Inertial navigation systems (INS) use a combination of accelerometers and angular rate sensors (gyroscopes) to detect altitude, location, and motion. They may also be capable of detecting attitude, position, velocity, temperature, or magnetic fields.

Inertial navigation relies on knowing the initial position, velocity, and attitude in order to measure attitude rates and acceleration. The operation of inertial navigation systems depends upon Newton's laws of classical mechanics. It is the only form of navigation that does not rely on external references.

In order to navigate with respect to the inertial reference frame, it is necessary to keep track of the direction in which the accelerometers are pointing. INS keeps track of all movements in all directions by using three accelerometers, one north-south, one east-west, and one up-down, all mounted on a stable platform. A magnetic field is produced by electricity between the two parts and any change in movement by the free part will disturb the magnetic field.

INS consists of an inertial measurement unit (IMU), instrument support electronics, and navigation computers to calculate the gravitational acceleration. IMUs typically contain three orthogonal rate gyroscopes and three orthogonal accelerometers, measuring angular velocity and linear acceleration respectively. These computers also integrate the net acceleration to maintain an estimate of the host vehicle's position.

Advantages and Disadvantages

INS systems are autonomous and do not rely on external aids or visibility conditions. This means they can operate in tunnels or underwater as well as in stealth applications since there is no external antenna that may be detectable by radar. They are well-suited for integrated navigation, guidance, and control of the host vehicle. A system's IMU measures variables such as position, velocity, and altitude.

One disadvantage of INS systems is the cost, including the acquisition cost, operations cost, and maintenance cost. Other disadvantages include increasing navigation errors over time and heat dissipation. Size, weight, and power requirements are still higher than those for GPS receivers but are shrinking with time and technological improvement. 

Performance Specifications

Inertial navigation systems generally fall into two different designs. The first design is gimbaled or stabilized in which the inertial sensors are mounted on a stable platform and mechanically isolated from the rotational motion of the vehicle. They are typically used in systems that require very precise navigation data, such as ships and submarines.

The second design is called a strapdown as the sensors are attached rigidly, or "strapped down", to the body of the host vehicle. The advantages of this approach are lower cost, reduced size, and greater reliability, but the disadvantage is an increase in computing complexity.

Angular rate specifications for inertial navigation systems include angular rate range, bandwidth, transverse sensitivity, and linearity.

• Angular rate range is the maximum rotary rate for which the gyro is rated. If one product or series can be configured for different rates, then the range of maxima is listed.
• Angular bandwidth is the frequency range over which a device meets accuracy specifications before rolling off. Because gyros are almost always capable of DC response, only the high-frequency 3-dB rolloff point is included.
• Angular transverse sensitivity is the maximum output signal due to rotation about an axis orthogonal to the sensitive axis under consideration. It is expressed as a percentage of the orthogonal input angular velocity.
• Angular linearity or rotary axis linearity is measured over an operating temperature range as a percentage (±) of full scale.

Additional specifications for inertial navigation systems include weight, maximum dimension, and operating temperature.

Measurement Technologies

Inertial navigation systems differ in terms of types of gyros and measurement methods.

Types of Gyros

Gyroscopes are sensors for measuring rotation. Optical, spinning-mass, or vibrating gyros are used to sense the angular or rotary rate.

• Optical gyros permit the reflection of a laser ray many times within an enclosure.
• Spinning mass gyros use a steadily moving mass with a free-moving axis (gimbal).
• Vibrating gyros use microelectro-mechanical system (MEMS) technology and a vibrating, quartz tuning-fork to measure Coriolis force.

Measurement Methods

There are many ways to measure linear acceleration, but most inertial navigation systems measure the displacement of a proof mass.

• Capacitance-based devices measure the variable capacitance between a support structure and proof mass.
• Null-balance devices keep the mass nearly centered with positional feedback and a servo-mechanism.
• Inductive position sensors are noncontact devices that determine an object's coordinates (linear or angular) with respect to a reference.
• Piezoelectric devices compress a piezoelectric material and generate a charge that is measured by a charge amplifier.
• Piezoresistive devices change resistance when the material is under pressure, stressed, or deflected. Resonant devices provide frequency-shift outputs. 

Specifications

Parameters for inertial navigation systems include outputs and features.

Output Options

Choices for electrical output are analog voltage, current loop, pulse or frequency, switch or relay outputs, serial or digital output, and network / fieldbus. Additional outputs provide measurements of magnetic fields, temperature, and linear velocity.

Features

Some inertial navigation systems include data recorders or global positioning system (GPS) features.

Standards

Inertial navigation systems that are RoHS compliance meet the requirements of the European Union’s (EU) Restriction of Hazardous Substances directive. Additional standards can be found at the IHS standards store.

AD 75-22-13: Inertial navigation systems\Litton systems.

FAA AC 25-4: Inertial navigation systems.

References

An Introduction to inertial navigation

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