by sensing the movement of the navigating platform from a precisely known starting point in space. The general approach is to use multiple gyroscopes as references for the axes of position, as well as for error correction, using accelerometers to sense the motion.
There are two major types of inertial navigation sensors, gimballed, where the sensors can move, and strapdown, where they are bonded to the moving vehicle.
The method is exceptionally important for precision-guided munitions, space vehicles, and military ships, submarines and aircraft, because it
- Does not require external navigation infrastructure
- Is resistant to electronic warfare
Inertial navigation is well-established technique; it is not to be confused with the concept of inertial propulsion developed within the Russian space research program.
Modes of operation
- Space stable
- Local level north slaved
- Local level wander or free azimuth
- Two accelerometer local level
In practice, inertial information is combined with other sources, such as GPS satellite.
Essential to inertial navigation are gyroscopes, accelerometers, and computers, as well as the mathematics of inertial position comutation.
Gyroscopes, first used as non-magnetic compasses and as true vertical indicators in aviation, are the fundamental enabling technology. Inertial navigation greatly benefited when gyroscope technology moved beyond mechanical to optical techniques, which do not require moving parts and can be much smaller.
Gyroscope-generated information includes:
- rate integrating
- single degree of freedom
- two degree of freedom gyro
Inertial navigation is referenced to a geodetic coordinate system.
Gyrocompasses were precursors of full inertial navigation.
A first major implementation was the Ship Inertial Navigation System aboard the first U.S. ballistic missile submarines, which needed absolutely precise position information for accurate missile launching, but whose operational security depended on not exposing themselves for star sights, radio navigation, etc. While the submarine systems have continued to improve, and intertial navigation became common on surface warships, the United States Navy became concerned that it had multiple systems (i.e., AN/WSN-5 aboard surface combatants, the AN/WSN-3 aboard submarines and the AN/WSN-1 aboard aircraft carriers) where one would do, and that much less expensive commercial-off-the-shelf components were available. The AN/WSN-7 Ring Laser Gyro Navigation System, will replace all three, which are 15-20 years old and for which it is increasingly difficult to obtain spare parts. 
- ↑ Ring Laser Gyro Navigator Project Team, Acquiring a Common NDI Ship/Submarine Inertial Navigation System, Naval Sea Systems Command, United States Navy