electronic system that continuously monitors the position, velocity, and acceleration of a vehicle, usually a submarine, missile, or airplane, and thus provides navigational data or control without need for communicating with a base station.

The basic components of an inertial guidance system are gyroscopes (gyroscope), accelerometers (accelerometer), and a computer. The gyroscopes provide fixed reference directions or turning rate measurements, and accelerometers measure changes in the velocity of the system. The computer processes information on changes in direction and acceleration and feeds its results to the vehicle's navigation system.

There are two fundamentally different types of inertial navigation systems: gimbaling systems and strapdown systems. A typical gimbaling inertial navigation system, such as might be used on board a missile, uses three gyroscopes and three accelerometers. The three gimbal-mounted gyroscopes establish a frame of reference for the vehicle's roll (rotation about the axis running from the front to the rear of the vehicle), pitch (rotation about the axis running left to right), and yaw (rotation about the axis running top to bottom). The accelerometers measure velocity changes in each of these three directions. The computer performs two separate numerical integrations (integration) on the data it receives from the inertial guidance system. First it integrates the acceleration data to get the current velocity of the vehicle, then it integrates the computed velocity to determine the current position. This information is compared continuously to the desired (predetermined and programmed) course.

In a strapdown inertial navigation system the accelerometers are rigidly mounted parallel to the body axes of the vehicle. In this application the gyroscopes do not provide a stable platform; they are instead used to sense the turning rates of the craft. Double numerical integration, combining the measured accelerations and the instantaneous turning rates, allows the computer to determine the craft's current velocity and position and to guide it along the desired trajectory.

In many modern inertial navigation systems, such as those used on commercial jetliners, booster rockets, and orbiting satellites, the turning rates are measured by ring laser gyroscopes (gyroscope) or by fibre-optic gyroscopes. Minute errors in the measuring capabilities of the accelerometers or in the balance of the gyroscopes can introduce large errors into the information that the inertial guidance system provides. These instruments must, therefore, be constructed and maintained to strict tolerances, carefully aligned, and reinitialized at frequent intervals using an independent navigation system such as the global positioning system (GPS) (GPS).