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INERTIAL NAVIGATION

General information

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 an object relative to a known departure point.A big advantage is that this is done without the need for external references like radio navigation beacons or global position satellites.Inertial navigation is used onboard ships, airplanes, submarines, guided missiles and space craft.

An inertial navigation system is also known as inertial guidance system, inertial reference system or inertial measurement unit.

The most important parts of an  inertial navigation system are 3 single-axis accelerometers.

Because an airplane can move in 3 directions, forward-backward (x), left-right (y) and upward-downward (z),
there is an accelerometer provided for each axis.

 

 

 

To measure the real airplane 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 the rotational motion (roll, pitch, yaw) of the airplane. The gimbals are mounted inside one another.  Gyroscopes, mounted on the platform, gives 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.

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

The computer contains the servo amplifiers and integrators and provides also special AC and DC voltages required for system operation.

Inertial platform unit with gimbals, gyroscopes, accelerometers and servo motors.

 

Before flight, the pilot has to enter the actual airplane position on the system control panel and started up the alignment phase, so the navigation system can measure the horizontal plane and orientation to north.

The complete alignment phase takes about 8 minutes.

 

Boeing 737  INS control panel

 

 

 

High speed digital computers and laser techniques permit to eliminate the gimbals and mechanical gyroscopes and creating a so called strap down inertial system.

It increases the reliability and accuracy by eliminating all the moving parts.

Litton LN 93 self-contained strap down inertial reference system

 

Laser angular rate sensors are used to measure the direction and velocity of the airplane attitude changes. Because a laser rate sensor can measure rotation in one plane, 3 sensors are used to sense changes in airplane roll, pitch and yaw.
The outputs of the laser sensors are used to correct the accelerometer measurements because they are simply tied down.

Strap down inertial sensor with 3 laser rate sensors

Inertial strap down systems are today widely used in commercial and military aviation.