What is an IMU?

An IMU (inertial measurement unit) is a small cluster of sensors that measures a body’s own motion: how it is rotating and how it is accelerating, reported many times per second, using nothing outside the vehicle. Feed that motion into a computer and you can work out which way the vehicle points and, with help, where it has gone. No satellites, no beacons, no infrastructure.

An IMU does not, by itself, tell you your position on a map. It tells you how you are moving. Turning that into position and heading is the job of the software and aiding sensors around it. That distinction is the whole reason the terms IMU, AHRS and INS exist.

What is inside an IMU: degrees of freedom

Every IMU carries at least gyroscopes and accelerometers: three of each, one per axis. Three axes of rotation plus three axes of acceleration is six signals, or six degrees of freedom (6-DoF). That is the floor. Many IMUs add more sensors to give the navigation software extra references:

ConfigurationSensorsWhat the extra sensors add6-DoF3 gyroscopes + 3 accelerometersthe core: rotation rate and acceleration on every axis9-DoF+ 3 magnetometersmagnetic field on three axes, a heading reference against magnetic north11-DoF+ 2 inclinometersinclination on two axes, a gravity-based level reference

More sensors is not automatically better. Magnetometers help heading but are thrown off by motors and steel; inclinometers help leveling but only when the vehicle is not accelerating hard. What almost always decides IMU quality is the gyroscope, not the sensor count.

What each sensor measures

• Gyroscopes measure rate of rotation. Integrate that over time and you know which way the vehicle is pointing. Gyro quality is the number that matters most, and the one this guide set keeps coming back to.
• Accelerometers measure acceleration, including gravity. They anchor which way is down and contribute to velocity and position estimates.
• Magnetometers measure magnetic field, giving an absolute heading reference when the local magnetic environment is clean.
• Inclinometers measure tilt relative to gravity, a stable level reference when the platform is still or moving smoothly.

IMU, AHRS, INS: the family it gets confused with

The same sensors sit at the core of three products that do increasingly more of the math for you:

• IMU: outputs raw motion (rotation and acceleration). What you do with it is your job.
• AHRS (attitude and heading reference system): computes orientation, which way you point, but not position.
• INS (inertial navigation system): wraps an IMU in a navigation computer that integrates the motion, usually fuses aiding sources, and outputs position, velocity and attitude directly.

Same sensors at the core; the difference is how much computation is done for you. See our guide IMU vs INS: which do you need?

What separates a good IMU from a poor one

Not the sensor count, and not the marketing grade name. It is gyro bias stability: how much the gyroscope’s zero-point wanders while it runs, measured in degrees per hour. Small wander means position error grows slowly when there is no GNSS to correct it. The industry sorts IMUs into rough grades by this number:

OSCP’s photonic units span tactical grade (0.5 °/hr, MK2E2) to a navigation-grade prototype (0.005 °/hr on the optical Z axis, MK2Z). The MK2M2 is a tactical MEMS unit.

For what each datasheet number means, see How to read an IMU datasheet.

Where IMUs are used

Anywhere a vehicle has to know how it is moving without relying on an outside signal: drones and other aircraft, ground robots and vehicles, ships and subsea craft, guided and stabilized platforms. The harder the environment (jamming, tunnels, underwater, contested airspace), the more the IMU carries the mission on its own, and the more the gyro grade matters.

Photonic, MEMS, FOG: one line each

• MEMS: smallest and cheapest; drift and vibration sensitivity limit it in demanding navigation.
• FOG: the performance benchmark; large, power-hungry, and priced like it.
• Photonic: FOG-class physics with chip-scale manufacturing, the middle that did not exist until recently.

See What is a photonic IMU? and Photonic vs FOG vs MEMS gyroscopes.

Frequently asked questions

How many sensors are in an IMU?

At least six: three gyroscopes and three accelerometers (6-DoF). Some add three magnetometers (9-DoF) and two inclinometers (11-DoF) to give the navigation software extra references.

What do 6-DoF, 9-DoF and 11-DoF mean?

Degrees of freedom, the count of independent motion signals. 6-DoF is gyroscopes plus accelerometers; 9-DoF adds magnetometers; 11-DoF adds inclinometers.

Is an IMU the same as an INS?

No. An IMU outputs raw motion. An INS adds a navigation computer that turns that motion, plus aiding sources, into position and heading. An IMU plus your own estimator plus aiding is, functionally, an INS you control.

Does an IMU need GPS?

No. An IMU needs nothing external. GNSS, when available, corrects the slow drift that inertial navigation accumulates, but the IMU keeps working when GNSS is jammed or absent. How long it stays accurate is set by the gyro grade and by any aiding sensors.